execve(2) — Linux manual pages

Linux manual pages	Section 2: System Calls

Name

execve — execute program

Synopsis

#include <unistd.h>

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

DESCRIPTION

execve() executes the program pointed to by filename. filename 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 argument strings passed to the new program. By convention, the first of these strings should contain the filename associated with the file being executed. envp is an array of strings, conventionally of the form key=value, which are passed as environment to the new program. Both argv and envp must be terminated by a NULL pointer. The argument vector and environment can be accessed by the called program's main function, when it is defined as:

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

execve() does not return on success, and the text, data, bss, and stack of the calling process are overwritten by that of the program loaded.

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

If the set-user-ID bit is set on the program file pointed to by filename, and the underlying filesystem is not mounted nosuid (the MS_NOSUID flag for mount(2)), and the calling process is not being ptraced, then the effective user ID of the calling process is changed to that of the owner of the program file. Similarly, when the set-group-ID bit of the program file is set the effective group ID of the calling process is set to the group of the program file.

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 permission bits.

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 libraries 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 libraries. This interpreter is typically /lib/ld-linux.so.2 for binaries linked with glibc 2. (For binaries linked with the old Linux libc5, the interpreter was typically /lib/ld-linux.so.1.)

All process attributes are preserved during an execve(), except the following:

The dispositions of any signals that are being caught are reset to the default (signal(7)).
Any alternate signal stack is not preserved (sigaltstack(2)).
Memory mappings are not preserved (mmap(2)).
Attached System V shared memory segments are detached (shmat(2)).
POSIX shared memory regions are unmapped (shm_open(3)).
Open POSIX message queue descriptors are closed (mq_overview(7)).
Any open POSIX named semaphores are closed (sem_overview(7)).
POSIX timers are not preserved (timer_create(2)).
Any open directory streams are closed (opendir(3)).
Memory locks are not preserved (mlock(2), mlockall(2)).
Exit handlers are not preserved (atexit(3), on_exit(3)).
The floating-point environment is reset to the default (see fenv(3)).

The process attributes in the preceding list are all specified in POSIX.1-2001. The following Linux-specific process attributes are also not preserved during an execve():

The prctl(2) PR_SET_DUMPABLE flag is set, unless a set-user-ID or set-group ID program is being executed, in which case it is cleared.
The prctl(2) PR_SET_KEEPCAPS flag is cleared.
(Since Linux 2.4.36 / 2.6.23) If a set-user-ID or set-group-ID program is being executed, then the parent death signal set by prctl(2) PR_SET_PDEATHSIG flag is cleared.
The process name, as set by prctl(2) PR_SET_NAME (and displayed by ps −o comm), is reset to the name of the new executable file.
The SECBIT_KEEP_CAPS securebits flag is cleared. See capabilities(7).
The termination signal is reset to SIGCHLD (see clone(2)).

Note the following further points:

All threads other than the calling thread are destroyed during an execve(). Mutexes, condition variables, and other pthreads objects are not preserved.
The equivalent of setlocale(LC_ALL, "C") is executed at program start-up.
POSIX.1-2001 specifies that the dispositions of any signals that are ignored or set to the default are left unchanged. POSIX.1-2001 specifies one exception: if SIGCHLD is being ignored, then an implementation may leave the disposition unchanged or reset it to the default; Linux does the former.
Any outstanding asynchronous I/O operations are canceled (aio_read(3), aio_write(3)).
For the handling of capabilities during execve(), see capabilities(7).
By default, file descriptors remain open across an execve(). File descriptors that are marked close-on-exec are closed; see the description of FD_CLOEXEC in fcntl(2). (If a file descriptor is closed, this will cause the release of all record locks obtained on the underlying file by this process. See fcntl(2) for details.) POSIX.1-2001 says that if file descriptors 0, 1, and 2 would otherwise be closed after a successful execve(), and the process would gain privilege because the set-user_ID or set-group_ID permission bit was set on the executed file, then the system may open an unspecified file for each of these file descriptors. As a general principle, no portable program, whether privileged or not, can assume that these three file descriptors will remain closed across an execve().

Interpreter scripts

An interpreter script is a text file that has execute permission enabled and whose first line is of the form:

#! interpreter [optional-arg]

The interpreter must be a valid pathname for an executable which is not itself a script. If the filename argument of execve() specifies an interpreter script, then interpreter will be invoked with the following arguments:

interpreter [optional-arg] filename arg...

where arg... is the series of words pointed to by the argv argument of execve().

