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    C –FAQ: C –FAQ: Document Transcript

    • C –FAQ:Q: How should I decide which integer type to use?A: If you might need large values (above 32,767 or below -32,767), uselong. Otherwise, if space is very important (i.e. if there are large arrays or manystructures), use short. Otherwise, use int. If well-defined overflow characteristicsare important and negative values are not, or if you want to steer clear of sign-extension problems when manipulating bits or bytes, use one of thecorresponding unsigned types.Although character types (especially unsigned char) can be used as ``tinyintegers, doing so is sometimes more trouble than its worth. The compiler willhave to emit extra code to convert between char and int (making the executablelarger), and unexpected sign extension can be troublesome.A similar space/time tradeoff applies when deciding between float and double.(Many compilers still convert all float values to double during expressionevaluation.) None of the above rules apply if pointers to the variable must have aparticular type.Base Minimum Minimum Maximum Maximumtype size(bits) value(signed) value(signed) value(unsigned)Char 8 -127 127 255Short 16 -32767 32767 65535Int 16 -32767 32767 65535long 32- -2,147,483,647 2,147,483,647 4,294,967,295Q: Why arent the sizes of the standard types precisely defined?A: Though C is considered relatively low-level as high-level languages go, itdoes take the position that the exact size of an object (i.e. in bits) is animplementation detail. Most programs do not need precise control over thesesizes; many programs that do try to achieve this control would be better off ifthey didnt.
    • Type int is supposed to represent a machines natural word size. Its the righttype to use for most integer variables;Q: Since C doesnt define sizes exactly, Ive been using typedefs like int16and int32. I can then define these typedefs to be int, short, long, etc. dependingon what machine Im using. That should solve everything, right?A: If you truly need control over exact type sizes, this is the right approach.There remain several things to be aware of: • There might not be an exact match on some machines. (There are, for example, 36-bit machines.) • A typedef like int16 or int32 accomplishes nothing if its intended meaning is ``at least the specified size, because types int and long are already essentially defined as being ``at least 16 bits and ``at least 32 bits, respectively. • Typedefs will never do anything about byte order problems (e.g. if youre trying to interchange data or conform to externally-imposed storage layouts). • You no longer have to define your own typedefs, because the Standard header <inttypes.h> contains a complete set.Q: What should the 64-bit type be on a machine that can support it?A: The new C99 Standard specifies type long long as effectively being atleast 64 bits, and this type has been implemented by a number of compilers forsome time. (Others have implemented extensions such as __longlong.) On theother hand, its also appropriate to implement type short int as 16, int as 32,and long int as 64 bits, and some compilers do.
    • Q: Whats wrong with this declaration?char* p1, p2;I get errors when I try to use p2.A: Nothing is wrong with the declaration--except that it doesnt do what youprobably want. The * in a pointer declaration is not part of the base type; it ispart of the declarator containing the name being declared . The correct form is:char *p1, *p2;Q: Im trying to declare a pointer and allocate some space for it, but its notworking. Whats wrong with this code?char *p;*p = malloc(10);A: The pointer you declared is p, not *p.Q: Why is this loop always executing once? 1.for(i = start; i < end; i++); 2. { 3. printf("%dn", i); 4. }
    • A: The accidental extra semicolon hiding at the end of the line 1 so thereoccurs an error.Q: I cant seem to define a linked list successfully. I tried typedef struct { char *item; NODEPTR next; } *NODEPTR;but the compiler gave me error messages. Cant a structure in C contain a pointerto itself?A: The correct form is that : 1. To fix this code, first give the structure a tag (e.g. ``struct node). Then, declare the next field as a simple struct node *, or disentangle the typedef declaration from the structure definition, or both. One corrected version would be: typedef struct node { char *item; struct node*next; } *NODEPTR; Or typedef struct node*NODEPTR; struct node { char *item; NODEPTRnext; }; 2. Finally, here is a rearrangement incorporating both suggestions: struct node { char *item; struct node*next; }; typedef struct node *NODEPTR;Q: Whats the difference between const char *p, char const *p?
    • A: The first two are interchangeable; they declare a pointer to a constantcharacter (you cant change any pointed-to characters). char * const p declares aconstant pointer to a (variable) character (i.e. you cant change the pointer).Q: This code, straight out of a book, isnt compiling:int f(){ char a[] = "Hello, world!";}A: Perhaps you have an old, pre-ANSI compiler, which doesnt allowinitialization of ``automatic aggregates (i.e. non-static local arrays, structures,or unions). You have four possible workarounds: • If the array wont be written to or if you wont need a fresh copy during any subsequent calls, you can declare it static (or perhaps make it global). • If the array wont be written to, you could replace it with a pointer: f() { char *a = "Hello, world!"; } You can always initialize local char * variables to point to string literals • If neither of the above conditions hold, youll have to initialize the array by hand with strcpy when the function is called: f() { char a[14]; strcpy(a, "Hello, world!"); } • Get an ANSI-compatible compiler.
    • Q: Whats wrong with this initialization?char *p = malloc(10);My compiler is complaining about an ``invalid initializer, or something.A: Is the declaration of a static or non-local variable? Function calls areallowed in initializers only for automatic variables (that is, for local, non-staticvariables).Q: This program works correctly, but it dumps core after it finishes. Why? 1. struct list { 1. char *item; 2. struct list *next; 2. } /* Here is the main program. */3.main(argc, argv) { ... }A: A missing semicolon at the end of the structure declaration(i.e., line 2)causes main to be declared as returning a structure.Q: Ive experimented with the codeint i = 3;
    • i = i++;on several compilers. Some gave i the value 3, and some gave 4. Which compileris correct?A: There is no correct answer; the expression is undefined. (Also, note thatneither i++ nor ++i is the same as i+1. If you want to increment i, use i=i+1, i+=1,i++, or ++i, not some combination. See also question)Under Cs integral promotion rules, the multiplication is carried out using intarithmetic, and the result may overflow or be truncated before being promotedand assigned to the long int left-hand side. Use an explicit cast on at least one ofthe operands to force long arithmetic: long int c = (long int)a * b;or perhaps long int c = (long int)a * (long int)b;Q: If you cant assign to arrays, then how can int f(char str[]) { if(str[0] == 0) str = "none"; ... }work?A: In this code, str is a function parameter, so its declaration is rewritten bythe compiler. In other words, str is a pointer (of type char *), and it is legal toassign to it.
    • How should I decide which integer type to use?A: If you might need large values (tens of thousands), use long. Otherwise, if space is very important, use short. Otherwise, use int.1.4: What should the 64-bit type be on a machine that can support it?A: C99 specifies long long.1.7: Whats the best way to declare and define global variables?A: The best arrangement is to place each definition in some relevant .c file, with an external declaration in a header file.1.11: What does extern mean in a function declaration?A: Nothing, really; the keyword extern is optional here.1.12: Whats the auto keyword good for?A: Nothing.1.14: I cant seem to define a linked list node which contains a pointer to itself.
