Tribblix: manual page: closefrom.3c

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