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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 mode 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	 objects  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
       objects.	 This interpreter is typically /lib/ld-linux.so.2 for binaries
       linked with glibc.

       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.	 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)).

       *  The file descriptor table is unshared, undoing  the  effect  of  the
	  CLONE_FILES flag of 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 specifies that the dispositions  of  any  signals  that  are
	  ignored or set to the default are left unchanged.  POSIX.1 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 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 says that if file descriptors 0, 1, and 2 would otherwise be
	  closed after a successful execve(), and the process would gain priv-
	  ilege	 because  the  set-user_ID or set-group_ID mode 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  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.

       EAGAIN (since Linux 3.1)
	      Having changed its real UID using one of	the  set*uid()	calls,
	      the   caller  was--and  is  now  still--above  its  RLIMIT_NPROC
	      resource limit (see setrlimit(2)).  For a more detailed explana-
	      tion of this error, see NOTES.

       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 per-process limit on the number of open file descriptors has
	      been reached.

       ENAMETOOLONG
	      filename is too long.

       ENFILE The system-wide 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
       POSIX.1-2001,  POSIX.1-2008, SVr4, 4.3BSD.  POSIX does not document the
       #! behavior, but it exists (with some variations) on  other  UNIX  sys-
       tems.

NOTES
       Set-user-ID and set-group-ID processes can not be ptrace(2)d.

       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.

       On  Linux, argv and envp can be specified as NULL.  In both cases, this
       has the same effect as specifying the argument as a pointer to  a  list
       containing  a  single null pointer.  Do not take advantage of this non-
       standard and nonportable misfeature!  On many other UNIX systems, spec-
       ifying  argv as NULL will result in an error (EFAULT).  Some other UNIX
       systems treat the envp==NULL case the same as Linux.

       POSIX.1 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.

       In  most	 cases	where  execve() fails, control returns to the original
       executable image, and the caller of execve() can then handle the error.
       However,	 in  (rare)  cases  (typically caused by resource exhaustion),
       failure may occur past the point of no return: the original  executable
       image  has  been	 torn  down, but the new image could not be completely
       built.  In such cases, the kernel kills the process with a SIGKILL sig-
       nal.

   Interpreter scripts
       A  maximum  line length of 127 characters is allowed for the first line
       in an interpreter scripts.

       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.

       Linux ignores the set-user-ID and set-group-ID bits on scripts.

   execve() and EAGAIN
       A  more	detailed explanation of the EAGAIN error that can occur (since
       Linux 3.1) when calling execve() is as follows.

       The EAGAIN  error  can  occur  when  a  preceding  call	to  setuid(2),
       setreuid(2),  or setresuid(2) caused the real user ID of the process to
       change, and that change caused the process to exceed  its  RLIMIT_NPROC
       resource limit (i.e., the number of processes belonging to the new real
       UID exceeds the resource limit).	 From Linux 2.6.0 to 3.0, this	caused
       the  set*uid() call to fail.  (Prior to 2.6, the resource limit was not
       imposed on processes that changed their user IDs.)

       Since Linux 3.1, the scenario  just  described  no  longer  causes  the
       set*uid()  call	to  fail,  because  it too often led to security holes
       where buggy applications didn't check the  return  status  and  assumed
       that--if the caller had root privileges--the call would always succeed.
       Instead, the set*uid() calls now successfully change the real UID,  but
       the kernel sets an internal flag, named PF_NPROC_EXCEEDED, to note that
       the  RLIMIT_NPROC  resource  limit   has	  been	 exceeded.    If   the
       PF_NPROC_EXCEEDED  flag is set and the resource limit is still exceeded
       at the time of a subsequent execve() call, that	call  fails  with  the
       error EAGAIN.  This kernel logic ensures that the RLIMIT_NPROC resource
       limit is still enforced for the	common	privileged  daemon  workflow--
       namely, fork(2) + set*uid() + execve().

       If  the	resource  limit	 was  not  still  exceeded  at the time of the
       execve() call (because other processes belonging to this real UID  ter-
       minated	between	 the  set*uid()	 call and the execve() call), then the
       execve() call succeeds and  the	kernel	clears	the  PF_NPROC_EXCEEDED
       process flag.  The flag is also cleared if a subsequent call to fork(2)
       by this process succeeds.

   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\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() returns only 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
	   #!./myecho script-arg
	   ^D
	   $ chmod +x script

       We can then use our program to exec the script:

	   $ ./execve ./script
	   argv[0]: ./myecho
	   argv[1]: script-arg
	   argv[2]: ./script
	   argv[3]: hello
	   argv[4]: world

SEE ALSO
       chmod(2),   execveat(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 4.04 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.



Linux				  2015-12-28			     EXECVE(2)