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    Command:

    execve

    
    
    

    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 file-
           name,  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 permission bits.
    
           If the executable is an a.out dynamically linked binary executable con-
           taining  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 inter-
           preter named in the PT_INTERP segment is used to load the needed shared
           libraries.   This interpreter is typically /lib/ld-linux.so.2 for bina-
    
           *  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.
    
           *  (Since Linux 2.4.36 / 2.6.23) If a set-user-ID or set-group-ID  pro-
              gram 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 capabili-
              ties(7).
    
           *  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 imple-
              mentation may leave the disposition unchanged or  reset  it  to  the
              wise  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(), 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  environ-
           ment  and argument strings was limited to 32 pages (defined by the ker-
           nel 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  archi-
           tectures,  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
                  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.
    
           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.
    
           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 can-
                  not 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 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.
    
           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, either argv or envp can be specified as NULL, which  has  the
           same  effect  as specifying these arguments as a pointer to a list con-
           taining a single null pointer.  Do not take advantage of  this  misfea-
           ture!   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  invari-
           ant  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++)
    
               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),  creden-
           tials(7), environ(7), path_resolution(7), ld.so(8)
    
    
    

    Linux 2014-01-08 EXECVE(2)

    
    
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