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undefined reference to...

This error is often generated because you have typed the name of a function or variable incorrectly. For example, the following code:

#include <stdio.h>

void print_hello()
  printf ("Hello!\n");

/* To shorten example, not using argp */
int main()
  return 0;

generates the following rather forbidding error:

/tmp/cc9KXhmV.o: In function `main':
/tmp/cc9KXhmV.o(.text+0x1f): undefined reference to `Print_hello'
collect2: ld returned 1 exit status

The answer, however, is very simple. C is case-sensitive. The main function calls the function Print_hello (with a capital P), but the correct name of the function is print_hello (with a lower-case p). The linker could not find a function with the name Print_hello.