pid = fork()

• pid is the process id of child process
• for parent, pid is the process id of the created child process
• for child, pid equals to zero
• if error, pid is a negative value

The implementation order of parent process and child process is random

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
 
int main()
{
    pid_t fpid;
 
    fpid = fork();
 
    if(fpid < 0)
    {
        printf("Not able to create child process ...");
        exit(1);
    }
 
    if(fpid == 0)
    {
        printf("I am the child process %d, child id is: %d\n", getpid(), fpid);
    }
    else
    {
        printf("I am the parent process %d, parent id is: %d\n", getpid(), fpid);
    }
 
    return 0;
}
		

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
 
int main()
{
    int i = 0;
    pid_t fpid;
 
    printf("%10s %10s %10s %10s\n", "index", "parent", "current", "child");
 
    for(i = 0; i < 2; i++)
    {
    	fpid = fork();
 
    	if(fpid == 0)
    	{
        	printf("%10d %10d %10d %10d\n", i, getppid(), getpid(), fpid);
    	}
    	else
    	{
        	printf("%10d %10d %10d %10d\n", i, getppid(), getpid(), fpid);
    	}
    }
 
    return 0;
}
		

let parent process implement after child process is implemented

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
 
int main()
{
    pid_t fpid;
 
    fpid = fork();
 
    if(fpid == 0)
    {
        printf("I am the child process %d, child id is: %d\n", getpid(), fpid);
    }
    else
    {
        wait(&fpid);
        printf("I am the parent process %d, child id is: %d\n", getpid(), fpid);
    }
 
    return 0;
}
		

int execl(const char *path, const char *arg, ...)

int execlp(const char *file, const char *arg, ...)

int execle(const char *path, const char *arg, ..., char *const envp[])

int execv(const char *path, char *const argv[])

int execvp(const char *file, char *const argv[])

int execve(const char *path, char *const argv[], char *const envp[])

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
 
int main()
{
    pid_t fpid;
 
    fpid = fork();
 
    if(fpid == 0)
    {
        printf("I am the child process %d, child id is: %d\n", getpid(), fpid);
	char * execv_str[] = {"ls", "-l", "/home", NULL}; // ended with NULL
	if (execv("/bin/ls",execv_str) <0 ){
		perror("error on exec");
		exit(1);
	}
    }
    else
    {
        wait(&fpid);
        printf("I am the parent process %d, parent id is: %d\n", getpid(), fpid);
    }
 
    return 0;
}
		

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
 
int main()
{
    pid_t fpid;
 
    fpid = fork();
 
    if(fpid == 0)
    {
	char * execve_str[] = {"env",NULL};
	char * env[] = {"PATH=/tmp", "USER=lchen", "STATUS=testing", NULL};
	if (execve("/usr/bin/env",execve_str,env) <0 ){
		perror("error on exec");
		exit(1);
        }
    }
    else
    {
        wait(&fpid);
        printf("I am the parent process %d, parent id is: %d\n", getpid(), fpid);
    }
 
    return 0;
}
		

system() automatically create a child process, does not support thread

exec functions support thread

#include <stdio.h>
#include <stdlib.h>
 
int main()
{
    printf("Call system() to implement a child process ...");
 
    system("ls -l /home");
 
    printf("Return from the child process ...");
 
    return 0;
}