    • A: Structures in C can certainly contain pointers to themselves; the discussion and example in section 6.5 of K&R make this clear. Problems arise if an attempt is made to define (and use) a typedef in the midst of such a declaration; avoid this.1.21: How do I declare an array of N pointers to functions returning pointers to functions returning pointers to char?A: char *(*(*a[N])())(); Using a chain of typedefs, or the cdecl program, makes these declarations easier.1.25: My compiler is complaining about an invalid redeclaration of a function, but I only define it once.A: Calling an undeclared function declares it implicitly as returning int.1.25b: Whats the right declaration for main()?A: See questions 11.12a through What am I allowed to assume about the initial values of variables which are not explicitly initialized?A: Uninitialized variables with "static" duration start out as 0,
    • as if the programmer had initialized them. Variables with "automatic" duration, and dynamically-allocated memory, start out containing garbage (with the exception of calloc).1.31: Why cant I initialize a local array with a string?A: Perhaps you have a pre-ANSI compiler.1.31b: Whats wrong with "char *p = malloc(10);" ?A: Function calls are not allowed in initializers for global or static variables.1.32: What is the difference between char a[] = "string"; and char *p = "string"; ?A: The first declares an initialized and modifiable array; the second declares a pointer initialized to a not-necessarily- modifiable constant string.1.34: How do I initialize a pointer to a function?A: Use something like "extern int func(); int (*fp)() = func;" .Section 2. Structures, Unions, and Enumerations
    • 2.1: Whats the difference between struct x1 { ... }; and typedef struct { ... } x2; ?A: The first structure is named by a tag, the second by a typedef name.2.2: Why doesnt "struct x { ... }; x thestruct;" work?A: C is not C++.2.3: Can a structure contain a pointer to itself?A: See question How can I implement opaque (abstract) data types in C?A: One good way is to use structure pointers which point to structure types which are not publicly defined.2.4b: Is there a good way of simulating OOP-style inheritance in C?A: There are some clumsy ways, but nothing like C++.2.6: I came across some code that declared a structure with the last member an array of one element, and then did some tricky
    • allocation to make it act like the array had several elements. Is this legal or portable?A: An official interpretation has deemed that it is not strictly conforming with the C Standard.2.8: Is there a way to compare structures automatically?A: No.2.10: Can I pass constant values to functions which accept structure arguments?A: In C99 you can use "compound literals".2.11: How can I read/write structures from/to data files?A: It is relatively straightforward to use fread and fwrite.2.12: How can I turn off structure padding?A: There is no standard method.2.13: Why does sizeof report a larger size than I expect for a structure type?
    • A: The alignment of arrays of structures must be preserved.2.14: How can I determine the byte offset of a field within a structure?A: ANSI C defines the offsetof() macro in <stddef.h>.2.15: How can I access structure fields by name at run time?A: Build a table of names and offsets, using the offsetof() macro.2.18: I have a program which works correctly, but dumps core after it finishes. Why?A: Check to see if main() is misdeclared, perhaps because a preceding structure type declaration is missing its trailing semicolon, causing main() to be declared as returning a structure. See also questions 10.9 and Can I initialize unions?A: In the original ANSI C, only the first-named member; in C99, using "designated initializers", yes, any member.2.22: Whats the difference between an enumeration and a set of preprocessor #defines?
    • A: There is little difference. The C Standard states that enumerations are compatible with integral types.2.24: Is there an easy way to print enumeration values symbolically?A: No.Section 3. Expressions3.1: Why doesnt the code "a[i] = i++;" work?A: The variable i is both modified and separately referenced in the same expression.3.2: Under my compiler, the code "int i = 7; printf("%dn", i++ * i++);" prints 49. Regardless of the order of evaluation, shouldnt it print 56?A: The operations implied by the postincrement and postdecrement operators ++ and -- are performed at some time after the operands former values are yielded and before the end of the expression, but not necessarily immediately after, or before other parts of the expression are evaluated.3.3: What should the code "int i = 3; i = i++;" do?
    • A: The expression is undefined.3.3b: Heres a slick expression: "a ^= b ^= a ^= b". It swaps a and b without using a temporary.A: Not portably; its behavior is undefined.3.4: Dont precedence and parentheses dictate order of evaluation?A: Operator precedence and explicit parentheses impose only a partial ordering on the evaluation of an expression, which does not generally include the order of side effects.3.5: But what about the && and || operators?A: There is a special exception for these operators: left-to-right evaluation is guaranteed.3.8: Whats a "sequence point"?A: A point (at the end of a full expression, or at the ||, &&, ?:, or comma operators, or just before a function call) at which all side effects are guaranteed to be complete.3.9: So given a[i] = i++; we dont know which cell of a[] gets written to, but i does get incremented by one, right?
    • A: Not necessarily! Once an expression or program becomes undefined, *all* aspects of it become undefined.3.12a: Whats the difference between ++i and i++?A: ++i adds one to i and "returns" the incremented value; i++ returns the prior, unincremented value.3.12b: If Im not using the value of the expression, should I use ++i or i++ to increment a variable?A: Since the two forms differ only in the value yielded, they are entirely equivalent when only their side effect is needed.3.14: Why doesnt the code "int a = 1000, b = 1000; long int c = a * b;" work?A: You must manually cast one of the operands to (long).3.16: Can I use ?: on the left-hand side of an assignment expression?A: No.Section 4. Pointers
    • 4.2: Whats wrong with "char *p; *p = malloc(10);"?A: The pointer you declared is p, not *p.4.3: Does *p++ increment p, or what it points to?A: *p++ increments p. To increment the value pointed to by p, use (*p)++ .4.5: I want to use a char * pointer to step over some ints. Why doesnt "((int *)p)++;" work?A: In C, a cast operator is a conversion operator, and by definition it yields an rvalue, which cannot be assigned to, or incremented with ++.4.8: I have a function which accepts, and is supposed to initialize, a pointer, but the pointer in the caller remains unchanged.A: The called function probably altered only the passed copy of the pointer.4.9: Can I use a void ** pointer as a parameter so that a function can accept a generic pointer by reference?A: Not portably.
    • 4.10: I have a function which accepts a pointer to an int. How can I pass a constant like 5 to it?A: In C99, you can use a "compound literal". Otherwise, declare a temporary variable.4.11: Does C even have "pass by reference"?A: Not really, though it can be simulated.4.12: Ive seen different syntax used for calling functions via pointers.A: The extra parentheses and explicit * are now officially optional, although some older implementations require them.4.15: How do I convert an int to a char *?A: See question 13.1, 8.6, or 19.25, depending on what youre trying to do.Section 5. Null Pointers5.1: What is this infamous null pointer, anyway?
