Beware of Implicit Declaration

For some compilation modes, the compiler might assume the type int for any function or variable that is used in a module and not defined or declared externally. Any longs and pointers used in this way are truncated by the compiler's implicit int declaration. The appropriate extern declaration for a function or variable should be placed in a header and not in the C module. The header should then be included by any C module that uses the function or variable. In the case of a function or variable defined by the system headers, the proper header should still be included in the code.

For example, because getlogin() is not declared, the following code:

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

produces the warnings:

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

For better results, use::

#include <unistd.h>
#include <stdio.h>
 
int
main(int argc, char *argv[])
{
		char *name = getlogin();
		(void) printf("login = %s\n", name);
		return (0);
}