close_range(2)

Synopsis

#define _GNU_SOURCE
#include <unistd.h>
#include <linux/close_range.h>

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

Rod's take

The pre-exec loop that calls close() on every number up to _SC_OPEN_MAX is a million syscalls returning EBADF, and it's sitting in code you depend on right now. close_range(3, ~0U, 0) does it in one. CLOSE_RANGE_UNSHARE does the work on a private copy of the fd table, so sibling threads don't get their descriptors yanked mid-use. CLOSE_RANGE_CLOEXEC is the clever one: leave everything open here, guarantee none of it leaks across exec. The BSDs had closefrom for years. Linux showed up late, but it brought flags.

Flags and arguments that matter

Errors you will actually hit

Example

/* in the child, right before exec: nothing above stderr survives */
if (close_range(3, ~0U, CLOSE_RANGE_UNSHARE) < 0)
    err(1, "close_range");
execvp(argv[0], argv);

/* or: keep fds open here, but stop them leaking into children */
close_range(3, ~0U, CLOSE_RANGE_CLOEXEC);
dup2(pipe_wr, 3);   /* dup2 clears FD_CLOEXEC on fd 3 */

Trimmed from the full walkthrough in close_range: one syscall instead of a million EBADFs.

BSD equivalent

FreeBSD: close_range(2) with CLOSE_RANGE_CLOEXEC (FreeBSD 12.2) and closefrom(2) (FreeBSD 8.0). OpenBSD: closefrom(2) (OpenBSD 3.5).