Provided by: manpages-dev_4.04-2_all bug


       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
       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  containing  shared-
       library  stubs,  the  Linux dynamic linker 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/ 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

       *  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

       *  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

       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 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
       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


       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 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 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.


       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.


       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

       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.

       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() 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),
       credentials(7), environ(7), path_resolution(7),


       This page is part of release 4.04 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