Man Pages

execve(2) - phpMan execve(2) - phpMan

Command: man perldoc info search(apropos)  


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



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.  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 effec-
       tive 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.1 for binaries linked
       with the Linux libc 5, or /lib/ld-linux.so.2 for binaries linked with the glibc 2.

       All process attributes are preserved during an execve(), except the following:

       *      The dispositions of any signals that are being caught are reset to being ignored.

       *      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.

       *  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 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 argu-
       ment of execve() specifies an interpreter script, then interpreter will be invoked  with  the  following  argu-
       ments:

           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.

   Limits on size of arguments and environment
       Most Unix implementations impose some limit on the total size of the command-line argument (argv) and  environ-
       ment  (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 man-
       agement  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 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.)  Additionally, the limit per string is 32 pages (the kernel constant MAX_ARG_STRLEN), and the max-
       imum number of strings is 0x7FFFFFFF.

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 inter-
              preter.  (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 inter-
              preter.

       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.

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

NOTES
       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 non-standard and
       non-portable: 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.   How-
       ever,  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  environ-
       ment variables has changed.

   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
       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>
           #include <assert.h>

           int
           main(int argc, char *argv[])
           {
               char *newargv[] = { NULL, "hello", "world", NULL };
               char *newenviron[] = { NULL };

               assert(argc == 2);  /* argv[1] identifies
                                      program to exec */
               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
       chmod(2), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), credentials(7), environ(7),  path_resolution(7),
       ld.so(8)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-04-21                         EXECVE(2)