.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992

.\" and Copyright (c) 2006 Michael Kerrisk <mtk.manpages@gmail.com>

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

.\"

.\" Permission is granted to copy and distribute modified versions of this

.\" manual under the conditions for verbatim copying, provided that the

.\" entire resulting derived work is distributed under the terms of a

.\" permission notice identical to this one.

.\"

.\" Since the Linux kernel and libraries are constantly changing, this

.\" manual page may be incorrect or out-of-date.  The author(s) assume no

.\" responsibility for errors or omissions, or for damages resulting from

.\" the use of the information contained herein.  The author(s) may not

.\" have taken the same level of care in the production of this manual,

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.\" %%%LICENSE_END

.\"

.\" Modified by Michael Haardt <michael@moria.de>

.\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>

.\" Modified 1994-08-21 by Michael Chastain <mec@shell.portal.com>:

.\" Modified 1997-01-31 by Eric S. Raymond <esr@thyrsus.com>

.\" Modified 1999-11-12 by Urs Thuermann <urs@isnogud.escape.de>

.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>

.\" 2006-09-04 Michael Kerrisk <mtk.manpages@gmail.com>

.\"     Added list of process attributes that are not preserved on exec().

.\" 2007-09-14 Ollie Wild <aaw@google.com>, mtk

.\"     Add text describing limits on command-line arguments + environment

.\"

.TH EXECVE 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

execve \- execute program

.SH SYNOPSIS

.B #include <unistd.h>

.PP

.BI "int execve(const char *" filename ", char *const " argv "[], "

.br

.BI "           char *const " envp []);

.SH DESCRIPTION

.BR execve ()

executes the program pointed to by \fIfilename\fP.

\fIfilename\fP must be either a binary executable, or a script

starting with a line of the form:

.PP

.in +4n

.EX

\fB#!\fP \fIinterpreter \fP[optional-arg]

.EE

.in

.PP

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

.PP

\fIargv\fP is an array of argument strings passed to the new program.

By convention, the first of these strings (i.e.,

.IR argv[0] )

should contain the filename associated with the file being executed.

\fIenvp\fP is an array of strings, conventionally of the form

\fBkey=value\fP, which are passed as environment to the new program.

The \fIargv\fP and \fIenvp\fP arrays must each include a null pointer

at the end of the array.

.PP

The argument vector and environment can be accessed by the

called program's main function, when it is defined as:

.PP

.in +4n

.EX

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

.EE

.in

.PP

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

.BR environ (7).

.PP

.BR 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.

.PP

If the current program is being ptraced, a \fBSIGTRAP\fP signal is sent to it

after a successful

.BR execve ().

.PP

If the set-user-ID bit is set on the program file pointed to by

\fIfilename\fP,

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.

.PP

The aforementioned transformations of the effective IDs are

.I not

performed (i.e., the set-user-ID and set-group-ID bits are ignored)

if any of the following is true:

.IP * 3

the

.I no_new_privs

attribute is set for the calling thread (see

.BR prctl (2));

.IP *

the underlying filesystem is mounted

.I nosuid

(the

.B MS_NOSUID

flag for

.BR mount (2));

or

.IP *

the calling process is being ptraced.

.PP

The capabilities of the program file (see

.BR capabilities (7))

are also ignored if any of the above are true.

.PP

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.

.PP

The process's real UID and real GID, as well its supplementary group IDs,

are unchanged by a call to

.BR execve ().

.PP

If the executable is an a.out dynamically linked

binary executable containing

shared-library stubs, the Linux dynamic linker

.BR ld.so (8)

is called at the start of execution to bring

needed shared objects into memory

and link the executable with them.

.PP

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

.I /lib/ld-linux.so.2

for binaries linked with glibc (see

.BR ld-linux.so (8)).

.PP

All process attributes are preserved during an

.BR execve (),

except the following:

.IP * 3

The dispositions of any signals that are being caught are

reset to the default

.RB ( signal (7)).

