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EXECVEAT(2) Linux Programmer's Manual EXECVEAT(2)

NAME

     execveat - execute program relative to a directory file descriptor

SYNOPSIS

     #include <unistd.h>

     int execveat(int dirfd, const char *pathname,
                  char *const argv[], char *const envp[],
                  int flags);

DESCRIPTION

     The execveat() system call executes the program referred to by the com-
     bination of dirfd and pathname.  It operates in exactly the same way as
     execve(2), except for the differences described in this manual page.

     If  the  pathname given in pathname is relative, then it is interpreted
     relative to the directory referred to  by  the  file  descriptor  dirfd
     (rather  than  relative to the current working directory of the calling
     process, as is done by execve(2) for a relative pathname).

     If pathname is relative and dirfd is the special value  AT_FDCWD,  then
     pathname  is  interpreted  relative to the current working directory of
     the calling process (like execve(2)).

     If pathname is absolute, then dirfd is ignored.

     If pathname is an empty string and the AT_EMPTY_PATH flag is specified,
     then the file descriptor dirfd specifies the file to be executed (i.e.,
     dirfd refers to an executable file, rather than a directory).

     The flags argument is a bit mask that can include zero or more  of  the
     following flags:

     AT_EMPTY_PATH
            If  pathname is an empty string, operate on the file referred to
            by dirfd (which may have been obtained using the open(2)  O_PATH
            flag).

     AT_SYMLINK_NOFOLLOW
            If  the  file  identified  by dirfd and a non-NULL pathname is a
            symbolic link, then the call fails with the error ELOOP.

RETURN VALUE

     On success, execveat() does not return.  On error, -1 is returned,  and
     errno is set appropriately.

ERRORS

     The same errors that occur for execve(2) can also occur for execveat().
     The following additional errors can occur for execveat():

     EBADF  dirfd is not a valid file descriptor.

     EINVAL Invalid flag specified in flags.

     ELOOP  flags includes AT_SYMLINK_NOFOLLOW and the  file  identified  by
            dirfd and a non-NULL pathname is a symbolic link.

     ENOENT The program identified by dirfd and pathname requires the use of
            an interpreter program (such as a script  starting  with  "#!"),
            but  the  file  descriptor  dirfd  was opened with the O_CLOEXEC
            flag, with the result that the program file is  inaccessible  to
            the launched interpreter.  See BUGS.

     ENOTDIR
            pathname is relative and dirfd is a file descriptor referring to
            a file other than a directory.

VERSIONS

     execveat() was added to Linux in kernel 3.19.  GNU C library support is
     pending.

CONFORMING TO

     The execveat() system call is Linux-specific.

NOTES

     In  addition to the reasons explained in openat(2), the execveat() sys-
     tem call is also needed to allow fexecve(3) to be implemented  on  sys-
     tems that do not have the /proc filesystem mounted.

     When  asked to execute a script file, the argv[0] that is passed to the
     script interpreter is a string of the form  /dev/fd/N  or  /dev/fd/N/P,
     where N is the number of the file descriptor passed via the dirfd argu-
     ment.  A  string  of  the  first  form  occurs  when  AT_EMPTY_PATH  is
     employed.  A string of the second form occurs when the script is speci-
     fied via both dirfd and pathname; in this case, P is the value given in
     pathname.

     For  the  same  reasons described in fexecve(3), the natural idiom when
     using execveat() is to set the close-on-exec flag on dirfd.   (But  see
     BUGS.)

BUGS

     The  ENOENT  error described above means that it is not possible to set
     the close-on-exec flag on the file descriptor given to a  call  of  the
     form:

         execveat(fd, "", argv, envp, AT_EMPTY_PATH);

     However,  the inability to set the close-on-exec flag means that a file
     descriptor referring to the script leaks through to the script  itself.
     As  well  as  wasting a file descriptor, this leakage can lead to file-
     descriptor exhaustion in scenarios  where  scripts  recursively  employ
     execveat().