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 (i.e., argv[0]) 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. The argv and envp arrays must each include a null pointer at the end of the array.

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[])

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 environ(7).

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.

If the current program is being ptraced, a SIGTRAP signal is sent to it after a successful execve().

If the set-user-ID bit is set on the program file pointed to by filename, 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 aforementioned transformations of the effective IDs are not performed (i.e., the set-user-ID and set-group-ID bits are ignored) if any of the following is true:

• the no_new_privs attribute is set for the calling thread (see prctl(2));
• the underlying filesystem is mounted nosuid (the MS_NOSUID flag for mount(2)); or
• the calling process is being ptraced.

The capabilities of the program file (see capabilities(7)) are also ignored if any of the above are true.

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.

The process's real UID and real GID, as well its supplementary group IDs, are unchanged by a call to execve().

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 objects 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 objects. This interpreter is typically /lib/ld-linux.so.2 for binaries linked with glibc (see ld-linux.so(8)).

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. 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)).
• The file descriptor table is unshared, undoing the effect of the CLONE_FILES flag of 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 specifies that the dispositions of any signals that are ignored or set to the default are left unchanged. POSIX.1 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 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 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 execve().

#! interpreter [optional-arg]

interpreter [optional-arg] filename arg...

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 filesystem is mounted noexec.

EAGAIN (since Linux 3.1)

Having changed its real UID using one of the set*uid() calls, the caller was—and is now still—above its RLIMIT_NPROC resource limit (see setrlimit(2)). For a more detailed explanation of this error, see NOTES.

EFAULT

filename or one of the pointers in the vectors argv or envp 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.

ELOOP

The maximum recursion limit was reached during recursive script interpretation (see "Interpreter scripts", above). Before Linux 3.8, the error produced for this case was ENOEXEC.

EMFILE

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

ENAMETOOLONG

filename is too long.

ENFILE

The system-wide 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 the 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 filesystem 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.

EPERM

A "capability-dumb" applications would not obtain the full set of permitted capabilities granted by the executable file. See capabilities(7).

ETXTBSY

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

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.

NOTES¶

Set-user-ID and set-group-ID processes can not be ptrace(2)d.

The result of mounting a filesystem 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.

On Linux, argv and 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. Do not take advantage of this nonstandard and nonportable misfeature! On many other UNIX systems, specifying argv as NULL will result in an error (EFAULT). Some other UNIX systems treat the envp==NULL case the same as Linux.

POSIX.1 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.

In most cases where execve() fails, control returns to the original executable image, and the caller of 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 SIGKILL signal.

EXAMPLE¶

The following program is designed to be execed by the second program below. It just echoes its command-line arguments, 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() returns only 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
#!./myecho script-arg
^D
$ chmod +x script

We can then use our program to exec the script:

$ ./execve ./script
argv[0]: ./myecho
argv[1]: script-arg
argv[2]: ./script
argv[3]: hello
argv[4]: world

SEE ALSO¶

chmod(2), execveat(2), fork(2), get_robust_list(2), ptrace(2), execl(3), fexecve(3), getopt(3), system(3), credentials(7), environ(7), path_resolution(7), ld.so(8)

COLOPHON¶

This page is part of release 4.15 of the Linux 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/.