    • A: For each pointer type, there is a special value -- the "null pointer" -- which is distinguishable from all other pointer values and which is not the address of any object or function.5.2: How do I get a null pointer in my programs?A: A constant 0 in a pointer context is converted into a null pointer at compile time. A "pointer context" is an initialization, assignment, or comparison with one side a variable or expression of pointer type, and (in ANSI standard C) a function argument which has a prototype in scope declaring a parameter as being of pointer type. In other contexts (function arguments without prototypes, or in the variable part of variadic function calls) a constant 0 with an appropriate explicit cast is required.5.3: Is the abbreviated pointer comparison "if(p)" to test for non- null pointers valid?A: Yes. The construction "if(p)" works, regardless of the internal representation of null pointers, because the compiler essentially rewrites it as "if(p != 0)" and goes on to convert 0 into the correct null pointer.5.4: What is NULL and how is it defined?A: NULL is simply a preprocessor macro, defined as a null pointer
    • constant, typically 0 or ((void *)0), which is used (as a stylistic convention, in preference to unadorned 0s) to generate null pointers.5.5: How should NULL be defined on a machine which uses a nonzero bit pattern as the internal representation of a null pointer?A: The same as on any other machine: as 0. (The compiler makes the translation, upon seeing a 0, not the preprocessor; see also question 5.4.)5.6: If NULL were defined as "((char *)0)," wouldnt that make function calls which pass an uncast NULL work?A: Not in the most general case. (A cast might still required to tell the compiler which kind of null pointer is required, since it may be different from (char *)0.)5.9: If NULL and 0 are equivalent as null pointer constants, which should I use?A: Either; the distinction is entirely stylistic.5.10: But wouldnt it be better to use NULL, in case the value of NULL changes?A: No. NULL is a constant zero, so a constant zero is equally
    • sufficient.5.12: I use the preprocessor macro "#define Nullptr(type) (type *)0" to help me build null pointers of the correct type.A: This trick, though valid, does not buy much.5.13: This is strange. NULL is guaranteed to be 0, but the null pointer is not?A: A "null pointer" is a language concept whose particular internal value does not matter. A null pointer is requested in source code with the character "0". "NULL" is a preprocessor macro, which is always #defined as 0 (or ((void *)0)).5.14: Why is there so much confusion surrounding null pointers?A: The fact that null pointers are represented both in source code, and internally to most machines, as zero invites unwarranted assumptions. The use of a preprocessor macro (NULL) may seem to suggest that the value could change some day, or on some weird machine.5.15: Im confused. I just cant understand all this null pointer stuff.A: A simple rule is, "Always use `0 or `NULL for null pointers,
    • and always cast them when they are used as arguments in function calls."5.16: Given all the confusion surrounding null pointers, wouldnt it be easier simply to require them to be represented internally by zeroes?A: Such a requirement would accomplish little.5.17: Seriously, have any actual machines really used nonzero null pointers?A: Machines manufactured by Prime, Honeywell-Bull, and CDC, as well as Symbolics Lisp Machines, have done so.5.20: What does a run-time "null pointer assignment" error mean?A: It means that youve written, via a null pointer, to an invalid location. (See also question 16.8.)Section 6. Arrays and Pointers6.1: I had the definition char a[6] in one source file, and in another I declared extern char *a. Why didnt it work?A: The declaration extern char *a simply does not match the actual
    • definition. Use extern char a[].6.2: But I heard that char a[] was identical to char *a.A: Not at all. Arrays are not pointers. A reference like x[3] generates different code depending on whether x is an array or a pointer.6.3: So what is meant by the "equivalence of pointers and arrays" in C?A: An lvalue of type array-of-T which appears in an expression decays into a pointer to its first element; the type of the resultant pointer is pointer-to-T. So for an array a and pointer p, you can say "p = a;" and then p[3] and a[3] will access the same element.6.4: Why are array and pointer declarations interchangeable as function formal parameters?A: Its supposed to be a convenience.6.7: How can an array be an lvalue, if you cant assign to it?A: An array is not a "modifiable lvalue."6.8: What is the real difference between arrays and pointers?
    • A: Arrays automatically allocate space which is fixed in size and location; pointers are dynamic.6.9: Someone explained to me that arrays were really just constant pointers.A: An array name is "constant" in that it cannot be assigned to, but an array is *not* a pointer.6.11: I came across some "joke" code containing the "expression" 5["abcdef"] . How can this be legal C?A: Yes, array subscripting is commutative in C. The array subscripting operation a[e] is defined as being identical to *((a)+(e)).6.12: Whats the difference between array and &array?A: The type.6.13: How do I declare a pointer to an array?A: Usually, you dont want to. Consider using a pointer to one of the arrays elements instead.6.14: How can I set an arrays size at run time?
    • A: Its straightforward to use malloc() and a pointer.6.15: How can I declare local arrays of a size matching a passed-in array?A: If you dont have a C99 compiler, you cant.6.16: How can I dynamically allocate a multidimensional array?A: The traditional solution is to allocate an array of pointers, and then initialize each pointer to a dynamically-allocated "row." See the full list for code samples.6.17: Can I simulate a non-0-based array with a pointer?A: Not if the pointer points outside of the block of memory it is intended to access.6.18: My compiler complained when I passed a two-dimensional array to a function expecting a pointer to a pointer.A: The rule by which arrays decay into pointers is *not* applied recursively. An array of arrays (i.e. a two-dimensional array in C) decays into a pointer to an array, not a pointer to a pointer.
    • 6.19: How do I write functions which accept two-dimensional arrays when the width is not known at compile time?A: Its not always particularly easy.6.20: How can I use statically- and dynamically-allocated multidimensional arrays interchangeably when passing them to functions?A: There is no single perfect method, but see the full list for some ideas.6.21: Why doesnt sizeof properly report the size of an array which is a parameter to a function?A: The sizeof operator reports the size of the pointer parameter which the function actually receives.Section 7. Memory Allocation7.1: Why doesnt the code "char *answer; gets(answer);" work?A: The pointer variable answer has not been set to point to any valid storage. The simplest way to correct this fragment is to use a local array, instead of a pointer.
    • 7.2: I cant get strcat() to work. I tried "char *s3 = strcat(s1, s2);" but I got strange results.A: Again, the main problem here is that space for the concatenated result is not properly allocated.7.3: But the man page for strcat() says that it takes two char *s as arguments. How am I supposed to know to allocate things?A: In general, when using pointers you *always* have to consider memory allocation, if only to make sure that the compiler is doing it for you.7.3b: I just tried the code "char *p; strcpy(p, "abc");" and it worked. Why didnt it crash?A: You got "lucky".7.3c: How much memory does a pointer variable allocate?A: Only enough memory to hold the pointer itself, not any memory for the pointer to point to.7.5a: I have a function that is supposed to return a string, but when it returns to its caller, the returned string is garbage.A: Make sure that the pointed-to memory is properly (i.e. not
    • locally) allocated.7.5b: So whats the right way to return a string?A: Return a pointer to a statically-allocated buffer, a buffer passed in by the caller, or memory obtained with malloc().7.6: Why am I getting "warning: assignment of pointer from integer lacks a cast" for calls to malloc()?A: Have you #included <stdlib.h>?7.7: Why does some code carefully cast the values returned by malloc to the pointer type being allocated?A: Before ANSI/ISO C, these casts were required to silence certain warnings.7.7c: In a call to malloc(), what does an error like "Cannot convert `void * to `int *" mean?A: It means youre using a C++ compiler.7.8: Why does so much code leave out the multiplication by sizeof(char) when allocating strings?A: Because sizeof(char) is, by definition, exactly 1.
