title: How does a Client (Written in C) listens to a request on Linux OS?
body: <p>I am a beginner and I want to understand some basics of Client-Server applications written in C (I understand web based server-client applications written in Java, PHP ,etc).</p>
  
  <p>To be more specific, I am talking about ZABBIX which is a client-server tool (Zabbix_server is a server and zabbix_agentd is a client, both written in C).</p>
  
  <p>Zabbix_server asks zabbix_agentd for data and zabbix_agents responds accordingly.</p>
  
  <p>I have imported zabbix codes on Eclipse and have tried to debug to understand codes. A complete execution of zabbix_agentd on Eclipse just launched the daemon successfully and created a child process. But when I check the log, I find that different functions are being called which had not come in the way while debugging. It means, these functions are called by some process, may be inetd, etc. (correct me if i am wrong). I tried to find inetd on RHEL 6.4 OS, but it was no where found (using "service inetd status", "find"). </p>
  
  <p>So, how those functions are being called? Can anyone please give me ideas about that?
  Please suggest me how to use those calling requests to further debug my client application.</p>
  
  <p>One thing I have noticed while debugging Eclipse is that, I get below error after calling fork():</p>
  
  <pre><code>No source available for "fork() at 0x36ca0acbc0"
  </code></pre>
  
  <p>I couldn't understand the impact of the above error as the daemon keeps on working after this error also.
  I browsed through similar error different people getting and found that their binaries were not linked to sources. However, my binaries are linked to sources also. I mean, expanding the binary of zabbix_agentd shows several *.h and *.c files.</p>
  
  <p>As this error terminates the debugger immediately but the line of code at this occurrence is the last line of the source file also. So, I cannot say if I need to take care for this error.</p>
  
  <p>Is this error the reason why I am not able to view other functions being called?</p>
  
  <p>Please let me know if more information is required.</p>
  
  <p>Thanks in advance.</p>
  
  <p>Regards,
  Rohit</p>