title: Why do I get "multiple definition" errors when compiling my project?
body: <p>Edit : initializing the constants of my project in a C module and declaring them in a corresponding header resolved the issue.</p>
  
  <p>I am working on making a C implementation of the AES cipher (link : <a href="https://github.com/SuperTotoGo/AES_Cipher" rel="nofollow noreferrer">https://github.com/SuperTotoGo/AES_Cipher</a>). Everything was working fine until I tried to separate my project in modules. Now when I compile I get this error :</p>
  
  <pre><code>gcc -std=c99 -g -o aes.out *.c 
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:8: multiple definition of `AES_SUB_BOX'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:8: first defined here
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:28: multiple definition of `INV_AES_SUB_BOX'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:28: first defined here
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:48: multiple definition of `AES_LOG_TABLE'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:48: first defined here
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:68: multiple definition of `AES_ALOG_TABLE'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:68: first defined here
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:88: multiple definition of `AES_MULT_MAT'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:88: first defined here
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:96: multiple definition of `INV_AES_MULT_MAT'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:96: first defined here
  /usr/bin/ld: /tmp/ccpeYNuO.o:/home/pipou/AES_Cipher/aes_const.h:104: multiple definition of `RCON'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:104: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:8: multiple definition of `AES_SUB_BOX'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:8: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:28: multiple definition of `INV_AES_SUB_BOX'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:28: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:48: multiple definition of `AES_LOG_TABLE'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:48: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:68: multiple definition of `AES_ALOG_TABLE'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:68: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:88: multiple definition of `AES_MULT_MAT'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:88: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:96: multiple definition of `INV_AES_MULT_MAT'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:96: first defined here
  /usr/bin/ld: /tmp/cczfD784.o:/home/pipou/AES_Cipher/aes_const.h:104: multiple definition of `RCON'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:104: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:8: multiple definition of `AES_SUB_BOX'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:8: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:28: multiple definition of `INV_AES_SUB_BOX'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:28: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:48: multiple definition of `AES_LOG_TABLE'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:48: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:68: multiple definition of `AES_ALOG_TABLE'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:68: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:88: multiple definition of `AES_MULT_MAT'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:88: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:96: multiple definition of `INV_AES_MULT_MAT'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:96: first defined here
  /usr/bin/ld: /tmp/ccxcDlRl.o:/home/pipou/AES_Cipher/aes_const.h:104: multiple definition of `RCON'; /tmp/ccHvhAUx.o:/home/pipou/AES_Cipher/aes_const.h:104: first defined here
  collect2: error: ld returned 1 exit status
  </code></pre>
  
  <p>My functions prototypes are defined in the headers and my functions are implemented in the corresponding c files, from what I understood about modular projects I normally only need to include the header files in the main file to have it working.</p>
  
  <p>So, would you have any idea what I did wrong?</p>
  
  <p>Also if you have any tips about making modular C projects or any advice conecerning how I have done it, please tell me!</p>