execlp( command, NULL );

child.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

int main(void)
{
    printf( "PID = %d\n", getpid() );
    printf( "PPID = %d\n", getppid() );
    return 0;
}

Parent.c
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
char command[256]="./child";
int main(void)
{
    printf( "Parent-PID = %d\n", getpid() );
    printf( "Parent-PPID = %d\n", getppid() );
    execlp( command, NULL );
    printf( " Parent process return \n");
 }
/*
# ./Parent
Parent-PID = 26012
Parent-PPID = 25427
PID = 26012
PPID = 25427
*/