7.3.8 Beware of Implicit Declarations

If you use -std=c90 or -xc99=none, the C compiler assumes that any function or variable that is used in a module and is not defined or declared externally is an integer. Any long and pointer data used in this way is 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. Even if the function or variable is 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 following 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);
}