How to get child PID in C?

As mentioned in previous answer that "fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process." So, the code can be written in this way:

pid = fork(); /* call fork() from parent process*/
if (0 == pid)
{
  /* fork returned 0. This part will be executed by child process*/
  /*  getpid() will give child process id here */
}
else
{
  /* fork returned child pid which is non zero. This part will be executed by parent process*/
  /*  getpid() will give parent process id here */
} 

This link is very helpful and explains in detail.

fork already returns the child's pid. Just store the return value.

look at man 2 fork:

RETURN VALUES

 Upon successful completion, fork() returns a value of 0 to the child process and
 returns the process ID of the child process to the parent process.  Otherwise, a
 value of -1 is returned to the parent process, no child process is created, and
 the global variable errno is set to indicate the error.

if fork() is successfully created then it returns 0 value in the child process.

int main(void)
{
    int id;
    id= fork();
    if(id==0)
    {
        printf("I am child process my ID is   =  %d\n" , getpid());
    }
}