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EXECVE(2)		   Linux Programmers Manual		    EXECVE(2)

       execve - execute program


       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.  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 programs main function, when it is defined

	   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

       *  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

       *  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

       *  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

	   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   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.

       On  success,  execve()  does  not  return, on error -1 is returned, and
       errno is set appropriately.

       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

       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 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.

	      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 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.

	      Executable was open for writing by one or more processes.

       SVr4,  4.3BSD,  POSIX.1-2001.   POSIX.1-2001  does  not document the #!
       behavior but is otherwise compatible.

       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

       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).

       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  execed by the second program
       below.  It just echoes its command-line one per line.

	   /* myecho.c */


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

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


       This program can be used to exec the program named in its  command-line

	   /* execve.c */


	   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 */

       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 3.05 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)

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