unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (Debian-5.0)
Page:
Section:
Apropos / Subsearch:
optional field

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



NAME
       execve - execute program

SYNOPSIS
       #include <&lt;unistd.h>&gt;

       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  file-
       name,  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 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 con-
       taining 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  inter-
       preter named in the PT_INTERP segment is used to load the needed shared
       libraries.  This interpreter is typically /lib/ld-linux.so.1 for  bina-
       ries  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 set of pending signals is cleared (sigpending(2)).

       *  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(3)).

       *  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 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 imple-
          mentation 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  capabili-
          ties(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  other-
          wise  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().

       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 &lt;limits.h&gt; 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  environ-
       ment  and argument strings was limited to 32 pages (defined by the ker-
       nel 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.  For these architec-
       tures, the total size is limited to 1/4 of the allowed stack size,  the
       limit  per string is 32 pages (the kernel constant MAX_ARG_STRLEN), and
       the maximum number of strings is 0x7FFFFFFF.  (This change allows  pro-
       grams to have a much larger argument and/or environment list.  Imposing
       the 1/4-limit ensures that  the  new  program  always  has  some  stack
       space.)   Architectures  with  no  memory management unit are excepted:
       they maintain the limit that was in effect before kernel 2.6.23.

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 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 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 can-
              not 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 supe-
              ruser, 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).

   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  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), creden-
       tials(7), environ(7), path_resolution(7), ld.so(8)

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-09-14                         EXECVE(2)