    • 7.11: How can I dynamically allocate arrays?A: See questions 6.14 and Ive heard that some operating systems dont actually allocate malloced memory until the program tries to use it. Is this legal?A: Its hard to say.7.16: Im allocating a large array for some numeric work, but malloc() is acting strangely.A: Make sure the number youre trying to pass to malloc() isnt bigger than a size_t can hold.7.17: Ive got 8 meg of memory in my PC. Why can I only seem to malloc 640K or so?A: Under the segmented architecture of PC compatibles, it can be difficult to use more than 640K with any degree of transparency. See also question My program is crashing, apparently somewhere down inside malloc.A: Make sure you arent using more memory than you malloced,
    • especially for strings (which need strlen(str) + 1 bytes).7.20: You cant use dynamically-allocated memory after you free it, can you?A: No. Some early documentation implied otherwise, but the claim is no longer valid.7.21: Why isnt a pointer null after calling free()?A: Cs pass-by-value semantics mean that called functions can never permanently change the values of their arguments.7.22: When I call malloc() to allocate memory for a local pointer, do I have to explicitly free() it?A: Yes.7.23: When I free a dynamically-allocated structure containing pointers, do I also have to free each subsidiary pointer?A: Yes.7.24: Must I free allocated memory before the program exits?A: You shouldnt have to.
    • 7.25: Why doesnt my programs memory usage go down when I free memory?A: Most implementations of malloc/free do not return freed memory to the operating system.7.26: How does free() know how many bytes to free?A: The malloc/free implementation remembers the size of each block as it is allocated.7.27: So can I query the malloc package to find out how big an allocated block is?A: Not portably.7.30: Is it legal to pass a null pointer as the first argument to realloc()?A: ANSI C sanctions this usage, although several earlier implementations do not support it.7.31: Whats the difference between calloc() and malloc()?A: calloc() takes two arguments, and initializes the allocated memory to all-bits-0.
    • 7.32: What is alloca() and why is its use discouraged?A: alloca() allocates memory which is automatically freed when the function which called alloca() returns. alloca() cannot be written portably, is difficult to implement on machines without a stack, and fails under certain conditions if implemented simply.Section 8. Characters and Strings8.1: Why doesnt "strcat(string, !);" work?A: strcat() concatenates *strings*, not characters.8.2: Why wont the test if(string == "value") correctly compare string against the value?A: Its comparing pointers. To compare two strings, use strcmp().8.3: Why cant I assign strings to character arrays?A: Strings are arrays, and you cant assign arrays directly. Use strcpy() instead.
    • 8.6: How can I get the numeric (character set) value corresponding to a character?A: In C, if you have the character, you have its value.8.9: Why is sizeof(a) not 1?A: Character constants in C are of type int.Section 9. Boolean Expressions and Variables9.1: What is the right type to use for Boolean values in C?A: Theres no one right answer; see the full list for some discussion.9.2: What if a built-in logical or relational operator "returns" something other than 1?A: When a Boolean value is generated by a built-in operator, it is guaranteed to be 1 or 0. (This is *not* true for some library routines such as isalpha.)9.3: Is if(p), where p is a pointer, valid?
    • A: Yes. See question 5.3.Section 10. C Preprocessor10.2: Ive got some cute preprocessor macros that let me write C code that looks more like Pascal. What do yall think?A: Bleah.10.3: How can I write a generic macro to swap two values?A: There is no good answer to this question. The best all-around solution is probably to forget about using a macro.10.4: Whats the best way to write a multi-statement macro?A: #define Func() do {stmt1; stmt2; ... } while(0) /* (no trailing ;) */10.6: What are .h files and what should I put in them?A: Header files (also called ".h files") should generally contain common declarations and macro, structure, and typedef definitions, but not variable or function definitions.10.7: Is it acceptable for one header file to #include another?
    • A: Its a question of style, and thus receives considerable debate.10.8a: Whats the difference between #include <> and #include "" ?A: Roughly speaking, the <> syntax is for Standard headers and "" is for project headers.10.8b: What are the complete rules for header file searching?A: The exact behavior is implementation-defined; see the full list for some discussion.10.9: Im getting strange syntax errors on the very first declaration in a file, but it looks fine.A: Perhaps theres a missing semicolon at the end of the last declaration in the last header file youre #including.10.10b: Im #including the header file for a function, but the linker keeps saying its undefined.A: See question Where can I get a copy of a missing header file?
    • A: Contact your vendor, or see question 18.16 or the full list.10.12: How can I construct preprocessor #if expressions which compare strings?A: You cant do it directly; try #defining several manifest constants and implementing conditionals on those.10.13: Does the sizeof operator work in preprocessor #if directives?A: No.10.14: Can I use an #ifdef in a #define line, to define something two different ways?A: No.10.15: Is there anything like an #ifdef for typedefs?A: Unfortunately, no.10.16: How can I use a preprocessor #if expression to detect endianness?A: You probably cant.
    • 10.18: How can I preprocess some code to remove selected conditional compilations, without preprocessing everything?A: Look for a program called unifdef, rmifdef, or scpp.10.19: How can I list all of the predefined identifiers?A: If the compiler documentation is unhelpful, try extracting printable strings from the compiler or preprocessor executable.10.20: I have some old code that tries to construct identifiers with a macro like "#define Paste(a, b) a/**/b", but it doesnt work any more.A: Try the ANSI token-pasting operator ##.10.22: What does the message "warning: macro replacement within a string literal" mean?A: See question Im having trouble using macro arguments inside string literals, using the `# operator.A: See questions 11.17 and Ive got this tricky preprocessing I want to do and I cant
    • figure out a way to do it.A: Consider writing your own little special-purpose preprocessing tool, instead.10.26: How can I write a macro which takes a variable number of arguments?A: Here is one popular trick. Note that the parentheses around printfs argument list are in the macro call, not the definition. #define DEBUG(args) (printf("DEBUG: "), printf args) if(n != 0) DEBUG(("n is %dn", n));Section 11. ANSI/ISO Standard C11.1: What is the "ANSI C Standard?"A: In 1983, the American National Standards Institute (ANSI) commissioned a committee to standardize the C language. Their work was ratified as ANS X3.159-1989, and has since been adopted as ISO/IEC 9899:1990, and later amended.11.2: How can I get a copy of the Standard?
