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Linux 2.6 - man page for execve (linux section 2)

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 filename, and the underly-
       ing 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 containing 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 interpreter named in the
       PT_INTERP segment is used to load the needed shared libraries.  This interpreter is  typi-
       cally  /lib/ld-linux.so.2 for binaries 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  (sig-
	  nal(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 fol-
       lowing 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 program is being exe-
	  cuted, 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 capabilities(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 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 capabilities(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 otherwise 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(), 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  environment  and  argument
       strings was limited to 32 pages (defined by the kernel constant MAX_ARG_PAGES).	On archi-
       tectures 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 architectures, 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 appropri-
       ately.

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 cannot 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 other-
       wise 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	implemen-
       tations.  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 spec-
       ifying these arguments as a pointer to a list containing a single NULL  pointer.   Do  not
       take  advantage of this misfeature!  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 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.

   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), credentials(7), environ(7),
       path_resolution(7), ld.so(8)

COLOPHON
       This page is part of release 3.55 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					    2013-10-26					EXECVE(2)


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