.IP *

Any alternate signal stack is not preserved

.RB ( sigaltstack (2)).

.IP *

Memory mappings are not preserved

.RB ( mmap (2)).

.IP *

Attached System\ V shared memory segments are detached

.RB ( shmat (2)).

.IP *

POSIX shared memory regions are unmapped

.RB ( shm_open (3)).

.IP *

Open POSIX message queue descriptors are closed

.RB ( mq_overview (7)).

.IP *

Any open POSIX named semaphores are closed

.RB ( sem_overview (7)).

.IP *

POSIX timers are not preserved

.RB ( timer_create (2)).

.IP *

Any open directory streams are closed

.RB ( opendir (3)).

.IP *

Memory locks are not preserved

.RB ( mlock (2),

.BR mlockall (2)).

.IP *

Exit handlers are not preserved

.RB ( atexit (3),

.BR on_exit (3)).

.IP *

The floating-point environment is reset to the default (see

.BR fenv (3)).

.PP

The process attributes in the preceding list are all specified

in POSIX.1.

The following Linux-specific process attributes are also

not preserved during an

.BR execve ():

.IP * 3

The

.BR prctl (2)

.B 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.

.IP *

The

.BR prctl (2)

.B PR_SET_KEEPCAPS

flag is cleared.

.IP *

(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

.BR prctl (2)

.B PR_SET_PDEATHSIG

flag is cleared.

.IP *

The process name, as set by

.BR prctl (2)

.B PR_SET_NAME

(and displayed by

.IR "ps\ \-o comm" ),

is reset to the name of the new executable file.

.IP *

The

.B SECBIT_KEEP_CAPS

.I securebits

flag is cleared.

See

.BR capabilities (7).

.IP *

The termination signal is reset to

.B SIGCHLD

(see

.BR clone (2)).

.IP *

The file descriptor table is unshared, undoing the effect of the

.B CLONE_FILES

flag of

.BR clone (2).

.PP

Note the following further points:

.IP * 3

All threads other than the calling thread are destroyed during an

.BR execve ().

Mutexes, condition variables, and other pthreads objects are not preserved.

.IP *

The equivalent of \fIsetlocale(LC_ALL, "C")\fP

is executed at program start-up.

.IP *

POSIX.1 specifies that the dispositions of any signals that

are ignored or set to the default are left unchanged.

POSIX.1 specifies one exception: if

.B SIGCHLD

is being ignored,

then an implementation may leave the disposition unchanged or

reset it to the default; Linux does the former.

.IP *

Any outstanding asynchronous I/O operations are canceled

.RB ( aio_read (3),

.BR aio_write (3)).

.IP *

For the handling of capabilities during

.BR execve (),

see

.BR capabilities (7).

.IP *

By default, file descriptors remain open across an

.BR execve ().

File descriptors that are marked close-on-exec are closed;

see the description of

.B FD_CLOEXEC

in

.BR 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

.BR fcntl (2)

for details.)

POSIX.1 says that if file descriptors 0, 1, and 2 would

otherwise be closed after a successful

.BR execve (),

and the process would gain privilege because the set-user-ID or

set-group_ID mode 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

.BR execve ().

.\" On Linux it appears that these file descriptors are

.\" always open after an execve(), and it looks like

.\" Solaris 8 and FreeBSD 6.1 are the same. -- mtk, 30 Apr 2007

.SS Interpreter scripts

An interpreter script is a text file that has execute

permission enabled and whose first line is of the form:

.PP

.in +4n

.EX

\fB#!\fP \fIinterpreter \fP[optional-arg]

.EE

.in

.PP

The

.I interpreter

must be a valid pathname for an executable file.

If the

.I filename

argument of

.BR execve ()

specifies an interpreter script, then

.I interpreter

will be invoked with the following arguments:

.PP

.in +4n

.EX

\fIinterpreter\fP [optional-arg] \fIfilename\fP arg...

