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
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.
struct rlimit rl;
for (i = lowfd; i < rl.rlim_max; i++)
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:
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.