    • A: Copies are available electronically from ansi.com, from ANSI in New York, or from Global Engineering Documents in Englewood, CO, or from any national standards body, or from ISO in Geneva, or republished within one or more books. See the unabridged list for details.11.2b: Where can I get information about updates to the Standard?A: See the full list for pointers.11.3: My ANSI compiler is complaining about prototype mismatches for parameters declared float.A: You have mixed the new-style prototype declaration "extern int func(float);" with the old-style definition "int func(x) float x;". "Narrow" types are treated differently according to which syntax is used. This problem can be fixed by avoiding narrow types, or by using either new-style (prototype) or old-style syntax consistently.11.4: Can you mix old-style and new-style function syntax?A: Doing so is currently legal, for most argument types (see question 11.3).11.5: Why does the declaration "extern int f(struct x *p);" give me a
    • warning message?A: A structure declared (or even mentioned) for the first time within a prototype cannot be compatible with other structures declared in the same source file.11.8: Why cant I use const values in initializers and array dimensions?A: The value of a const-qualified object is *not* a constant expression in the full sense of the term.11.8b: If you cant modify string literals, why arent they defined as being arrays of const characters?A: It would break a lot of existing code.11.9: Whats the difference between "const char *p" and "char * const p"?A: The former declares a pointer to a constant character; the latter declares a constant pointer to a character.11.10: Why cant I pass a char ** to a function which expects a const char **?A: The rule which permits slight mismatches in qualified pointer
    • assignments is not applied recursively.11.12a: Whats the correct declaration of main()?A: int main(int argc, char *argv[]) .11.12b: Can I declare main() as void, to shut off these annoying "main returns no value" messages?A: No.11.13: But what about mains third argument, envp?A: Its a non-standard (though common) extension.11.14a: I believe that declaring void main() cant fail, since Im calling exit() instead of returning.A: It doesnt matter whether main() returns or not, the problem is that its caller may not even be able to *call* it correctly.11.14b: Are there systems where void main() doesnt work?A: Yes.11.15: The book Ive been using always uses void main().
    • A: Its wrong.11.16: Is exit(status) truly equivalent to returning the same status from main()?A: Yes and no. (See the full list for details.)11.17: How do I get the ANSI "stringizing" preprocessing operator `# to stringize the macros value instead of its name?A: You can use a two-step #definition to force a macro to be expanded as well as stringized.11.18: What does the message "warning: macro replacement within a string literal" mean?A: Some pre-ANSI compilers/preprocessors expanded macro parameters even inside string literals and character constants.11.19: Im getting strange syntax errors inside lines Ive #ifdeffed out.A: Under ANSI C, #ifdeffed-out text must still consist of "valid preprocessing tokens." This means that there must be no newlines inside quotes, and no unterminated comments or quotes (i.e. no single apostrophes).
    • 11.20: What are #pragmas ?A: The #pragma directive provides a single, well-defined "escape hatch" which can be used for extensions.11.21: What does "#pragma once" mean?A: It is an extension implemented by some preprocessors to help make header files idempotent.11.22: Is char a[3] = "abc"; legal?A: Yes, in ANSI C.11.24: Why cant I perform arithmetic on a void * pointer?A: The compiler doesnt know the size of the pointed-to objects.11.25: Whats the difference between memcpy() and memmove()?A: memmove() offers guaranteed behavior if the source and destination arguments overlap.11.26: What should malloc(0) do?
    • A: The behavior is implementation-defined.11.27: Why does the ANSI Standard place limits on the length and case- significance of external identifiers?A: The problem is older linkers which cannot be forced (by mere words in a Standard) to upgrade.11.29: My compiler is rejecting the simplest possible test programs, with all kinds of syntax errors.A: Perhaps it is a pre-ANSI compiler.11.30: Why are some ANSI/ISO Standard library functions showing up as undefined, even though Ive got an ANSI compiler?A: Perhaps you dont have ANSI-compatible headers and libraries.11.31: Does anyone have a tool for converting old-style C programs to ANSI C, or for automatically generating prototypes?A: See the full list for details.11.32: Why wont frobozz-cc, which claims to be ANSI compliant, accept this code?A: Are you sure that the code being rejected doesnt rely on some
    • non-Standard extension?11.33: Whats the difference between implementation-defined, unspecified, and undefined behavior?A: If youre writing portable code, ignore the distinctions. Otherwise, see the full list.11.33b: What does it really mean for a program to be "conforming"?A: The Standard talks about three kinds of conformance: conforming programs, strictly conforming programs, and conforming implementations. (See the full list for definitions.)11.34: Im appalled that the ANSI Standard leaves so many issues undefined.A: In most of these cases, the Standard is simply codifying existing practice.11.35: I just tried some allegedly-undefined code on an ANSI-conforming compiler, and got the results I expected.A: A compiler may do anything it likes when faced with undefined behavior, including doing what you expect.
    • Section 12. Stdio12.1: Whats wrong with the code "char c; while((c = getchar()) != EOF) ..."?A: The variable to hold getchars return value must be an int.12.1b: How do I enter EOF from the keyboard?A: Depending on your operating system, usually either control-D or control-Z.12.2: Why wont the code "while(!feof(infp)) { fgets(buf, MAXLINE, infp); fputs(buf, outfp); }" work?A: EOF is only indicated *after* an input routine fails.12.4: My programs prompts and intermediate output dont always show up on the screen.A: Its best to use an explicit fflush(stdout) whenever output should definitely be visible.12.5: How can I read one character at a time, without waiting for the RETURN key?A: See question 19.1.
    • 12.6: How can I print a % character with printf?A: "%%".12.9: How can printf() use %f for type double, if scanf() requires %lf?A: Cs "default argument promotions" mean that values of type float are promoted to double.12.9b: What printf format should I use for a typedef when I dont know the underlying type?A: Use a cast to convert the value to a known type, then use the printf format matching that type.12.10: How can I implement a variable field width with printf?A: Use printf("%*d", width, x).12.11: How can I print numbers with commas separating the thousands?A: There is no standard function (but see <locale.h>).12.12: Why doesnt the call scanf("%d", i) work?
    • A: The arguments you pass to scanf() must always be pointers.12.12b: Why *does* the call "char s[30]; scanf("%s", s);" work?A: What scanf() needs is pointers, and arrays are always passed to functions as pointers. See question Why doesnt the code "double d; scanf("%f", &d);" work?A: Unlike printf(), scanf() uses %lf for double, and %f for float.12.15: How can I specify a variable width in a scanf() format string?A: You cant.12.17: When I read numbers from the keyboard with scanf "%dn", it seems to hang until I type one extra line of input.A: Try using "%d" instead of "%dn".12.18a: Im reading a number with scanf %d and then a string with gets(), but the compiler seems to be skipping the call to gets()!A: scanf() and gets() do not work well together.
    • 12.19: Im re-prompting the user if scanf() fails, but sometimes it seems to go into an infinite loop.A: scanf() tends to "jam" on bad input since it does not discard it.12.20: Why does everyone say not to use scanf()? What should I use instead?A: scanf() has a number of problems. Usually, its easier to read entire lines and then interpret them.12.21: How can I tell how much destination buffer space Ill need for an arbitrary sprintf call? How can I avoid overflowing the destination buffer with sprintf()?A: Use the new snprintf() function, if you can.12.23: Why does everyone say not to use gets()?A: It cannot be prevented from overflowing the input buffer.12.24: Why does errno contain ENOTTY after a call to printf()?A: Dont worry about it. It is only meaningful for a program to inspect the contents of errno after an error has been reported.
