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EXECVE(2)		   Linux Programmer's Manual		     EXECVE(2)

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 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
       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 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‐
	  ties(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 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).

       *  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
       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.

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 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 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 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
       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:

	   /* 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>\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 3.54 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/.

Linux				  2013-07-04			     EXECVE(2)
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