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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 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

       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

       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.

       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‐

       *  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.

       On success, execve() does not return, on error -1 is returned, and errno is set  appropri‐

       E2BIG  The total number of bytes in the environment (envp) and argument list (argv) is too

       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 explanation 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.

       EMFILE The process has the maximum number of files open.

              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.

              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.

              A component of the path prefix of filename or a script or ELF interpreter is not  a

       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.

              Executable was open for writing by one or more processes.

       SVr4, 4.3BSD, POSIX.1-2001.  POSIX.1-2001 does not document the #!  behavior but is other‐
       wise compatible.

       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 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.

       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  origi‐
       nal  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 signal.

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

       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.

       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 setre‐
       suid(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 ker‐
       nel 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 terminated 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 argument:

           /* 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() only returns 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), fork(2), ptrace(2), execl(3), fexecve(3), getopt(3), credentials(7), environ(7),
       path_resolution(7), ld.so(8)

       This  page  is  part of release 3.74 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                                       2014-10-02                                  EXECVE(2)

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