    • 12.25: Whats the difference between fgetpos/fsetpos and ftell/fseek?A: fgetpos() and fsetpos() use a special typedef which may allow them to work with larger files than ftell() and fseek().12.26a: Will fflush(stdin) flush unread characters from the standard input stream?A: No.12.26b: If fflush() wont work, what can I use to flush input?A: It depends on what youre trying to do; see the full list for details. (But first see question 12.20.)12.27: fopen() is failing for certain pathnames.A: See questions 19.17 and 19.17b.12.30: Im trying to update a file in place, by using fopen mode "r+", but its not working.A: Be sure to call fseek between reading and writing.12.33: How can I redirect stdin or stdout from within a program?
    • A: Use freopen().12.34: Once Ive used freopen(), how can I get the original stream back?A: There isnt a good way. Try avoiding freopen.12.36b: How can I arrange to have output go two places at once?A: You could write your own printf variant which printed everything twice. See question How can I read a binary data file properly?A: Be sure to specify "rb" mode when calling fopen().Section 13. Library Functions13.1: How can I convert numbers to strings?A: Just use sprintf().13.2: Why does strncpy() not always write a 0?
    • A: For mildly-interesting historical reasons.13.5: Why do some versions of toupper() act strangely if given an upper-case letter?A: Older versions of toupper() and tolower() did not always work as expected in this regard.13.6: How can I split up a string into whitespace-separated fields?A: Try strtok().13.7: I need some code to do regular expression and wildcard matching.A: regexp libraries abound; see the full list for details.13.8: Im trying to sort an array of strings with qsort(), using strcmp() as the comparison function, but its not working.A: Youll have to write a "helper" comparison function which takes two generic pointer arguments, converts them to char **, and dereferences them, yielding char *s which can be usefully compared.13.9: Now Im trying to sort an array of structures, but the compiler is complaining that the function is of the wrong type for qsort().
    • A: The comparison function must be declared as accepting "generic pointers" (const void *) which it then converts to structure pointers.13.10: How can I sort a linked list?A: Algorithms like insertion sort and merge sort work well, or you can keep the list in order as you build it.13.11: How can I sort more data than will fit in memory?A: You want an "external sort"; see the full list for details.13.12: How can I get the time of day in a C program?A: Just use the time(), ctime(), localtime() and/or strftime() functions.13.13: How can I convert a struct tm or a string into a time_t?A: The ANSI mktime() function converts a struct tm to a time_t. No standard routine exists to parse strings.13.14: How can I perform calendar manipulations?
    • A: The ANSI/ISO Standard C mktime() and difftime() functions provide some (limited) support for both problems.13.15: I need a random number generator.A: The Standard C library has one: rand().13.16: How can I get random integers in a certain range?A: One method is something like (int)((double)rand() / ((double)RAND_MAX + 1) * N)13.17: Each time I run my program, I get the same sequence of numbers back from rand().A: You can call srand() to seed the pseudo-random number generator with a truly random initial value.13.18: I need a random true/false value, so Im just taking rand() % 2, but its alternating 0, 1, 0, 1, 0...A: Try using the higher-order bits: see question How can I generate random numbers with a normal or Gaussian distribution?
    • A: See the longer versions of this list for ideas.13.25: I get errors due to library functions being undefined even though I #include the right header files.A: You may have to explicitly ask for the correct libraries to be searched.13.26: Im still getting errors due to library functions being undefined, even though Im requesting the right libraries.A: Library search order is significant; usually, you must search the libraries last.13.28: What does it mean when the linker says that _end is undefined?A: You generally get that message only when other symbols are undefined, too.13.29: My compiler is complaining that printf is undefined!A: See if you can create a "console application".Section 14. Floating Point
    • 14.1: When I set a float variable to 3.1, why is printf printing it as 3.0999999?A: Most computers use base 2 for floating-point numbers, and many fractions (including 0.1 decimal) are not exactly representable in base 2.14.2: Why is sqrt(144.) giving me crazy numbers?A: Make sure that you have #included <math.h>, and correctly declared other functions returning double.14.3: I keep getting "undefined: sin" compilation errors.A: Make sure youre actually linking with the math library.14.4a: My floating-point calculations are acting strangely and giving me different answers on different machines.A: First, see question 14.2 above. If the problem isnt that simple, see the full list for a brief explanation, or any good programming book for a better one.14.5: Whats a good way to check for "close enough" floating-point equality?
    • A: The best way is to use an accuracy threshold which is relative to the magnitude of the numbers being compared.14.6: How do I round numbers?A: For positive numbers, try (int)(x + 0.5) .14.7: Where is Cs exponentiation operator?A: Try using the pow() function.14.8: The predefined constant M_PI seems to be missing from <math.h>.A: That constant is not standard.14.9: How do I test for IEEE NaN and other special values?A: There is not yet a portable way, but see the full list for ideas.14.11: Whats a good way to implement complex numbers in C?A: It is straightforward to define a simple structure and some arithmetic functions to manipulate them.
    • 14.13: Im having trouble with a Turbo C program which crashes and says something like "floating point formats not linked."A: You may have to insert a dummy call to a floating-point library function to force loading of floating-point support.Section 15. Variable-Length Argument Lists15.1: I heard that you have to #include <stdio.h> before calling printf(). Why?A: So that a proper prototype for printf() will be in scope.15.2: How can %f be used for both float and double arguments in printf()?A: In variable-length argument lists, types char and short int are promoted to int, and float is promoted to double.15.3: Why dont function prototypes guard against mismatches in printfs arguments?A: Function prototypes do not provide any information about the number and types of variable arguments.15.4: How can I write a function that takes a variable number of
    • arguments?A: Use the <stdarg.h> header.15.5: How can I write a function that takes a format string and a variable number of arguments, like printf(), and passes them to printf() to do most of the work?A: Use vprintf(), vfprintf(), or vsprintf().15.6: How can I write a function analogous to scanf(), that calls scanf() to do most of the work?A: C99 supports vscanf().15.8: How can I discover how many arguments a function was actually called with?A: Any function which takes a variable number of arguments must be able to determine *from the arguments values* how many of them there are.15.9: My compiler isnt letting me declare a function that accepts *only* variable arguments.A: Standard C requires at least one fixed argument.
    • 15.10: Why isnt "va_arg(argp, float)" working?A: Because the "default argument promotions" apply in variable- length argument lists, you should always use va_arg(argp, double).15.11: I cant get va_arg() to pull in an argument of type pointer-to- function.A: Use a typedef.15.12: How can I write a function which takes a variable number of arguments and passes them to some other function ?A: In general, you cannot.15.13: How can I call a function with an argument list built up at run time?A: You cant.Section 16. Strange Problems16.1b: Im getting baffling syntax errors which make no sense at all, and it seems like large chunks of my program arent being compiled.
    • A: Check for unclosed comments or mismatched preprocessing directives.16.1c: Why isnt my procedure call working?A: Function calls always require parenthesized argument lists.16.3: This program crashes before it even runs!A: Look for very large, local arrays. (See also questions 11.12b, 16.4, 16.5, and 18.4.)16.4: I have a program that seems to run correctly, but then crashes as its exiting.A: See the full list for ideas.16.5: This program runs perfectly on one machine, but I get weird results on another.A: See the full list for a brief list of possibilities.16.6: Why does the code "char *p = "hello, world!"; p[0] = H;" crash?
