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NAME

       close_range - close all file descriptors in a given range

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <unistd.h>

       #include <linux/close_range.h> /* Definition of CLOSE_RANGE_*
                                         constants */

       int close_range(unsigned int first, unsigned int last, int flags);

DESCRIPTION

       The  close_range()  system  call  closes  all  open  file  descriptors  from first to last
       (included).

       Errors closing a given file descriptor are currently ignored.

       flags is a bit mask containing 0 or more of the following:

       CLOSE_RANGE_CLOEXEC (since Linux 5.11)
              Set  the  close-on-exec  flag  on  the  specified  file  descriptors,  rather  than
              immediately closing them.

       CLOSE_RANGE_UNSHARE
              Unshare  the  specified  file  descriptors  from any other processes before closing
              them, avoiding races with other threads sharing the file descriptor table.

RETURN VALUE

       On success, close_range() returns 0.  On error,  -1  is  returned  and  errno  is  set  to
       indicate the error.

ERRORS

       EINVAL flags is not valid, or first is greater than last.

       The  following  can  occur  with CLOSE_RANGE_UNSHARE (when constructing the new descriptor
       table):

       EMFILE The  number  of  open   file   descriptors   exceeds   the   limit   specified   in
              /proc/sys/fs/nr_open  (see proc(5)).  This error can occur in situations where that
              limit was lowered before a call to close_range() where the CLOSE_RANGE_UNSHARE flag
              is specified.

       ENOMEM Insufficient kernel memory was available.

STANDARDS

       None.

HISTORY

       FreeBSD.  Linux 5.9, glibc 2.34.

NOTES

   Closing all open file descriptors
       To  avoid blindly closing file descriptors in the range of possible file descriptors, this
       is sometimes implemented (on Linux) by listing open file descriptors in /proc/self/fd/ and
       calling close(2) on each one.  close_range() can take care of this without requiring /proc
       and within a single system call, which provides significant performance benefits.

   Closing file descriptors before exec
       File descriptors can be closed safely using

           /* we don't want anything past stderr here */
           close_range(3, ~0U, CLOSE_RANGE_UNSHARE);
           execve(....);

       CLOSE_RANGE_UNSHARE is conceptually equivalent to

           unshare(CLONE_FILES);
           close_range(first, last, 0);

       but can be more efficient: if the unshared range extends past the current  maximum  number
       of  file descriptors allocated in the caller's file descriptor table (the common case when
       last is ~0U), the kernel will unshare a new file descriptor table for  the  caller  up  to
       first, copying as few file descriptors as possible.  This avoids subsequent close(2) calls
       entirely; the whole operation is complete once the table is unshared.

   Closing files on exec
       This is particularly useful in cases where multiple pre-exec setup steps risk  conflicting
       with  each  other.   For  example,  setting  up  a  seccomp(2) profile can conflict with a
       close_range() call: if the file descriptors are closed before the  seccomp(2)  profile  is
       set  up,  the  profile  setup can't use them itself, or control their closure; if the file
       descriptors are closed afterwards, the seccomp profile can't block the close_range()  call
       or  any  fallbacks.   Using CLOSE_RANGE_CLOEXEC avoids this: the descriptors can be marked
       before the  seccomp(2)  profile  is  set  up,  and  the  profile  can  control  access  to
       close_range() without affecting the calling process.

EXAMPLES

       The  program shown below opens the files named in its command-line arguments, displays the
       list of files that it has opened (by iterating through the entries in /proc/PID/fd),  uses
       close_range() to close all file descriptors greater than or equal to 3, and then once more
       displays the process's list of open files.  The following example demonstrates the use  of
       the program:

           $ touch /tmp/a /tmp/b /tmp/c
           $ ./a.out /tmp/a /tmp/b /tmp/c
           /tmp/a opened as FD 3
           /tmp/b opened as FD 4
           /tmp/c opened as FD 5
           /proc/self/fd/0 ==> /dev/pts/1
           /proc/self/fd/1 ==> /dev/pts/1
           /proc/self/fd/2 ==> /dev/pts/1
           /proc/self/fd/3 ==> /tmp/a
           /proc/self/fd/4 ==> /tmp/b
           /proc/self/fd/5 ==> /tmp/b
           /proc/self/fd/6 ==> /proc/9005/fd
           ========= About to call close_range() =======
           /proc/self/fd/0 ==> /dev/pts/1
           /proc/self/fd/1 ==> /dev/pts/1
           /proc/self/fd/2 ==> /dev/pts/1
           /proc/self/fd/3 ==> /proc/9005/fd

       Note  that  the  lines  showing  the  pathname  /proc/9005/fd  result  from  the  calls to
       opendir(3).

   Program source

       #define _GNU_SOURCE
       #include <dirent.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/types.h>
       #include <unistd.h>

       /* Show the contents of the symbolic links in /proc/self/fd */

       static void
       show_fds(void)
       {
           DIR            *dirp;
           char           path[PATH_MAX], target[PATH_MAX];
           ssize_t        len;
           struct dirent  *dp;

           dirp = opendir("/proc/self/fd");
           if (dirp  == NULL) {
               perror("opendir");
               exit(EXIT_FAILURE);
           }

           for (;;) {
               dp = readdir(dirp);
               if (dp == NULL)
                   break;

               if (dp->d_type == DT_LNK) {
                   snprintf(path, sizeof(path), "/proc/self/fd/%s",
                            dp->d_name);

                   len = readlink(path, target, sizeof(target));
                   printf("%s ==> %.*s\n", path, (int) len, target);
               }
           }

           closedir(dirp);
       }

       int
       main(int argc, char *argv[])
       {
           int  fd;

           for (size_t j = 1; j < argc; j++) {
               fd = open(argv[j], O_RDONLY);
               if (fd == -1) {
                   perror(argv[j]);
                   exit(EXIT_FAILURE);
               }
               printf("%s opened as FD %d\n", argv[j], fd);
           }

           show_fds();

           printf("========= About to call close_range() =======\n");

           if (close_range(3, ~0U, 0) == -1) {
               perror("close_range");
               exit(EXIT_FAILURE);
           }

           show_fds();
           exit(EXIT_FAILURE);
       }

SEE ALSO

       close(2)