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

       execve - execute program

       #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 (i.e., argv[0])  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.  The argv and envp arrays must each
       include a null pointer at the end of the array.

       The argument vector and environment can be accessed by the called  pro-
       gram's main function, when it is defined as:

	   int main(int argc, char *argv[], char *envp[])

       Note, however, that the use of a third argument to the main function is
       not specified in POSIX.1; according to POSIX.1, the environment	should
       be accessed via the external variable environ(7).

       execve()	 does  not  return on success, and the text, initialized data,
       uninitialized data (bss), and stack of the calling  process  are	 over-
       written according to the contents of the newly loaded program.

       If the current program is being ptraced, a SIGTRAP signal 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,  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	aforementioned
       transformations	of the effective IDs are not performed (i.e., the set-
       user-ID and set-group-ID bits are ignored) if any of the	 following  is

       *  the  no_new_privs  attribute	is  set	 for  the  calling thread (see

       *  the underlying filesystem is mounted nosuid (the MS_NOSUID flag  for
	  mount(2)); or

       *  the calling process is being ptraced.

       The  capabilities of the program file (see capabilities(7)) are also if
       any of the above are true.

       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.

       The  process's  read  UID and real GID, as well its supplementary group
       IDs, are unchanged by a call to execve().

       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 (see ld-linux.so(8)).

       All process attributes are preserved during  an	execve(),  except  the

       *  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

       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-

       *  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

       *  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-

       *  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  file.   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

       Since  Linux  2.6.28, the kernel permits the interpreter of a script to
       itself be a script.  This permission is recursive, up  to  a  limit  of
       four  recursions,  so  that  the	 interpreter  may be a script which is
       interpreted by a script, and so on.

   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.

       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 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.

	      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.

       ELOOP  The maximum recursion limit was reached during recursive	script
	      interpretation (see "Interpreter scripts", above).  Before Linux
	      3.8, the error produced for this case was ENOEXEC.

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

	      filename is too long.

       ENFILE The system-wide 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 the 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.

       EPERM  A	 "capability-dumb"  applications would not obtain the full set
	      of permitted capabilities granted by the executable  file.   See

	      The  specified  executable  was  open for writing by one or more

       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-

       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

       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-

   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

       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.

       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 arguments, 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]);


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

	   /* execve.c */

	   #include <stdio.h>
	   #include <stdlib.h>
	   #include <unistd.h>

	   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() returns only 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
	   #!./myecho script-arg
	   $ 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

       chmod(2),  execveat(2),	fork(2),  ptrace(2),   execl(3),   fexecve(3),
       getopt(3),  system(3),  credentials(7), environ(7), path_resolution(7),

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Linux				  2017-03-13			     EXECVE(2)