Beware of Implicit Declarations

The C compiler assumes that any function or variable that is used in a module and not defined or declared externally is an integer. Any longs and pointers used in this way are truncated by the compiler's implicit integer declaration. Place the appropriate extern declaration for the function or variable in a header and not in the C module. Include this header in any C module that uses the function or variable. If this is a function or variable defined by the system headers, you still need to include the proper header in the code. Consider the following example:

int
main(int argc, char *argv[])
{
  char *name = getlogin()
  printf("login = %s\n", name);
  return (0);
}

%
warning: improper pointer/integer combination: op "="
warning: cast to pointer from 32-bit integer
implicitly declared to return int
getlogin        printf  

The proper headers are now in the modified version

#include <unistd.h>
#include <stdio.h>

int
main(int argc, char *argv[])
{
  char *name = getlogin();
  (void) printf("login = %s\n", name);
  return (0);
}