🧠 POSIX/Unix Process Creation Cheatsheet

🎯 Visual Process Creation Flowchart

PARENT PROCESS
    |
    |-- fork() / vfork() / clone()
    |        |
    |        +-- CHILD
    |              |
    |              +-- exec*() to load new program
    |
    |-- posix_spawn() / posix_spawnp()
    |        |
    |        +-- CHILD already loaded with new program
    |
    +-- Parent monitors child (wait(), waitpid(), waitid())

✅ Key point:

• fork()/vfork()/clone() ➔ Create child, then child manually execs.
  
• posix_spawn() / posix_spawnp() ➔ Spawn and exec combined, no manual exec needed.
• After child creation, parent typically uses wait() / waitpid() / waitid() to monitor and reap the child.

🗺️ Logical Flow Summary

1. 👶 Child Creation (Parent Spawns a Child Process)

⚡ Quick Comparison: fork() vs vfork() vs clone() vs posix_spawn() vs posix_spawnp()

🛠️ When to Prefer fork() vs posix_spawn()

Examples:

1.1 fork()

pid_t pid = fork();

1.2 vfork()

pid_t pid = vfork();

• Child runs in parent memory until exec() or _exit().
• Faster and more memory-efficient than fork().
• Use only if child does not touch memory except exec()/_exit().

1.3 clone() (Simple Example)

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

int child_fn(void *arg) {
    printf("Child says hi!\n");
    _exit(0);
}

#define STACK_SIZE 1024*1024

int main() {
    char *stack = malloc(STACK_SIZE);
    if (!stack) { perror("malloc"); exit(1); }

    pid_t pid = clone(child_fn, stack + STACK_SIZE, SIGCHLD, NULL);
    if (pid == -1) { perror("clone"); exit(1); }

    waitpid(pid, NULL, 0);
    free(stack);
}

• Minimal working clone.
• SIGCHLD → child generates signal on exit (like fork()).
• You must allocate your own stack!

1.4 posix_spawn() (Simple Example)

#include <spawn.h>
#include <unistd.h>

extern char **environ;

int main() {
    pid_t pid;
    char *argv[] = {"ls", "-l", NULL};
    posix_spawn(&pid, "/bin/ls", NULL, NULL, argv, environ);
    waitpid(pid, NULL, 0);
}

• Spawns and execs in one call.
• Does NOT search $PATH.

1.5 posix_spawnp() (Simple Example)

#include <spawn.h>
#include <unistd.h>

extern char **environ;

int main() {
    pid_t pid;
    char *argv[] = {"ls", "-l", NULL};
    posix_spawnp(&pid, "ls", NULL, NULL, argv, environ);
    waitpid(pid, NULL, 0);
}

• Like posix_spawn(), but searches $PATH for the executable.
• High-level alternative for dynamic command lookup.

2. 📦 Loading a New Program (Child Replaces Itself)

Examples:

2.1 execl()

execl("/bin/ls", "ls", "-l", (char *)NULL);

2.2 execv()

char *args[] = {"ls", "-l", NULL};
execv("/bin/ls", args);

2.3 execle()

char *envp[] = {"MYVAR=value", NULL};
execle("/bin/ls", "ls", "-l", (char *)NULL, envp);

Environment behavior:
🔵 Fully replaces existing environment.
🔵 Only what you specify is visible inside the new process.

2.4 execve()

char *args[] = {"ls", "-l", NULL};
char *envp[] = {"MYVAR=value", NULL};
execve("/bin/ls", args, envp);

Environment behavior:
🔵 Fully replaces existing environment.

2.5 execlp()

execlp("ls", "ls", "-l", (char *)NULL);

• Searches $PATH.

2.6 execvp()

char *args[] = {"ls", "-l", NULL};
execvp("ls", args);

• Searches $PATH.

2.7 execvpe() (GNU only)

char *args[] = {"ls", "-l", NULL};
char *envp[] = {"MYVAR=value", NULL};
execvpe("ls", args, envp);

Environment behavior:
🔵 Fully replaces environment, and searches executable in $PATH.

3. 🕵️ Monitoring and Reaping Children

Examples:

3.1 wait()

int status;
pid_t pid = wait(&status);

3.2 waitpid()

int status;
pid_t pid = waitpid(child_pid, &status, 0);

3.3 waitid()

siginfo_t info;
waitid(P_PID, child_pid, &info, WEXITED);

📋 Notes:

• posix_spawn() and posix_spawnp() internally do not duplicate memory like fork().
• posix_spawnp() additionally searches $PATH, like execlp/execvp.
• Systems like iOS and modern macOS discourage or prohibit fork() in some contexts.