man-pages/man2/execve.2

.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
.\" and Copyright (c) 2006 Michael Kerrisk <mtk-manpages@gmx.net>
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" 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.\" manual page may be incorrect or out-of-date.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,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" 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@gmx.net>
.\" 2006-09-04 Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added list of process attributes that are not preserved on exec().
.\"
.TH EXECVE 2 2006-09-04 "Linux 2.6.17" "Linux Programmer's Manual"
.SH NAME
execve \- execute program
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "int execve(const char *" filename ", char *const " argv "[], "
.br
.BI "           char *const " envp []);
.SH DESCRIPTION
\fBexecve\fP() 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:

.in +0.5i
.nf
\fB#! \fIinterpreter \fR[optional-arg]
.fi
.in

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

\fIargv\fP is an array of argument strings passed to the new program.
\fIenvp\fP is an array of strings, conventionally of the form
\fBkey=value\fR, which are passed as environment to the new program.
Both \fIargv\fP and \fIenvp\fP 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:

.in +0.5i
.nf
int main(int argc, char *argv[], char *envp[])\fR.
.fi
.in

\fBexecve\fP() 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 \fBSIGTRAP\fP is sent to it
after a successful \fBexecve\fP().

If the set-user-ID bit is set on the program file pointed to by
\fIfilename\fP, 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
.BR 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
\fI/lib/ld-linux.so.1\fR for binaries linked with the Linux libc
version 5, or \fI/lib/ld-linux.so.2\fR for binaries linked with the
GNU libc version 2.

All process attributes are preserved during an
.BR execve (),
except the following:
.IP * 4
File descriptors that are marked close-on-exec are closed
; see the description of
.BR 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.)
.\" FIXME add some statement about the effect on record locks (fcntl()).
.IP * 4
The set of pending signals is cleared
.RB ( sigpending (2)).
.IP * 4
The dispositions of any signals that are being caught are
reset to being ignored.
.IP * 4
Any alternate signal stack is not preserved
.RB ( sigaltstack (2)).
.IP * 4
Memory mappings are not preserved
.RB ( mmap (2)).
.IP * 4
Attached System V shared memory segments are detached
.RB ( shmat (2)).
.IP * 4
POSIX shared memory regions are unmapped
.RB ( shm_open (3)).
.IP * 4
Open POSIX message queue descriptors are closed
.RB ( mq_overview (7)).
.IP * 4
Any open POSIX named semaphores are closed
.RB ( sem_overview (7)).
.IP * 4
POSIX timers are not preserved
.RB ( timer_create (3)).
.IP * 4
Any open directory streams are closed
.RB ( opendir (3)).
.IP * 4
Memory locks are not preserved
.RB ( mlock (2),
.BR mlockall (2)).
.IP * 4
Exit handlers are not preserved
.RB ( atexit (3),
.BR on_exit (3)).
.PP
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
.BR execve ():
.IP * 4
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 * 4
The
.BR prctl (2)
.B PR_SET_KEEPCAPS
flag is cleared.
.IP * 4
The process name, as set by
.BR prctl (2)
.BR PR_SET_NAME
(and displayed by
.IR "ps -o comm" ),
is reset to the name of the new executable file.
.IP * 4
The termination signal is reset to SIGCHLD
(see
.BR clone (2)).
.PP
Note the following further points:
.IP * 4
All threads other than the calling thread are destroyed during an
.BR execve ().
Mutexes, condition variables, and other pthreads objects are not preserved.
.IP * 4
The equivalent of \fIsetlocale(LC_ALL, "C")\fP
is executed at program start-up.
.IP * 4
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.
.IP * 4
Any outstanding asynchronous I/O operations are cancelled
.RB ( aio_read (3),
.BR aio_write (3)).
.IP * 4
For the handling of capabilities during
.BR execve (2),
see
.BR capabilities (7).
.SS Interpreter scripts
An interpreter script is a text file that has execute 
permission enabled and whose first line is of the form:

.in +0.5i
.nf
\fB#! \fIinterpreter \fR[optional-arg]
.fi
.in

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

.in +0.5i
.nf
\fIinterpreter\fR [optional-arg] \fIfilename\fR arg...
.fi
.in

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

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.
.SH "RETURN VALUE"
On success, \fBexecve\fP() 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 (2).)
.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 file system is mounted
.IR noexec .
.TP
.B EFAULT
.I filename
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 recognised 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 EMFILE
The process has the maximum number of files open.
.TP
.B ENAMETOOLONG
.I filename
is too long.
.TP
.B ENFILE
The system 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
needed for file or interpreter cannot be found.
.TP
.B ENOEXEC
An executable is not in a recognised 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 file system 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 ETXTBSY
Executable was open for writing by one or more processes.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
POSIX.1-2001 does not document the #!  behavior
but is otherwise compatible.
.\" 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 \fBptrace\fP()d.

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

The result of mounting a filesystem
.I nosuid
vary between 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
.BR exec ()
successfully.

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

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 onwards
an interpreter script can have multiple arguments,
and white spaces in
.I optional-arg
are used to delimit the arguments.
.\" For further info, see
.\" http://homepages.cwi.nl/~aeb/std/hashexclam-1.html
.\" http://www.in-ulm.de/~mascheck/various/shebang/
.\" http://lists.freebsd.org/pipermail/freebsd-arch/2005-February/003525.html

On Linux, 
.I argv
and
.I envp
can be specified as NULL,
which has the same effect as specifying these arguments
as pointers to lists containing a single NULL pointer.
.BR "Do not take advantage of this misfeature!"
It is non-standard and non-portable:
on most other Unix systems doing this will result in an error.
.\" e.g. EFAULT on Solaris 8 and FreeBSD 6.1; but
.\" HP-UX 11 is like Linux -- mtk, Apr 2007
.\" FIXME .
.\" Bug filed 30 Apr 2007: http://bugzilla.kernel.org/show_bug.cgi?id=8408
.\" Maybe this will get fixed (but it would constitute an ABI change).
.\"
.\" .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 \fBexecve\fP(),
.\" 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 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 EXAMPLE
The following program is designed to execed by the second program below.
It just echoes its command-line one  per line.

.in +0.5i
.nf
/* 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);
}
.fi
.in

This program can be used to exec the program named in its command-line
argument:
.in +0.5i
.nf

/* execve.c */

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

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

    assert(argc == 2);  /* argv[1] identifies 
                           program to exec */
    newargv[0] = argv[1];

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

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

.in +0.5i
.nf
$ cc myecho.c -o myecho
$ cc execve.c -o execve
$ ./exceve ./myecho
argv[0]: ./myecho
argv[1]: hello
argv[2]: world
.fi
.in

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:

.in +0.5i
.nf
$ cat > script.sh
#! ./myecho script-arg
^D
$ chmod +x script.sh
.fi
.in

We can then use our program to exec the script:

.in +0.5i
.nf
$ ./execve ./script.sh
argv[0]: ./myecho
argv[1]: script-arg
argv[2]: ./script.sh
argv[3]: hello
argv[4]: world
.fi
.in
.SH "SEE ALSO"
.BR chmod (2),
.BR fork (2),
.BR path_resolution (2),
.BR ptrace (2),
.BR execl (3),
.BR fexecve (3),
.BR environ (7),
.BR ld.so (8)