NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       execve - execute program

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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 filesystem 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().

   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 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(), starting at argv[1].

       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.

   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)).

       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.

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 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.

       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 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.

       ETXTBSY
              Executable was open for writing by one or more processes.

CONFORMING TO         top

       SVr4, 4.3BSD, POSIX.1-2001.  POSIX.1-2001 does not document the #!
       behavior but is otherwise compatible.

NOTES         top

       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 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.

       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.

   Interpreter scripts
       A maximum line length of 127 characters is allowed for the first line
       in an interpreter scripts.

       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 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
       IDs.)

       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.

   Historical
       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.

EXAMPLE         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>

           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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.70 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-06-13                        EXECVE(2)