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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.  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 func-
       tion, 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
       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  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  inter-
       preter  is  typically /lib/ld-linux.so.1 for binaries 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 following:

       *      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(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.

       *  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 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  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 environment and argu-
       ment   strings   was   limited   to  32  pages  (defined  by  the  kernel  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 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.)  Addition-
       ally, 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 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.

       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  file  system  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 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 #! exe-
       cutable 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, 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).

       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>
           #include <assert.h>

           int
           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 */
               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), env-
       iron(7), path_resolution(7), ld.so(8)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2009-04-21                         EXECVE(2)

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