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

       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  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
       below.

       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.

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.

       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.

       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 the 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.

       EPERM  A  "capability-dumb"  applications would not obtain the full set
              of permitted capabilities granted by the executable  file.   See
              capabilities(7).

       ETXTBSY
              The  specified  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),  system(3),  credentials(7), environ(7), path_resolution(7),
       ld.so(8)

COLOPHON
       This page is part of release 4.06 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2016-03-15                         EXECVE(2)

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