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
   program'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
   overwritten 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
   filename,  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.

   The  process's  read  UID and real GID, as well its supplementary group
   IDs, are unchanged by a call to execve(2).

   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  objects  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  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
      program  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

   *  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

   *  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
      privilege  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
   environment  and  argument  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.)  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  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.

   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
          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 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


   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
   nonstandard  and  nonportable  misfeature!  On many other UNIX systems,
   specifying 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

   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
   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

       /* 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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   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at

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