Manpages - execve.2

execve - execute program

  #include <unistd.h>

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

*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.

pathname must be either a binary executable, or a script starting with a line of the form:

  #!interpreter [optional-arg]

For details of the latter case, see “Interpreter scripts” below.

argv is an array of pointers to strings passed to the new program as its command-line arguments. By convention, the first of these strings (i.e., argv[0]) should contain the filename associated with the file being executed. The argv array must be terminated by a NULL pointer. (Thus, in the new program, argv[argc] will be NULL.)

envp is an array of pointers to strings, conventionally of the form key=value, which are passed as the environment of the new program. The envp array must be terminated by a NULL pointer.

The argument vector and environment can be accessed by the new program’s main function, when it is defined as:

  int main(int argc, char *argv[], char *envp[])

Note, however, that the use of a third argument to the main function is not specified in POSIX.1; according to POSIX.1, the environment should be accessed via the external variable *environ*(7).

*execve*() does not return on success, and the text, initialized data, uninitialized data (bss), and stack of the calling process are overwritten according to the contents of the newly loaded program.

If the current program is being ptraced, a SIGTRAP signal is sent to it after a successful *execve*().

If the set-user-ID bit is set on the program file referred to by pathname, then the effective user ID of the calling process is changed to that of the owner of the program file. Similarly, if the set-group-ID bit is set on the program file, then the effective group ID of the calling process is set to the group of the program file.

The aforementioned transformations of the effective IDs are not performed (i.e., the set-user-ID and set-group-ID bits are ignored) if any of the following is true:

• the no_new_privs attribute is set for the calling thread (see *prctl*(2));
• the underlying filesystem is mounted nosuid (the MS_NOSUID flag for *mount*(2)); or
• the calling process is being ptraced.

The capabilities of the program file (see *capabilities*(7)) are also ignored if any of the above are true.

The effective user ID of the process is copied to the saved set-user-ID; similarly, the effective group ID is copied to the saved set-group-ID. This copying takes place after any effective ID changes that occur because of the set-user-ID and set-group-ID mode bits.

The process’s real UID and real GID, as well as its supplementary group IDs, are unchanged by a call to *execve*().

If the executable is an a.out dynamically linked binary executable containing shared-library stubs, the Linux dynamic linker *ld.so*(8) is called at the start of execution to bring needed shared objects into memory and link the executable with them.

If the executable is a dynamically linked ELF executable, the interpreter named in the PT_INTERP segment is used to load the needed shared objects. This interpreter is typically /lib/ld-linux.so.2 for binaries linked with glibc (see *ld-linux.so*(8)).

On success, *execve*() does not return, on error -1 is returned, and errno is set to indicate the error.

E2BIG

The total number of bytes in the environment (envp) and argument list (argv) is too large.

EACCES

Search permission is denied on a component of the path prefix of pathname or the name of a script interpreter. (See also *path_resolution*(7).)

EACCES

The file or a script interpreter is not a regular file.

EACCES

Execute permission is denied for the file or a script or ELF interpreter.

EACCES

The filesystem is mounted noexec.

EAGAIN (since Linux 3.1)

Having changed its real UID using one of the set*uid*() calls, the caller was—and is now still—above its *RLIMIT_NPROC resource limit (see *setrlimit*(2)). For a more detailed explanation of this error, see NOTES.

EFAULT

pathname or one of the pointers in the vectors argv or envp points outside your accessible address space.

EINVAL

An ELF executable had more than one PT_INTERP segment (i.e., tried to name more than one interpreter).

EIO

An I/O error occurred.

EISDIR

An ELF interpreter was a directory.

ELIBBAD

An ELF interpreter was not in a recognized format.

ELOOP

Too many symbolic links were encountered in resolving pathname or the name of a script or ELF interpreter.

ELOOP

The maximum recursion limit was reached during recursive script interpretation (see “Interpreter scripts”, above). Before Linux 3.8, the error produced for this case was ENOEXEC.

EMFILE

The per-process limit on the number of open file descriptors has been reached.

ENAMETOOLONG

pathname is too long.

ENFILE

The system-wide limit on the total number of open files has been reached.

ENOENT

The file pathname or a script or ELF interpreter does not exist.

ENOEXEC

An executable is not in a recognized format, is for the wrong architecture, or has some other format error that means it cannot be executed.

ENOMEM

Insufficient kernel memory was available.

ENOTDIR

A component of the path prefix of pathname or a script or ELF interpreter is not a directory.

EPERM

The filesystem is mounted nosuid, the user is not the superuser, and the file has the set-user-ID or set-group-ID bit set.

EPERM

The process is being traced, the user is not the superuser and the file has the set-user-ID or set-group-ID bit set.

EPERM

A “capability-dumb” applications would not obtain the full set of permitted capabilities granted by the executable file. See *capabilities*(7).

ETXTBSY

The specified executable was open for writing by one or more processes.

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. POSIX does not document the #! behavior, but it exists (with some variations) on other UNIX systems.

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.

Set-user-ID and set-group-ID processes can not be *ptrace*(2)d.

The result of mounting a filesystem nosuid varies across Linux kernel versions: some will refuse execution of set-user-ID and set-group-ID executables when this would give the user powers they did not have already (and return EPERM), some will just ignore the set-user-ID and set-group-ID bits and *exec*() successfully.

On Linux, argv and envp can be specified as NULL. In both cases, this has the same effect as specifying the argument as a pointer to a list containing a single null pointer. Do not take advantage of this nonstandard and nonportable misfeature! On many other UNIX systems, specifying argv as NULL will result in an error (EFAULT). Some other UNIX systems treat the envp==NULL case the same as Linux.

POSIX.1 says that values returned by sysconf*(3) should be invariant over the lifetime of a process. However, since Linux 2.6.23, if the *RLIMIT_STACK resource limit changes, then the value reported by _SC_ARG_MAX will also change, to reflect the fact that the limit on space for holding command-line arguments and environment variables has changed.

In most cases where execve*() fails, control returns to the original executable image, and the caller of *execve*() can then handle the error. However, in (rare) cases (typically caused by resource exhaustion), failure may occur past the point of no return: the original executable image has been torn down, but the new image could not be completely built. In such cases, the kernel kills the process with a *SIGSEGV (SIGKILL until Linux 3.17) signal.

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

We can also use these programs to demonstrate the use of a script interpreter. To do this we create a script whose “interpreter” is our myecho program:

  $ cat > script
  #!./myecho script-arg
  ^D
  $ chmod +x script

We can then use our program to exec the script:

  $ ./execve ./script
  argv[0]: ./myecho
  argv[1]: script-arg
  argv[2]: ./script
  argv[3]: hello
  argv[4]: world

*chmod*(2), *execveat*(2), *fork*(2), *get_robust_list*(2), *ptrace*(2), *exec*(3), *fexecve*(3), *getauxval*(3), *getopt*(3), *system*(3), *capabilities*(7), *credentials*(7), *environ*(7), *path_resolution*(7), *ld.so*(8)

This page is part of release 5.13 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Information about execve.2 is found in manpage for: ld.so (8) ld­linux.so (8)). ld.so (8)