CLOSEFROM(3C) Standard C Library Functions CLOSEFROM(3C)
NAME
closefrom, fdwalk - close or iterate over open file descriptors
SYNOPSIS
#include <stdlib.h>
void closefrom(
int lowfd);
int fdwalk(
int (*
func)(
void *,
int),
void *cd);
DESCRIPTION
The
closefrom() function calls
close(2) on all open file descriptors
greater than or equal to
lowfd.
The effect of
closefrom(
lowfd) is the same as the code
#include <sys/resource.h>
struct rlimit rl;
int i;
getrlimit(RLIMIT_NOFILE, &rl);
for (i = lowfd; i < rl.rlim_max; i++)
(void) close(i);
except that
close() is called only on file descriptors that are
actually open, not on every possible file descriptor greater than or
equal to
lowfd, and
close() is also called on any open file
descriptors greater than or equal to
rl.rlim_max (and
lowfd), should
any exist.
The
fdwalk() function first makes a list of all currently open file
descriptors. Then for each file descriptor in the list, it calls the
user-defined function,
func(
cd,
fd), passing it the pointer to the
callback data,
cd, and the value of the file descriptor from the
list,
fd. The list is processed in file descriptor value order,
lowest numeric value first.
If
func() returns a non-zero value, the iteration over the list is
terminated and
fdwalk() returns the non-zero value returned by
func(). Otherwise,
fdwalk() returns 0 after having called
func() for
every file descriptor in the list.
The
fdwalk() function can be used for fine-grained control over the
closing of file descriptors. For example, the
closefrom() function
can be implemented as:
static int
close_func(void *lowfdp, int fd)
{
if (fd >= *(int *)lowfdp)
(void) close(fd);
return (0);
}
void
closefrom(int lowfd)
{
(void) fdwalk(close_func, &lowfd);
}
The
fdwalk() function can then be used to count the number of open
files in the process.
RETURN VALUES
No return value is defined for
closefrom(). If
close() fails for any
of the open file descriptors, the error is ignored and the file
descriptors whose
close() operation failed might remain open on
return from
closefrom().
The
fdwalk() function returns the return value of the last call to
the callback function
func(), or 0 if
func() is never called (no open
files).
ERRORS
No errors are defined. The
closefrom() and
fdwalk() functions do not
set
errno but
errno can be set by
close() or by another function
called by the callback function,
func().
FILES
/proc/self/fd directory (list of open files)
USAGE
The act of closing all open file descriptors should be performed only
as the first action of a daemon process. Closing file descriptors
that are in use elsewhere in the current process normally leads to
disastrous results.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+
|MT-Level | Unsafe |
+---------------+-----------------+
SEE ALSO
close(2),
getrlimit(2),
proc(5),
attributes(7) April 27, 2000 CLOSEFROM(3C)
