execve


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  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.
              (shmat(2)).

       *      POSIX shared memory regions are unmapped (shm_open(3)).

       *      Open  POSIX  message  queue  descriptors  are  closed  (mq_over-
              view(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 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).
          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 <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 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.  (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 archi-
       tectures, 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
       On success, execve() does not return, on  error  -1  is  returned,  and
       errno is set appropriately.
              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.
       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  invari-
       ant  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.

   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:

               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
       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 3.23 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                             2009-09-15                         EXECVE(2)
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