#include <unistd.h>

       int execve(const char *filename, char *const argv[],
                  char *const envp[]);

       execve() executes the program pointed to by filename.  filename must be
       either a binary executable, or a script starting with  a  line  of  the

           #! 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 file-
       name,  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 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.2 for bina-
       ries linked with glibc 2.  (For binaries  linked  with  the  old  Linux
       *  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

       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 pro-
          gram 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  capabili-

       *  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

       *  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.
          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

           interpreter [optional-arg] filename arg...

       where arg...  is the series of words pointed to by the argv argument of

       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

   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,
              filename  or  the  name  of  a  script  interpreter.   (See also

       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

       EACCES The filesystem 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.

              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.

              filename is too long.

       ENFILE The  system  limit  on  the  total number of open files has been

       ENOENT The file filename or a script or ELF interpreter does not exist,
              or  a  shared  library  needed for file or interpreter cannot be

              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.

              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.

       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

       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 con-
       taining a single NULL pointer.  Do not take advantage of  this  misfea-
       ture!   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  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

       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.

       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>

           main(int argc, char *argv[])
               int j;

               for (j = 0; j < argc; j++)
                   printf("argv[%d]: %s\n", j, argv[j]);

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

               if (argc != 2) {
                fprintf(stderr, "Usage: %s <file-to-exec>\n", argv[0]);

               newargv[0] = argv[1];

               execve(argv[1], newargv, newenviron);
               perror("execve");   /* execve() only returns on error */

       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
           $ 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

       chmod(2), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3),  creden-
       tials(7), environ(7), path_resolution(7), ld.so(8)

       This  page  is  part of release 3.54 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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