NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON
EXECVE(2) Linux Programmer's Manual EXECVE(2)
execve - execute program
#include <unistd.h>
int execve(const char *filename, char *const argv[],
char *const envp[]);
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. 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 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 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.1 for binaries linked with the
Linux libc 5, or /lib/ld-linux.so.2 for binaries linked with the glibc 2.
All process attributes are preserved during an execve(), except the following:
* The set of pending signals is cleared (sigpending(2)).
* The dispositions of any signals that are being caught are reset to being
ignored.
* 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.
* 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 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().
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.
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.
On success, execve() does not return, on error -1 is returned, and errno is
set appropriately.
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 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 file system 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.
SVr4, 4.3BSD, POSIX.1-2001. POSIX.1-2001 does not document the #! behavior
but is otherwise compatible.
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, argv can be specified as NULL, which has the same effect as
specifying this argument as a pointer to a list containing a single NULL
pointer. 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
(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.
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.
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>
#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);
}
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
chmod(2), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), credentials(7),
environ(7), path_resolution(7), ld.so(8)
This page is part of release 3.21 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 2008-11-28 EXECVE(2)