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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.  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 filename, and the
       underlying file system 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 permission
       bits.

       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 libraries 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  libraries.   This  interpreter  is
       typically  /lib/ld-linux.so.1  for  binaries  linked  with  the  Linux libc 5, or /lib/ld-
       linux.so.2 for binaries linked with the glibc 2.

       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.1-2001.  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.

       *  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 termination signal is reset to SIGCHLD (see 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.1-2001  specifies that the dispositions of any signals that are ignored or set to
          the default are left unchanged.  POSIX.1-2001 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.1-2001 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
          permission 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().

       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.

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 file system is mounted noexec.

       EFAULT filename 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.

       EMFILE The process has the maximum number of files open.

       ENAMETOOLONG
              filename is too long.

       ENFILE The system 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.

       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  file system 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.

       ETXTBSY
              Executable was open for writing by one or more processes.

CONFORMING TO

       SVr4,  4.3BSD,  POSIX.1-2001.   POSIX.1-2001  does  not  document  the #!  behavior but is
       otherwise compatible.

NOTES

       Set-user-ID and set-group-ID processes can not be ptrace(2)d.

       Linux ignores the set-user-ID and set-group-ID bits on scripts.

       The result of mounting a file system 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.

       A  maximum  line length of 127 characters is allowed for the first line in a #! executable
       shell 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.

       On  Linux,  argv  can  be  specified as NULL, which has the same effect as specifying this
       argument as a pointer to a list containing a single NULL pointer.  Do not  take  advantage
       of  this  misfeature!  It is nonstandard and nonportable: on most other UNIX systems doing
       this will result in an error (EFAULT).

       POSIX.1-2001 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.

   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 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() only returns 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.sh
           #! ./myecho script-arg
           ^D
           $ chmod +x script.sh

       We can then use our program to exec the script:

           $ ./execve ./script.sh
           argv[0]: ./myecho
           argv[1]: script-arg
           argv[2]: ./script.sh
           argv[3]: hello
           argv[4]: world

SEE ALSO

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

COLOPHON

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       pages/.