execve - execute program

#include <unistd.h>
int execve(const char *pathname, char *const _Nullable argv[],
 char *const _Nullable envp[]);

execve() executes the program referred to by pathname. This causes the program that is currently being run by the calling process to be replaced with a new program, with newly initialized stack, heap, and (initialized and uninitialized) data segments.

pathname 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 pointers to strings passed to the new program as its command-line arguments. By convention, the first of these strings (i.e., argv[0]) should contain the filename associated with the file being executed. The argv array must be terminated by a null pointer. (Thus, in the new program, argv[argc] will be a null pointer.)

envp is an array of pointers to strings, conventionally of the form key=value, which are passed as the environment of the new program. The envp array must be terminated by a null pointer.

This manual page describes the Linux system call in detail; for an overview of the nomenclature and the many, often preferable, standardised variants of this function provided by libc, including ones that search the PATH environment variable, see exec(3).

The argument vector and environment can be accessed by the new 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 referred to by pathname, then the effective user ID of the calling process is changed to that of the owner of the program file. Similarly, if the set-group-ID bit is set on the program file, then 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 as 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)).

Effect on process attributes

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 process's "dumpable" attribute is set to the value 1, unless a set-user-ID program, a set-group-ID program, or a program with capabilities is being executed, in which case the dumpable flag may instead be reset to the value in /proc/sys/fs/suid_dumpable, in the circumstances described under PR_SET_DUMPABLE in prctl(2). Note that changes to the "dumpable" attribute may cause ownership of files in the process's /proc/pid directory to change to root:root, as described in proc(5).

• 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 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 file.

If the pathname argument of execve() specifies an interpreter script, then interpreter will be invoked with the following arguments:

interpreter [optional-arg] pathname arg...

where pathname is the pathname of the file specified as the first argument of execve(), and arg... is the series of words pointed to by the argv argument of execve(), starting at argv[1]. Note that there is no way to get the argv[0] that was passed to the execve() call.

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.

Since Linux 2.6.28, the kernel permits the interpreter of a script to itself be a script. This permission is recursive, up to a limit of four recursions, so that the interpreter may be a script which is interpreted by a script, and so on.

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)).

Before Linux 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 Linux 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 Linux 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.) Additionally, the total size is limited to 3/4 of the value of the kernel constant _STK_LIM (8 MiB). Since Linux 2.6.25, the kernel also 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 2.6.22 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 to indicate the error.

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[])
{
    for (size_t j = 0; j < argc; j++)
        printf("argv[%zu]: %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[])
{
    static char *newargv[] = { NULL, "hello", "world", NULL };
    static 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

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