Decoding the Name: What Each Letter Means

All 6 exec() Functions — Side by Side

27.2.1 How PATH Search Works (execlp / execvp)

When you use execlp() or execvp(), you only give a filename like "ls" — no full path needed. The function searches directories listed in the PATH environment variable, one by one, until it finds the executable.

Example 1: execlp() — Search PATH for Program

Instead of writing /bin/echo, we just say "echo" and let execlp() find it via PATH.

/* example1_execlp.c
 * gcc -o ex1 example1_execlp.c && ./ex1
 */
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main(void)
{
    printf("Using execlp to run 'echo'...\n");

    /* execlp: just give filename, PATH is searched automatically */
    /* Arguments: filename, arg0(program name), arg1, ..., NULL */
    execlp("echo",          /* filename to search in PATH */
           "echo",          /* argv[0] — program name */
           "Hello from",
           "execlp!",
           (char *)NULL);   /* MUST end with NULL — cast required */

    /* Only reached on error */
    fprintf(stderr, "execlp failed: %s\n", strerror(errno));
    return 1;
}

/* -----------------------------------------------
 * Try breaking it: remove /bin from PATH
 * $ PATH=/tmp ./ex1
 * → execlp fails: No such file or directory
 * ----------------------------------------------- */

Example 2: execvp() — Array Args + PATH Search

Same as execlp but arguments are in an array. Useful when you build args dynamically at runtime.

/* example2_execvp.c
 * gcc -o ex2 example2_execvp.c && ./ex2
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

int main(int argc, char *argv[])
{
    /* Build argument array dynamically */
    char *cmd_args[] = {
        "grep",         /* argv[0]: program name */
        "-r",           /* recursive */
        "--include=*.c",/* only .c files */
        "main",         /* search for "main" */
        "/usr/src",     /* search in this dir */
        NULL            /* terminator */
    };

    printf("Running: grep -r --include=*.c main /usr/src\n");
    printf("Using execvp (PATH search, array args)\n\n");

    /* execvp: filename + array, PATH searched for "grep" */
    execvp("grep", cmd_args);

    fprintf(stderr, "execvp failed: %s\n", strerror(errno));
    return 1;
}

/* -----------------------------------------------
 * execvp is perfect when arg count varies:
 *
 * void run_cmd(char *name, char **args) {
 *     execvp(name, args);
 *     perror("exec");
 * }
 * ----------------------------------------------- */

Example 3: execle() — List Args + Custom Environment

Use execle() when you want to pass a specific environment AND use list-style arguments.

/* example3_execle.c
 * gcc -o ex3 example3_execle.c && ./ex3
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

int main(void)
{
    /* Custom environment to pass to printenv */
    char *my_env[] = {
        "DEVICE=nRF52840",
        "STACK=BlueZ",
        "COMPANY=Thundersoft",
        NULL
    };

    printf("Launching printenv with custom environment...\n\n");

    /* execle: full pathname + list args + custom envp at END */
    /* Note: NULL before envp is crucial! */
    execle("/usr/bin/printenv",   /* full path (no PATH search) */
           "printenv",            /* argv[0] */
           "DEVICE",              /* print only DEVICE variable */
           (char *)NULL,          /* NULL terminates arg list */
           my_env);               /* envp comes AFTER the NULL */

    fprintf(stderr, "execle failed: %s\n", strerror(errno));
    return 1;
}

/* Expected output:
 * Launching printenv with custom environment...
 *
 * nRF52840
 *
 * Only our custom env is visible to the child — not the
 * caller's environment!
 */

Quick Decision Guide: Which exec() to Use?