How execve() Works — Step by Step

Function Signature

#include <unistd.h>

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

/* Returns: never on success, -1 on error */

Example 1: Basic execve() — Run /bin/ls

This is the simplest possible use of execve(). We launch the ls command from a C program.

/* example1_execve_basic.c
 * Run: gcc -o example1 example1_execve_basic.c
 *      ./example1
 */
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main(void)
{
    /* argv[0] = program name, argv[1] = flag, NULL = end of list */
    char *argv[] = { "ls", "-l", "/tmp", NULL };

    /* Simple environment: just one variable */
    char *envp[] = { "HOME=/root", "PATH=/bin:/usr/bin", NULL };

    printf("Before execve: PID = %d\n", getpid());
    printf("About to exec /bin/ls ...\n");

    /* Replace this process with /bin/ls */
    execve("/bin/ls", argv, envp);

    /* If we reach here, execve() failed */
    printf("execve failed: %s\n", strerror(errno));
    return 1;
}

Example 2: execve() with fork() — Classic Pattern

The most common real-world use: fork() first, then exec() in the child. This way the parent process keeps running.

/* example2_fork_execve.c
 * Run: gcc -o example2 example2_fork_execve.c
 *      ./example2
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <string.h>
#include <errno.h>

int main(void)
{
    pid_t child_pid;
    int status;

    printf("Parent PID: %d — starting fork\n", getpid());

    child_pid = fork();

    if (child_pid == -1) {
        perror("fork failed");
        exit(1);
    }

    if (child_pid == 0) {
        /* ---- CHILD PROCESS ---- */
        printf("Child PID: %d — about to exec /bin/date\n", getpid());

        char *argv[] = { "date", NULL };
        char *envp[] = { "TZ=Asia/Kolkata", NULL };  /* IST timezone */

        execve("/bin/date", argv, envp);

        /* Only reached if exec fails */
        fprintf(stderr, "exec failed: %s\n", strerror(errno));
        exit(127);
    }

    /* ---- PARENT PROCESS ---- */
    printf("Parent: waiting for child %d to finish...\n", child_pid);

    if (waitpid(child_pid, &status, 0) == -1) {
        perror("waitpid failed");
        exit(1);
    }

    if (WIFEXITED(status))
        printf("Parent: child exited with status %d\n", WEXITSTATUS(status));

    printf("Parent: continuing after child done\n");
    return 0;
}

Example 3: execve() Environment Variables

Here we show how the child program receives and can read the environment variables we pass via envp.

/* example3_exec_env.c
 * TWO files: the launcher and the receiver
 * File 1: launcher.c — calls execve() with custom env
 * File 2: show_env.c — prints its env and argv
 */

/* ===== FILE 1: launcher.c ===== */
/* gcc -o launcher launcher.c && gcc -o show_env show_env.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

int main(int argc, char *argv[])
{
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <path-to-show_env>\n", argv[0]);
        exit(1);
    }

    /* Build argument vector */
    char *new_argv[] = {
        "show_env",        /* argv[0]: program name */
        "hello",           /* argv[1] */
        "world",           /* argv[2] */
        NULL               /* Must end with NULL */
    };

    /* Build environment vector — "name=value" pairs */
    char *new_envp[] = {
        "GREETING=Namaste",
        "PLACE=Hyderabad",
        "LANG=en_IN",
        NULL               /* Must end with NULL */
    };

    printf("Launcher PID: %d\n", getpid());
    printf("Launching: %s\n", argv[1]);

    execve(argv[1], new_argv, new_envp);

    /* Should not reach here */
    fprintf(stderr, "execve failed: %s\n", strerror(errno));
    exit(1);
}

/* ===== FILE 2: show_env.c ===== */
#include <stdio.h>
#include <stdlib.h>

extern char **environ;  /* Global environment pointer */

int main(int argc, char *argv[])
{
    int i;
    char **ep;

    printf("=== show_env: PID = %d ===\n", getpid());

    printf("\n--- Command-line arguments ---\n");
    for (i = 0; i < argc; i++)
        printf("  argv[%d] = %s\n", i, argv[i]);

    printf("\n--- Environment variables ---\n");
    for (ep = environ; *ep != NULL; ep++)
        printf("  %s\n", *ep);

    return 0;
}

Common Errors from execve()

What Changes vs What Stays After execve()

/* Check setUID bit before exec */
#include <sys/stat.h>

struct stat sb;
stat("/usr/bin/passwd", &sb);

if (sb.st_mode & S_ISUID)
    printf("passwd has SUID bit set — runs as root!\n");