[SOLVED] Standard functions in C99 being called "Implicit declarations"

Standard functions in C99 being called "Implicit declarations"

I have a C program that I am running on my MacOS terminal. All command line tools and GCC compiler have been installed. However for using functions like getpid() or execv() it gives the following error:

execv-test.c:7:35: error: implicit declaration of function 'getpid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        printf("Pid before execv: %d\n", getpid());
                                         ^
execv-test.c:8:2: error: implicit declaration of function 'execv' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        execv("print",NULL);
        ^
2 errors generated.

The code:

#include <stdio.h>
#include <stdlib.h>

int main (void)
{
        printf("The game is never over, John. But there may be some new players now.\n");
        printf("Pid before execv: %d\n", getpid());
        execv("print",NULL);
        printf("Returned from execv call.\n");
        return 0;
}

The following Stack Overflow exchange suggested that I write helper functions for the ones that were taken as implicit declarations. However, I am not sure you could do the same with getpid() or execv(). What should I do to make sure this doesn't happen?

PLEASE NOTE: "print" is just another helper file that is supposed to be run once execv() is called.

Solution

Note that you are using system calls that are defined in the unistd.h header file. Therefore calling them without including the std library #include <unistd.h> amounts to "implicit declaration" = "calling a function without defining it first".