execve(2) — Linux manual page


EXECVE(2)                 Linux Programmer's Manual                EXECVE(2)

NAME         top

       execve - execute program

SYNOPSIS         top

       #include <unistd.h>

       int execve(const char *pathname, char *const argv[],
                  char *const envp[]);

DESCRIPTION         top

       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 NULL.)

       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.

       The argument vector and environment can be accessed by the new pro‐
       gram'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 over‐
       written 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 per‐
       formed (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

       *  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 inter‐
       preter 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

       *  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

       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 ca‐
          pabilities 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

       *  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 ob‐
          jects are not preserved.

       *  The equivalent of setlocale(LC_ALL, "C") is executed at program

       *  POSIX.1 specifies that the dispositions of any signals that are
          ignored or set to the default are left unchanged.  POSIX.1 speci‐
          fies one exception: if SIGCHLD is being ignored, then an implemen‐
          tation may leave the disposition unchanged or reset it to the de‐
          fault; 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

       *  By default, file descriptors remain open across an execve().  File
          descriptors that are marked close-on-exec are closed; see the de‐
          scription 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 de‐
          tails.)  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 un‐
          specified file for each of these file descriptors.  As a general
          principle, no portable program, whether privileged or not, can as‐
          sume that these three file descriptors will remain closed across
          an execve().

   Interpreter scripts
       An interpreter script is a text file that has execute permission en‐
       abled 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 absolute 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 speci‐
       fied 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 in‐
       terpreted 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 ad‐
       vertise this limit using the ARG_MAX constant (either defined in
       <limits.h> or available at run time using the call

       On Linux prior to kernel 2.6.23, the memory used to store the envi‐
       ronment 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 pro‐
       grams 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 al‐
       ways has some stack space.)  Additionally, the total size is limited
       to 3/4 of the value of the kernel constant _STK_LIM (8 Mibibytes).
       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, ap‐
       plications are guaranteed to have at least as much argument and envi‐
       ronment 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.)  Addition‐
       ally, the limit per string is 32 pages (the kernel constant
       MAX_ARG_STRLEN), and the maximum number of strings is 0x7FFFFFFF.

RETURN VALUE         top

       On success, execve() does not return, on error -1 is returned, and
       errno is set appropriately.

ERRORS         top

       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 pathname or the name of a script interpreter.  (See also

       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

       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 pathname 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.

              An ELF interpreter was not in a recognized format.

       ELOOP  Too many symbolic links were encountered in resolving pathname
              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

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

              pathname is too long.

       ENFILE The system-wide limit on the total number of open files has
              been reached.

       ENOENT The file pathname or a script or ELF interpreter does not

              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.

              A component of the path prefix of pathname 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

              The specified executable was open for writing by one or more

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.  POSIX does not document
       the #! behavior, but it exists (with some variations) on other UNIX

NOTES         top

       One sometimes sees execve() (and the related functions described in
       exec(3)) described as "executing a new process" (or similar).  This
       is a highly misleading description: there is no new process; many
       attributes of the calling process remain unchanged (in particular,
       its PID).  All that execve() does is arrange for an existing process
       (the calling process) to execute a new program.

       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 they 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 SIGSEGV (SIGKILL until Linux 3.17) signal.

   Interpreter scripts
       The kernel imposes a maximum length on the text that follows the "#!"
       characters at the start of a script; characters beyond the limit are
       ignored.  Before Linux 5.1, the limit is 127 characters.  Since Linux
       5.1, the limit is 255 characters.

       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.

       Linux (like most other modern UNIX systems) ignores the set-user-ID
       and set-group-ID bits on scripts.

   execve() and EAGAIN
       A more detailed explanation of the EAGAIN error that can occur (since
       Linux 3.1) when calling execve() is as follows.

       The EAGAIN error can occur when a preceding call to setuid(2),
       setreuid(2), or setresuid(2) caused the real user ID of the process
       to change, and that change caused the process to exceed its
       RLIMIT_NPROC resource limit (i.e., the number of processes belonging
       to the new real UID exceeds the resource limit).  From Linux 2.6.0 to
       3.0, this caused the set*uid() call to fail.  (Prior to 2.6, the
       resource limit was not imposed on processes that changed their user

       Since Linux 3.1, the scenario just described no longer causes the
       set*uid() call to fail, because it too often led to security holes
       where buggy applications didn't check the return status and assumed
       that—if the caller had root privileges—the call would always succeed.
       Instead, the set*uid() calls now successfully change the real UID,
       but the kernel sets an internal flag, named PF_NPROC_EXCEEDED, to
       note that the RLIMIT_NPROC resource limit has been exceeded.  If the
       PF_NPROC_EXCEEDED flag is set and the resource limit is still
       exceeded at the time of a subsequent execve() call, that call fails
       with the error EAGAIN.  This kernel logic ensures that the
       RLIMIT_NPROC resource limit is still enforced for the common
       privileged daemon workflow—namely, fork(2) + set*uid() + execve().

       If the resource limit was not still exceeded at the time of the
       execve() call (because other processes belonging to this real UID
       terminated between the set*uid() call and the execve() call), then
       the execve() call succeeds and the kernel clears the
       PF_NPROC_EXCEEDED process flag.  The flag is also cleared if a
       subsequent call to fork(2) by this process succeeds.

       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.

EXAMPLES         top

       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>

           main(int argc, char *argv[])
               for (int j = 0; j < argc; j++)
                   printf("argv[%d]: %s\n", j, argv[j]);


       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>

           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]);

               newargv[0] = argv[1];

               execve(argv[1], newargv, newenviron);
               perror("execve");   /* execve() returns only on error */

       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 in‐
       terpreter.  To do this we create a script whose "interpreter" is our
       myecho program:

           $ cat > script
           #!./myecho script-arg
           $ 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         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-08-13                        EXECVE(2)

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