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NAME

       execve - execute program

SYNOPSIS

       #include <unistd.h>

       int execve(const char *pathname, char *const argv[],
                  char *const envp[]);

DESCRIPTION

       execve()  executes  the  program referred to by pathname.  This causes the program that is
       currently being run by the calling process to be replaced with a new program,  with  newly
       initialized stack, heap, and (initialized and uninitialized) data segments.

       pathname 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  pointers to strings passed to the new program as its command-line
       arguments.  By convention, the first of these strings (i.e., argv[0]) should  contain  the
       filename  associated with the file being executed.  The argv array must be terminated by a
       NULL pointer.  (Thus, in the new program, argv[argc] will be NULL.)

       envp is an array of pointers to strings, conventionally of the form key=value,  which  are
       passed as the environment of the new program.  The envp array must be terminated by a NULL
       pointer.

       The argument vector and environment can be accessed by the new  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  referred  to  by  pathname,  then  the
       effective  user  ID  of the calling process is changed to that of the owner of the program
       file.  Similarly, if the set-group-ID bit is set on the program file, then  the  effective
       group ID of the calling process is set to the group of the program file.

       The  aforementioned transformations of the effective IDs are not performed (i.e., the set-
       user-ID and set-group-ID bits are ignored) if any of the following is true:

       *  the no_new_privs attribute is set for the calling thread (see prctl(2));

       *  the underlying filesystem is mounted nosuid (the MS_NOSUID flag for mount(2)); or

       *  the calling process is being ptraced.

       The capabilities of the program file (see capabilities(7)) are also ignored if any of  the
       above are true.

       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 real UID and real GID, as well its supplementary group IDs, are unchanged by
       a call to execve().

       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)).

   Effect on process attributes
       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 process's "dumpable" attribute is set to the value 1, unless a set-user-ID program,
          a set-group-ID program, or a program with capabilities is being executed, in which case
          the dumpable flag may instead be reset to the value in  /proc/sys/fs/suid_dumpable,  in
          the  circumstances  described  under PR_SET_DUMPABLE in prctl(2).  Note that changes to
          the "dumpable" attribute may cause ownership of  files  in  the  process's  /proc/[pid]
          directory to change to root:root, as described in proc(5).

       *  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 capabilities(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 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 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  pathname  argument  of execve() specifies an interpreter script, then interpreter
       will be invoked with the following arguments:

           interpreter [optional-arg] pathname arg...

       where pathname is the absolute pathname of the file specified as  the  first  argument  of
       execve(),  and arg...  is the series of words pointed to by the argv argument of execve(),
       starting at argv[1].  Note that there is no way to get the argv[0] that was passed to  the
       execve() call.

       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 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.)  Additionally, the total size is limited to 3/4 of
       the value of the kernel constant _STK_LIM (8 Mibibytes).  Since Linux 2.6.25,  the  kernel
       also 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 pathname 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 pathname 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 pathname 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
              pathname is too long.

       ENFILE The system-wide limit on the total number of open files has been reached.

       ENOENT The file pathname or a script or ELF interpreter does not exist.

       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 pathname 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 systems.

NOTES

       One  sometimes sees execve() (and the related functions described in exec(3)) described as
       "executing a new process" (or similar).  This is a highly misleading description: there is
       no  new  process;  many attributes of the calling process remain unchanged (in particular,
       its PID).  All that execve() does is arrange for an existing process (the calling process)
       to execute a new program.

       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 they 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 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 SIGSEGV (SIGKILL until Linux
       3.17) signal.

   Interpreter scripts
       The kernel imposes a maximum length on the text that follows the "#!"  characters  at  the
       start  of  a script; characters beyond the limit are ignored.  Before Linux 5.1, the limit
       is 127 characters.  Since Linux 5.1, the limit is 255 characters.

       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  (like most other modern UNIX systems) 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.

   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.

EXAMPLES

       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),  get_robust_list(2),  ptrace(2),  exec(3),   fexecve(3),
       getopt(3),  system(3),  capabilities(7),  credentials(7),  environ(7), path_resolution(7),
       ld.so(8)

COLOPHON

       This page is part of release 5.08 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/.