man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

fdwalk(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(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
MT-Level
Unsafe

See Also

close(2), getrlimit(2), proc(4), attributes(5)