    • A: String literals are not modifiable, except (in effect) when they are used as array initializers.16.8: What does "Segmentation violation" mean?A: It generally means that your program tried to access memory it shouldnt have, invariably as a result of stack corruption or improper pointer use.Section 17. Style17.1: Whats the best style for code layout in C?A: There is no one "best style," but see the full list for a few suggestions.17.3: Is the code "if(!strcmp(s1, s2))" good style?A: Not particularly.17.4: Why do some people write if(0 == x) instead of if(x == 0)?A: Its a trick to guard against the common error of writing if(x = 0) .
    • 17.4b: What are the extra parentheses and underscores in "extern int func __((int, int));" for?A: Theyre part of a trick which allows prototypes to be turned off for a pre-ANSI compiler.17.5: I came across some code that puts a (void) cast before each call to printf(). Why?A: To suppress warnings about otherwise discarded return values.17.8: What is "Hungarian Notation"?A: Its a naming convention which encodes information about a variables type in its name.17.9: Where can I get the "Indian Hill Style Guide" and other coding standards?A: See the unabridged list.17.10: Some people say that gotos are evil and that I should never use them. Isnt that a bit extreme?A: Yes. Absolute rules are an imperfect approach to good programming style.
    • Section 18. Tools and Resources18.1: Im looking for C development tools (cross-reference generators, code beautifiers, etc.).A: See the full list for a few names.18.2: How can I track down these pesky malloc problems?A: See the full list for a list of tools.18.3: Whats a free or cheap C compiler I can use?A: See the full list for a brief catalog.18.4: I just typed in this program, and its acting strangely. Can you see anything wrong with it?A: See if you can run lint first.18.7: Where can I get an ANSI-compatible lint?A: See the unabridged list for two commercial products.18.8: Dont ANSI function prototypes render lint obsolete?
    • A: Not really. A good compiler may match most of lints diagnostics; few provide all.18.9: Are there any C tutorials or other resources on the net?A: There are several of them.18.9b: Where can I find some good code examples to study and learn from?A: See the longer versions of this list for some pointers.18.10: Whats a good book for learning C?A: There are far too many books on C to list here; the full list contains a few pointers.18.13: Where can I find the sources of the standard C libraries?A: Several possibilites are listed in the full list.18.13c: Where can I get a copy of the ANSI/ISO C Standard?A: See question 11.2.
    • 18.14: I need code to parse and evaluate expressions.A: Several available packages are mentioned in the full list.18.15: Where can I get a BNF or YACC grammar for C?A: See the ANSI Standard, or the unabridged list.18.15b: Does anyone have a C compiler test suite I can use?A: See the full list for several sources.18.15c: Where are some collections of useful code fragments and examples?A: See the full list for a few sources.18.15d: I need code for performing multiple precision arithmetic.A: See the full list for a few ideas.18.16: Where and how can I get copies of all these freely distributable programs?A: See the regular postings in the comp.sources.unix and
    • comp.sources.misc newsgroups, or the full version of this list, for information.Section 19. System Dependencies19.1: How can I read a single character from the keyboard without waiting for the RETURN key?A: Alas, there is no standard or portable way to do this sort of thing in C.19.2: How can I find out how many characters are available for reading, or do a non-blocking read?A: These, too, are entirely operating-system-specific.19.3: How can I display a percentage-done indication that updates itself in place, or show one of those "twirling baton" progress indicators?A: The character r is a carriage return, and b is a backspace.19.4: How can I clear the screen, or print text in color, or move the cursor?
    • A: The only halfway-portable solution is the curses library.19.5: How do I read the arrow keys? What about function keys?A: Such things depend on the keyboard, operating system, and library youre using.19.6: How do I read the mouse?A: What system are you using?19.7: How can I do serial ("comm") port I/O?A: Its system-dependent.19.8: How can I direct output to the printer?A: See the full list for ideas.19.9: How do I send escape sequences to control a terminal or other device?A: By sending them. ESC is 033 in ASCII.19.9b: How can I access an I/O board directly?
    • A: Use inport() and outport() functions, or memory-mapped I/O (see question 19.25).19.10: How can I do graphics?A: There is no portable way.19.10b: How can I display GIF and JPEG images?A: It depends on your display environment.19.11: How can I check whether a file exists?A: You can try the access() or stat() functions. Otherwise, the only guaranteed and portable way is to try opening the file.19.12: How can I find out the size of a file, prior to reading it in?A: You might be able to get an estimate using stat() or fseek/ftell (but see the full list for caveats).19.12b: How can I find the modification date of a file?A: Try stat().19.13: How can a file be shortened in-place without completely clearing
    • or rewriting it?A: There are various ways to do this, but there is no portable solution.19.14: How can I insert or delete a line in the middle of a file?A: Short of rewriting the file, you probably cant.19.15: How can I recover the file name given an open file descriptor?A: This problem is, in general, insoluble. It is best to remember the names of files yourself as you open them19.16: How can I delete a file?A: The Standard C Library function is remove().19.16b: How do I copy files?A: Open the source and destination files and copy a character or block at a time, or see question Whats wrong with the call fopen("c:newdirfile.dat", "r")?A: You probably need to double those backslashes.
    • 19.17b: fopen() isnt letting me open files like "$HOME/.profile".A: fopen() doesnt expand environment variables.19.18: How can I increase the allowable number of simultaneously open files?A: Check your system documentation.19.20: How can I read a directory in a C program?A: See if you can use the opendir() and readdir() functions.19.22: How can I find out how much memory is available?A: Your operating system may provide a routine which returns this information.19.23: How can I allocate arrays or structures bigger than 64K?A: Some operating systems wont let you.19.24: What does the error message "DGROUP exceeds 64K" mean?
    • A: It means that you have too much static data.19.25: How can I access memory located at a certain address?A: Set a pointer to the absolute address.19.27: How can I invoke another program from within a C program?A: Use system().19.30: How can I invoke another program and trap its output?A: Unix and some other systems provide a popen() function.19.31: How can my program discover the complete pathname to the executable from which it was invoked?A: argv[0] may contain all or part of the pathname. You may be able to duplicate the command language interpreters search path logic to locate the executable.19.32: How can I automatically locate a programs configuration files in the same directory as the executable?A: Its hard; see also question 19.31 above.
    • 19.33: How can a process change an environment variable in its caller?A: If its possible to do so at all, its system dependent.19.36: How can I read in an object file and jump to locations in it?A: You want a dynamic linker or loader.19.37: How can I implement a delay, or time a users response, with sub-second resolution?A: Unfortunately, there is no portable way.19.38: How can I trap or ignore keyboard interrupts like control-C?A: Use signal().19.39: How can I handle floating-point exceptions gracefully?A: Take a look at matherr() and signal(SIGFPE).19.40: How do I... Use sockets? Do networking? Write client/server applications?A: These questions have more to do with the networking facilities you have available than they do with C.
