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A microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, thread management, and inter-process communication (IPC).
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closefrom(2) [freebsd man page]

CLOSEFROM(2)						      BSD System Calls Manual						      CLOSEFROM(2)

closefrom -- delete open file descriptors LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> void closefrom(int lowfd); DESCRIPTION
The closefrom() system call deletes all open file descriptors greater than or equal to lowfd from the per-process object reference table. Any errors encountered while closing file descriptors are ignored. SEE ALSO
close(2) HISTORY
The closefrom() function first appeared in FreeBSD 8.0. BSD
June 12, 2009 BSD

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closefrom(3C)						   Standard C Library Functions 					     closefrom(3C)

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) SunOS 5.10 27 Apr 2000 closefrom(3C)

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