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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.  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

       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 filesystem is mounted noexec.

       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

       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,  either argv or envp can be specified as NULL, which has the
       same effect as specifying these  arguments  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.

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

COLOPHON

       This page is part of release 3.65 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/.



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