    • 19.40b: How do I... Use BIOS calls? Write ISRs? Create TSRs?A: These are very particular to a particular system.19.40c: Im trying to compile a program in which "union REGS" and int86() are undefined.A: Those have to do with MS-DOS interrupt programming.19.40d: What are "near" and "far" pointers?A: Obsolete and unnecessary.19.41: But I cant use all these nonstandard, system-dependent functions, because my program has to be ANSI compatible!A: Thats an impossible requirement. Any real program requires at least a few services which ANSI doesnt define.Section 20. Miscellaneous20.1: How can I return multiple values from a function?A: Either pass pointers to several locations which the function can
    • fill in, or have the function return a structure containing the desired values.20.3: How do I access command-line arguments?A: Via main()s argv parameter.20.5: How can I write data files which can be read on other machines with different data formats?A: The most portable solution is to use text files.20.6: How can I call a function, given its name as a string?A: The most straightforward thing to do is to maintain a correspondence table of names and function pointers.20.8: How can I implement sets or arrays of bits?A: Use arrays of char or int, with a few macros to access the desired bit at the proper index.20.9: How can I determine whether a machines byte order is big-endian or little-endian?A: The usual tricks involve pointers or unions.
    • 20.9b: How do I swap bytes?A: You can write code using pointers or unions; see the full list for details.20.10: How can I convert integers to binary or hexadecimal?A: Internally, integers are already in binary. During I/O, you may be able to select a base.20.11: Can I use base-2 constants (something like 0b101010)? Is there a printf() format for binary?A: No, on both counts.20.12: What is the most efficient way to count the number of bits which are set in an integer?A: Many "bit-fiddling" problems like this one can be sped up and streamlined using lookup tables.20.13: Whats the best way of making my program efficient?A: By picking good algorithms and implementing them carefully.
    • 20.14: Are pointers really faster than arrays? How much do function calls slow things down?A: Precise answers to these and many similar questions depend on the processor and compiler in use.20.15b: People claim that optimizing compilers are good, but mine cant even replace i/=2 with a shift.A: Was i signed or unsigned?20.15c: How can I swap two values without using a temporary?A: The "clever" trick is a ^= b; b ^= a; a ^= b; see also question 3.3b.20.17: Is there a way to switch on strings?A: Not directly.20.18: Is there a way to have non-constant case labels (i.e. ranges or arbitrary expressions)?A: No.20.19: Are the outer parentheses in return statements really optional?
    • A: Yes.20.20: Why dont C comments nest? Are they legal inside quoted strings?A: C comments dont nest because PL/Is comments dont either. The character sequences /* and */ are not special within double- quoted strings.20.21b: What does a+++++b mean ?A: Nothing. Its interpreted as "a ++ ++ + b", and cannot be parsed.20.24: Why doesnt C have nested functions?A: They were deliberately left out of C as a simplification.20.24b: What is assert()?A: It is a macro which documents an assumption being made by the programmer; it terminates the program if the assumption is violated.20.25: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions from C?
    • A: The answer is entirely dependent on the machine and the specific calling sequences of the various compilers in use.20.26: Does anyone know of a program for converting Pascal or FORTRAN to C?A: Several freely distributable programs are available, namely ptoc, p2c, and f2c. See the full list for details.20.27: Can I use a C++ compiler to compile C code?A: Not necessarily; C++ is not a strict superset of C.20.28: I need to compare two strings for close, but not necessarily exact, equality.A: See the full list for ideas.20.29: What is hashing?A: A mapping of strings (or other data structures) to integers, for easier searching.20.31: How can I find the day of the week given the date?
    • A: Use mktime(), Zellers congruence, or some code in the full list.20.32: Is (year % 4 == 0) an accurate test for leap years?A: No.20.34: How do you write a program which produces its own source code as output?A: Heres one: char*s="char*s=%c%s%c;main(){printf(s,34,s,34);}"; main(){printf(s,34,s,34);}20.35: What is "Duffs Device"?A: Its a devastatingly devious way of unrolling a loop. See the full list for details.20.37: What was the entry keyword mentioned in K&R1?A: It was reserved to allow functions with multiple, differently- named entry points, but it has been withdrawn.
    • 20.38: Where does the name "C" come from, anyway?A: C was derived from B, which was inspired by BCPL, which was a simplification of CPL.20.39: How do you pronounce "char"?A: Like the English words "char," "care," or "car" (your choice).20.39b: What do "lvalue" and "rvalue" mean?A: An "lvalue" denotes an object that has a location; an "rvalue" is any expression that has a value. 1. What will print out? main() { char *p1=“name”; char *p2; p2=(char*)malloc(20); memset (p2, 0, 20); while(*p2++ = *p1++); printf(“%sn”,p2); } Answer:empty string. 2. What will be printed as the result of the operation below: main() { int x=20,y=35;
    • x=y++ + x++; y= ++y + ++x; printf(“%d%dn”,x,y); } Answer : 57943. What will be printed as the result of the operation below: main() { int x=5; printf(“%d,%d,%dn”,x,x< <2,x>>2); } Answer: 5,20,14. What will be printed as the result of the operation below: #define swap(a,b) a=a+b;b=a-b;a=a-b; void main() { int x=5, y=10; swap (x,y); printf(“%d %dn”,x,y); swap2(x,y); printf(“%d %dn”,x,y); } int swap2(int a, int b) { int temp; temp=a; b=a; a=temp; return 0; } Answer: 10, 5 10, 55. What will be printed as the result of the operation below:
    • main() { char *ptr = ” Cisco Systems”; *ptr++; printf(“%sn”,ptr); ptr++; printf(“%sn”,ptr); } Answer:Cisco Systems isco systems6. What will be printed as the result of the operation below: main() { char s1[]=“Cisco”; char s2[]= “systems”; printf(“%s”,s1); } Answer: Cisco7. What will be printed as the result of the operation below: main() { char *p1; char *p2; p1=(char *)malloc(25); p2=(char *)malloc(25); strcpy(p1,”Cisco”); strcpy(p2,“systems”); strcat(p1,p2); printf(“%s”,p1); } Answer: Ciscosystems8. The following variable is available in file1.c, who can access it?: 9. static int average; Answer: all the functions in the file1.c can access the variable.
    • 10. WHat will be the result of the following code? #define TRUE 0 // some code while(TRUE) { // some code } Answer: This will not go into the loop as TRUE is defined as 0.11. What will be printed as the result of the operation below: int x; int modifyvalue() { return(x+=10); } int changevalue(int x) { return(x+=1); } void main() { int x=10; x++; changevalue(x); x++; modifyvalue(); printf("First output:%dn",x); x++; changevalue(x); printf("Second output:%dn",x); modifyvalue(); printf("Third output:%dn",x); } Answer: 12 , 13 , 1312. What will be printed as the result of the operation below:
    • main() { int x=10, y=15; x = x++; y = ++y; printf(“%d %dn”,x,y); } Answer: 11, 1613. What will be printed as the result of the operation below: main() { int a=0; if(a==0) printf(“Cisco Systemsn”); printf(“Cisco Systemsn”); } Answer: Two lines with “Cisco Systems” will be printed.