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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 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.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 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
          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().

       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)).
       For these architectures, 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  programs  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
              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 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.

       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 and envp can be specified as NULL, which  has  the  same
       effect  as specifying these arguments as pointers to lists 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.

   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),
       environ(7), credentials(7), path_resolution(7), ld.so(8)

COLOPHON

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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.