For portable use, optional-arg should either be absent, or be specified as a single word (i.e., it should not contain white space); see NOTES below.

Limits on size of arguments and environment

Most UNIX implementations impose some limit on the total size of the command-line argument (argv) and environment (envp) strings that may be passed to a new program. POSIX.1 allows an implementation to advertise this limit using the ARG_MAX constant (either defined in <limits.h> or available at run time using the call sysconf(_SC_ARG_MAX)).

On Linux prior to kernel 2.6.23, the memory used to store the environment and argument strings was limited to 32 pages (defined by the kernel constant MAX_ARG_PAGES). On architectures with a 4-kB page size, this yields a maximum size of 128 kB.

On kernel 2.6.23 and later, most architectures support a size limit derived from the soft RLIMIT_STACK resource limit (see getrlimit(2)) that is in force at the time of the execve() call. (Architectures with no memory management unit are excepted: they maintain the limit that was in effect before kernel 2.6.23.) This change allows programs to have a much larger argument and/or environment list. For these architectures, the total size is limited to 1/4 of the allowed stack size. (Imposing the 1/4-limit ensures that the new program always has some stack space.) Since Linux 2.6.25, the kernel places a floor of 32 pages on this size limit, so that, even when RLIMIT_STACK is set very low, applications are guaranteed to have at least as much argument and environment space as was provided by Linux 2.6.23 and earlier. (This guarantee was not provided in Linux 2.6.23 and 2.6.24.) Additionally, the limit per string is 32 pages (the kernel constant MAX_ARG_STRLEN), and the maximum number of strings is 0x7FFFFFFF.

RETURN VALUE

On success, execve() does not return, on error −1 is returned, and errno is set appropriately.

ERRORS

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 filename 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.
EFAULT: filename 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 filename or the name of a script or ELF interpreter.
EMFILE: The process has the maximum number of files open.
ENAMETOOLONG: filename is too long.
ENFILE: The system limit on the total number of open files has been reached.
ENOENT: The file filename or a script or ELF interpreter does not exist, or a shared library needed for file or interpreter cannot be found.
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 filename 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.
ETXTBSY: Executable was open for writing by one or more processes.

CONFORMING TO

SVr4, 4.3BSD, POSIX.1-2001. POSIX.1-2001 does not document the #! behavior but is otherwise compatible.

NOTES

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

Linux ignores the set-user-ID and set-group-ID bits on scripts.

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 she did not have already (and return EPERM), some will just ignore the set-user-ID and set-group-ID bits and exec() successfully.

A maximum line length of 127 characters is allowed for the first line in a #! executable shell script.

The semantics of the optional-arg argument of an interpreter script vary across implementations. On Linux, the entire string following the interpreter name is passed as a single argument to the interpreter, and this string can include white space. However, behavior differs on some other systems. Some systems use the first white space to terminate optional-arg. On some systems, an interpreter script can have multiple arguments, and white spaces in optional-arg are used to delimit the arguments.

On Linux, either argv or envp can be specified as NULL, which has the same effect as specifying these arguments as a pointer to a list containing a single NULL pointer. Do not take advantage of this misfeature! It is nonstandard and nonportable: on most other UNIX systems doing this will result in an error (EFAULT).

POSIX.1-2001 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.

Historical

With UNIX V6 the argument list of an exec() call was ended by 0, while the argument list of main was ended by −1. Thus, this argument list was not directly usable in a further exec() call. Since UNIX V7 both are NULL.

EXAMPLE

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

/* myecho.c */

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

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

    for (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() only returns 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.sh
#! ./myecho script-arg
^D
$ chmod +x script.sh

We can then use our program to exec the script:

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

COLOPHON

This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man−pages/.

  Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992
and Copyright (c) 2006 Michael Kerrisk <mtk.manpagesgmail.com>

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Modified by Michael Haardt <michaelmoria.de>
Modified 1993-07-21 by Rik Faith <faithcs.unc.edu>
Modified 1994-08-21 by Michael Chastain <mecshell.portal.com>:
Modified 1997-01-31 by Eric S. Raymond <esrthyrsus.com>
Modified 1999-11-12 by Urs Thuermann <ursisnogud.escape.de>
Modified 2004-06-23 by Michael Kerrisk <mtk.manpagesgmail.com>
2006-09-04 Michael Kerrisk <mtk.manpagesgmail.com>
    Added list of process attributes that are not preserved on exec().
2007-09-14 Ollie Wild <aawgoogle.com>, mtk
    Add text describing limits on command-line arguments + environment