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System call execve

Description​

execve() executes the program referred to by pathname. This causes the program that is currently being run by the calling process to be replaced with a new program, with newly initialized stack, heap, and (initialized and uninitialized) data segments.

note

One sometimes sees execve() (and the related functions described in exec(3)) described as "executing a new process" (or similar). This is a highly misleading description: there is no new process; many attributes of the calling process remain unchanged (in particular, its PID). All that execve() does is arrange for an existing process (the calling process) to execute a new program.

tip

exec() is standard C library (libc, -lc), execve() is systemcall.

The exec() family of functions replaces the current process image with a new process image.

Example​

The following program is designed to be execed by the second program below. It just echoes its command-line arguments, one per line.

/* myecho.c */

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

int
main(int argc, char *argv[])
{
   for (int j = 0; j < argc; j++)
       printf("argv[%d]: %s\n", j, argv[j]);

   exit(EXIT_SUCCESS);
}

This program can be used to exec the program named in its command-line argument:

/* execve.c */

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

int
main(int argc, char *argv[])
{
   char *newargv[] = { NULL, "hello", "world", NULL };
   char *newenviron[] = { NULL };

   if (argc != 2) {
       fprintf(stderr, "Usage: %s <file-to-exec>\n", argv[0]);
       exit(EXIT_FAILURE);
   }

   newargv[0] = argv[1];

   execve(argv[1], newargv, newenviron);
   perror("execve");   /* execve() returns only on error */
   exit(EXIT_FAILURE);
}

We can use the second program to exec the first as follows:

$ cc myecho.c -o myecho
$ cc execve.c -o execve
$ ./execve ./myecho
argv[0]: ./myecho
argv[1]: hello
argv[2]: world

References​