.EE

.in

.PP

where

.I arg...

is the series of words pointed to by the

.I argv

argument of

.BR execve (),

starting at

.IR argv [1].

.PP

For portable use,

.I optional-arg

should either be absent, or be specified as a single word (i.e., it

should not contain white space); see NOTES below.

.PP

Since Linux 2.6.28,

.\" commit bf2a9a39639b8b51377905397a5005f444e9a892

the kernel permits the interpreter of a script to itself be a script.

This permission is recursive, up to a limit of four recursions,

so that the interpreter may be a script which is interpreted by a script,

and so on.

.SS Limits on size of arguments and environment

Most UNIX implementations impose some limit on the total size

of the command-line argument

.RI ( argv )

and environment

.RI ( envp )

strings that may be passed to a new program.

POSIX.1 allows an implementation to advertise this limit using the

.B ARG_MAX

constant (either defined in

.I <limits.h>

or available at run time using the call

.IR "sysconf(_SC_ARG_MAX)" ).

.PP

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

.BR MAX_ARG_PAGES ).

On architectures with a 4-kB page size,

this yields a maximum size of 128\ kB.

.PP

On kernel 2.6.23 and later, most architectures support a size limit

derived from the soft

.B RLIMIT_STACK

resource limit (see

.BR getrlimit (2))

that is in force at the time of the

.BR 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 some background on the changes to ARG_MAX in kernels 2.6.23 and

.\" 2.6.25, see:

.\"     http://sourceware.org/bugzilla/show_bug.cgi?id=5786

.\"     http://bugzilla.kernel.org/show_bug.cgi?id=10095

.\"     http://thread.gmane.org/gmane.linux.kernel/646709/focus=648101,

.\"     checked into 2.6.25 as commit a64e715fc74b1a7dcc5944f848acc38b2c4d4ee2.

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.)

.\" Ollie: That doesn't include the lists of pointers, though,

.\" so the actual usage is a bit higher (1 pointer per argument).

Since Linux 2.6.25,

the kernel places a floor of 32 pages on this size limit,

so that, even when

.BR 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

.BR MAX_ARG_STRLEN ),

and the maximum number of strings is 0x7FFFFFFF.

.SH RETURN VALUE

On success,

.BR execve ()

does not return, on error \-1 is returned, and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B E2BIG

The total number of bytes in the environment

.RI ( envp )

and argument list

.RI ( argv )

is too large.

.TP

.B EACCES

Search permission is denied on a component of the path prefix of

.I filename

or the name of a script interpreter.

(See also

.BR path_resolution (7).)

.TP

.B EACCES

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

.TP

.B EACCES

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

.TP

.B EACCES

The filesystem is mounted

.IR noexec .

.TP

.BR EAGAIN " (since Linux 3.1)"

.\" commit 72fa59970f8698023045ab0713d66f3f4f96945c

Having changed its real UID using one of the

.BR set*uid ()

calls, the caller was\(emand is now still\(emabove its

.BR RLIMIT_NPROC

resource limit (see

.BR setrlimit (2)).

For a more detailed explanation of this error, see NOTES.

.TP

.B EFAULT

.I filename

or one of the pointers in the vectors

.I argv

or

.I envp

points outside your accessible address space.

.TP

.B EINVAL

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

name more than one interpreter).

.TP

.B EIO

An I/O error occurred.

.TP

.B EISDIR

An ELF interpreter was a directory.

.TP

.B ELIBBAD

An ELF interpreter was not in a recognized format.

.TP

.B ELOOP

Too many symbolic links were encountered in resolving

.I filename

or the name of a script or ELF interpreter.

.TP

.B ELOOP

The maximum recursion limit was reached during recursive script

interpretation (see "Interpreter scripts", above).

Before Linux 3.8,

.\" commit d740269867021faf4ce38a449353d2b986c34a67

the error produced for this case was

.BR ENOEXEC .

