close_range(2)

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/c
    /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)