EXECVE(2) Linux Programmer's Manual EXECVE(2)
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 file-
name, and the underlying file system 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 con-
taining 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 inter-
preter named in the PT_INTERP segment is used to load the needed shared
libraries. This interpreter is typically /lib/ld-linux.so.2 for bina-
ries 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 pro-
gram 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 capabili-
ties(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 imple-
mentation 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 capabili-
ties(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 other-
wise 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 environ-
ment and argument strings was limited to 32 pages (defined by the ker-
nel 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 archi-
tectures, 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 file system 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 can-
not 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 file system is mounted nosuid, the user is not the supe-
ruser, 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 file system 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 con-
taining a single NULL pointer. Do not take advantage of this misfea-
ture! 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 invari-
ant 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
SEE ALSO
chmod(2), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), creden-
tials(7), environ(7), path_resolution(7), ld.so(8)
COLOPHON
This page is part of release 3.53 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/.
Linux 2013-07-04 EXECVE(2)
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