About fork () function and execve () function

--Personal memo --Leave what you learned about kernel process generation --Run in C

About the fork () function

--When you execute the fork () function, it spawns a child process from the parent process.

fork.c

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

static void child()
{
    printf("Child. pid: %d.\n", getpid());
    exit(EXIT_SUCCESS);
}

static void parent(pid_t pid_child)
{
    printf("Parent. parent's pid: %d. child's pid: %d\n", getpid(), pid_child);
    exit(EXIT_SUCCESS);
}

int main()
{
    pid_t pid;
    pid = fork();
    
    if(pid == -1) err(EXIT_FAILURE, "fork() failed");
    
    //Child process returns 0
    if(pid == 0)
        child();
    //The process ID of the child process whose return value is the parent process(> 0)
    else 
        parent(pid);
}

• Execution result

Parent. parent's pid: 27701. child's pid: 27702
Child. pid: 27702.

You can see that the child process is spawned from the parent process and each has a different process ID

About the execv () function

--The kernel executes the process according to the following flow

1. Read the executable file
2. Overwrite the memory of the current process with the data of the new process
3. Run a new process

execve.c

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

static void child()
{
    char *args[] = {"/bin/echo", "Hello", NULL};
    printf("child. child's pid: %d.\n", getpid());
    execve("/bin/echo", args, NULL);
    err(EXIT_FAILURE, "execve() failed");
}

static void parent(pid_t pid_child)
{
    printf("parent. parent's pid: %d. child's pid: %d\n", getpid(), pid_child);
    exit(EXIT_SUCCESS);
}


int main(void)
{
    pid_t pid;
    pid = fork();
    if(pid == -1)
        err(EXIT_FAILURE, "fork() failed");
    
    if(pid == 0)
        child();
    else
        parent(pid);
    
}

• Execution result

parent. parent's pid: 28034. child's pid: 28035
child. child's pid: 28035.
Hello

After the process branches with the fork () function, you can see that / bin / echo is executed by the execve () function in the child process.