CLOSEFROM(3C) Standard C Library Functions CLOSEFROM(3C)


closefrom, fdwalk - close or iterate over open file descriptors


#include <stdlib.h>

void closefrom(int lowfd);

int fdwalk(int (*func)(void *, int), void *cd);


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);

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.


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

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).


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().


directory (list of open files)


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


See attributes(7) for descriptions of the following attributes:

|MT-Level | Unsafe |


close(2), getrlimit(2), proc(5), attributes(7)

illumos April 27, 2000 CLOSEFROM(3C)