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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. 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 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.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().
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
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.
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>
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
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 3.51 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 2012-10-27 EXECVE(2)
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