# execve

## NAME



           execve - execute program
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <unistd.h>**
    
           **int execve(const char ***_path_**, char *const _Nullable** _argv_**[],**
                      **char *const _Nullable** _envp_**[]);**
    


## DESCRIPTION



           **execve**() executes the program referred to by _path_.  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.
    
           _path_ 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
           a null pointer.)
    
           _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.
    
           This manual page describes the Linux system call in detail; for an
           overview of the nomenclature and the many, often preferable,
           standardised variants of this function provided by libc, including
           ones that search the **PATH** environment variable, see exec(3).
    
           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
           _path_ , 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 as 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.
    
           _interpreter_ will be invoked with the following arguments:
    
           _interpreter_ [_optional-arg_] _path arg_...
    
           where _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 VERSIONS 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)_).
    
           Before Linux 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 Linux 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 Linux 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 MiB).  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 2.6.22 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 to indicate the error.
    


## ERRORS



           **E2BIG** The total number of bytes in the environment (_envp_) and
                  argument list (_argv_) is too large, an argument or
                  environment string is too long, or the full _path_ of the
                  executable is too long.  The terminating null byte is
                  counted as part of the string length.
    
           **EACCES** Search permission is denied on a component of the path
                  prefix of _path_ 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** _path_ 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 _path_
                  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**
                  _path_ is too long.
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENOENT** _path_ 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 _path_ 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.
    


## VERSIONS



           POSIX does not document the #! behavior, but it exists (with some
           variations) on other UNIX systems.
    
           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.
    
       **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.
    


## STANDARDS



           POSIX.1-2008.
    


## 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.
    
           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.
    
       **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 Linux 3.0, this caused the **set*uid**() call to fail.
           (Before Linux 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.
    


## 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[])
               {
                   for (size_t j = 0; j < argc; j++)
                       printf("argv[%zu]: %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[])
               {
                   static char *newargv[] = { NULL, "hello", "world", NULL };
                   static 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
    


* * *