closefrom, fdwalk - close or iterate over open file descrip-
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
struct rlimit rl;
for (i = lowfd; i < rl.rlim_max; 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
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:
close_func(void *lowfdp, int fd)
if (fd >= *(int *)lowfdp)
(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 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).
No errors are defined. The closefrom() and fdwalk() func-
tions 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 per-
formed only as the first action of a daemon process. Clos-
ing file descriptors that are in use elsewhere in the
current process normally leads to disastrous results.
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | Unsafe |
close(2), getrlimit(2), proc(4), attributes(5)
Man(1) output converted with