.TP

.B EMFILE

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

.TP

.B ENAMETOOLONG

.I filename

is too long.

.TP

.B ENFILE

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

.TP

.B ENOENT

The file

.I filename

or a script or ELF interpreter does not exist, or a shared library

.\" FIXME but see http://sourceware.org/bugzilla/show_bug.cgi?id=12241

needed for the file or interpreter cannot be found.

.TP

.B 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.

.TP

.B ENOMEM

Insufficient kernel memory was available.

.TP

.B ENOTDIR

A component of the path prefix of

.I filename

or a script or ELF interpreter is not a directory.

.TP

.B EPERM

The filesystem is mounted

.IR nosuid ,

the user is not the superuser,

and the file has the set-user-ID or set-group-ID bit set.

.TP

.B 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.

.TP

.B EPERM

A "capability-dumb" applications would not obtain the full set of

permitted capabilities granted by the executable file.

See

.BR capabilities (7).

.TP

.B ETXTBSY

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

.SH CONFORMING TO

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.

.\" SVr4 documents additional error

.\" conditions EAGAIN, EINTR, ELIBACC, ENOLINK, EMULTIHOP; POSIX does not

.\" document ETXTBSY, EPERM, EFAULT, ELOOP, EIO, ENFILE, EMFILE, EINVAL,

.\" EISDIR or ELIBBAD error conditions.

.SH NOTES

Set-user-ID and set-group-ID processes can not be

.BR ptrace (2)d.

.PP

The result of mounting a filesystem

.I 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

.BR EPERM ),

some will just ignore the set-user-ID and set-group-ID bits and

.BR exec ()

successfully.

.PP

On Linux,

.I argv

and

.I 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.

.B "Do not take advantage of this nonstandard and nonportable misfeature!"

On many other UNIX systems, specifying

.I argv

as NULL will result in an error

.RB ( EFAULT ).

.I Some

other UNIX systems treat the

.I envp==NULL

case the same as Linux.

.\" e.g., EFAULT on Solaris 8 and FreeBSD 6.1; but

.\" HP-UX 11 is like Linux -- mtk, Apr 2007

.\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408

.\" Bug rejected (because fix would constitute an ABI change).

.\"

.PP

POSIX.1 says that values returned by

.BR sysconf (3)

should be invariant over the lifetime of a process.

However, since Linux 2.6.23, if the

.BR RLIMIT_STACK

resource limit changes, then the value reported by

.B _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.

.PP

In most cases where

.BR execve ()

fails, control returns to the original executable image,

and the caller of

.BR 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

.BR SIGKILL

signal.

.\"

.SS Interpreter scripts

A maximum line length of 127 characters is allowed for the first line in

an interpreter script.

.PP

The semantics of the

.I optional-arg

argument of an interpreter script vary across implementations.

On Linux, the entire string following the

.I 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

.\" e.g., Solaris 8

use the first white space to terminate

.IR optional-arg .

On some systems,

.\" e.g., FreeBSD before 6.0, but not FreeBSD 6.0 onward

an interpreter script can have multiple arguments,

and white spaces in

.I optional-arg

are used to delimit the arguments.

.PP

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

.\"

.\" .SH BUGS

.\" Some Linux versions have failed to check permissions on ELF

.\" interpreters.  This is a security hole, because it allows users to

.\" open any file, such as a rewinding tape device, for reading.  Some

.\" Linux versions have also had other security holes in

.\" .BR execve ()

.\" that could be exploited for denial of service by a suitably crafted

.\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.

.SS execve() and EAGAIN

A more detailed explanation of the

.BR EAGAIN

error that can occur (since Linux 3.1) when calling

.BR execve ()

is as follows.

