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class method · added in Ruby 3.3

Dir.fchdir

(::int) -> ::Integer

[T] (::int) { () -> T } -> T

Dir.fchdir(fd) → 0

Dir.fchdir(fd) { ... } → object

static VALUE
dir_s_fchdir(VALUE klass, VALUE fd_value)
{
    int fd = RB_NUM2INT(fd_value);

    if (chdir_alone_block_p()) {
        struct fchdir_data args;
        args.old_dir = dir_s_alloc(klass);
        dir_initialize(NULL, args.old_dir, rb_fstring_cstr("."), Qnil);
        args.fd = fd;
        args.done = FALSE;
        return rb_ensure(fchdir_yield, (VALUE)&args, fchdir_restore, (VALUE)&args);
    }
    else {
        int r = IO_WITHOUT_GVL_INT(nogvl_fchdir, &fd);
        if (r < 0)
            rb_sys_fail("fchdir");
    }

    return INT2FIX(0);
}

Changes the current working directory to the directory specified by the integer file descriptor fd.

When passing a file descriptor over a UNIX socket or to a child process, using fchdir instead of chdir avoids the time-of-check to time-of-use vulnerability

With no block, changes to the directory given by fd:

Example

Dir.chdir('/var/spool/mail')
Dir.pwd # => "/var/spool/mail"
dir  = Dir.new('/usr')
fd = dir.fileno
Dir.fchdir(fd)
Dir.pwd # => "/usr"

With a block, temporarily changes the working directory:

Calls the block with the argument.
Changes to the given directory.
Executes the block (yields no args).
Restores the previous working directory.
Returns the block’s return value.

Example:

Example

Dir.chdir('/var/spool/mail')
Dir.pwd # => "/var/spool/mail"
dir  = Dir.new('/tmp')
fd = dir.fileno
Dir.fchdir(fd) do
  Dir.pwd # => "/tmp"
end
Dir.pwd # => "/var/spool/mail"

This method uses the fchdir() function defined by POSIX 2008; the method is not implemented on non-POSIX platforms (raises NotImplementedError).

Raises an exception if the file descriptor is not valid.

In a multi-threaded program an error is raised if a thread attempts to open a chdir block while another thread has one open, or a call to chdir without a block occurs inside a block passed to chdir (even in the same thread).

Ruby version changes

3.3
- Dir.fchdir added for changing the directory to the directory specified by the provided directory file descriptor. [Feature #19347]

See also .exist? .foreach .entries .for_fd also in 4.0 · 3.4 · 3.3

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