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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.  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.

       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 (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, 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 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)).

       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  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 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 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.

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 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.

       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 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  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(2) 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  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 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 signal.

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

       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.

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

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.