.PP

The

.BR EAGAIN

error can occur when a

.I preceding

call to

.BR setuid (2),

.BR setreuid (2),

or

.BR setresuid (2)

caused the real user ID of the process to change,

and that change caused the process to exceed its

.BR RLIMIT_NPROC

resource limit (i.e., the number of processes belonging

to the new real UID exceeds the resource limit).

From Linux 2.6.0 to 3.0, this caused the

.BR set*uid ()

call to fail.

(Prior to 2.6,

.\" commit 909cc4ae86f3380152a18e2a3c44523893ee11c4

the resource limit was not imposed on processes that

changed their user IDs.)

.PP

Since Linux 3.1, the scenario just described no longer causes the

.BR set*uid ()

call to fail,

because it too often led to security holes where buggy applications

didn't check the return status and assumed

that\(emif the caller had root privileges\(emthe call would always succeed.

Instead, the

.BR set*uid ()

calls now successfully change the real UID,

but the kernel sets an internal flag, named

.BR PF_NPROC_EXCEEDED ,

to note that the

.BR RLIMIT_NPROC

resource limit has been exceeded.

If the

.BR PF_NPROC_EXCEEDED

flag is set and the resource limit is still

exceeded at the time of a subsequent

.BR execve ()

call, that call fails with the error

.BR EAGAIN .

This kernel logic ensures that the

.BR RLIMIT_NPROC

resource limit is still enforced for the

common privileged daemon workflow\(emnamely,

.BR fork (2)

+

.BR set*uid ()

+

.BR execve ().

.PP

If the resource limit was not still exceeded at the time of the

.BR execve ()

call

(because other processes belonging to this real UID terminated between the

.BR set*uid ()

call and the

.BR execve ()

call), then the

.BR execve ()

call succeeds and the kernel clears the

.BR PF_NPROC_EXCEEDED

process flag.

The flag is also cleared if a subsequent call to

.BR fork (2)

by this process succeeds.

.SS Historical

With UNIX\ V6, the argument list of an

.BR exec ()

call was ended by 0,

while the argument list of

.I main

was ended by \-1.

Thus, this argument list was not directly usable in a further

.BR exec ()

call.

Since UNIX\ V7, both are NULL.

.\"

.\" .SH BUGS

.\" Some Linux versions have failed to check permissions on ELF

.\" interpreters.  This is a security hole, because it allows users to

.\" open any file, such as a rewinding tape device, for reading.  Some

.\" Linux versions have also had other security holes in

.\" .BR execve ()

.\" that could be exploited for denial of service by a suitably crafted

.\" ELF binary. There are no known problems with 2.0.34 or 2.2.15.

.SH EXAMPLE

The following program is designed to be execed by the second program below.

It just echoes its command-line arguments, one per line.

.PP

.in +4n

.EX

/* 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);

}

.EE

.in

.PP

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

argument:

.PP

.in +4n

.EX

/* 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);

}

.EE

.in

.PP

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

.PP

.in +4n

.EX

.RB "$" " cc myecho.c \-o myecho"

.RB "$" " cc execve.c \-o execve"

.RB "$" " ./execve ./myecho"

argv[0]: ./myecho

argv[1]: hello

argv[2]: world

.EE

.in

.PP

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

.I myecho

program:

.PP

.in +4n

.EX

.RB "$" " cat > script"

.B #!./myecho script-arg

.B ^D

.RB "$" " chmod +x script"

.EE

.in

.PP

We can then use our program to exec the script:

.PP

.in +4n

.EX

.RB "$" " ./execve ./script"

argv[0]: ./myecho

argv[1]: script-arg

argv[2]: ./script

argv[3]: hello

argv[4]: world

.EE

.in

.SH SEE ALSO

.BR chmod (2),

.BR execveat (2),

.BR fork (2),

.BR get_robust_list (2),

.BR ptrace (2),

.BR execl (3),

.BR fexecve (3),

.BR getopt (3),

.BR system (3),

.BR credentials (7),

.BR environ (7),

.BR path_resolution (7),

.BR ld.so (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.