76 lines
2.7 KiB
Plaintext
76 lines
2.7 KiB
Plaintext
Contributed by Robert Collins
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fhandlers are the core mechanism by which cygwin provides a file descripter (fd)
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interface to things such as a random number generated, winsock sockets, raw disk
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devices, the clipboard, the console and so on. Under unix access to all such
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devices is via a combination of IOCTL's and open/close/read/write calls. Some
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special functions do exist - such as bind () and listen () for sockets, but
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these consistently operate on fd's. Under Win32 there are disparate interfaces
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that have little in common with each other. See for example Direct Sound and
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the Clipboard.
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The fhandler class provides all open,read,write,close, ioctl and fork()/exec()
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functionality for the fd interface. The base class operates on win32 backed
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files. The various derived classes utilise win32 primitives to provide their
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specific functionality.
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When a file is opened - not necesarily via open() a fd is assigned to it. The fd
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includes a pointer to the actual fhandler that operates this specific file. All
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file-oriented system calls then operate off this basic structure.
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For example, lets examine lseek ().
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extern "C" off_t
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_lseek (int fd, off_t pos, int dir)
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{
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off_t res;
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sigframe thisframe (mainthread);
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if (dir != SEEK_SET && dir != SEEK_CUR && dir != SEEK_END)
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{
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set_errno (EINVAL);
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res = -1;
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}
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else if (cygheap->fdtab.not_open (fd))
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{
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set_errno (EBADF);
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res = -1;
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}
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else
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{
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res = cygheap->fdtab[fd]->lseek (pos, dir);
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}
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syscall_printf ("%d = lseek (%d, %d, %d)", res, fd, pos, dir);
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return res;
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}
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The sigframe thisframe (mainthread); is signal related - see
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"how_signals_work.txt".
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The if, else if, else tests (in order)
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* the validity of the dir parameter,
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* is the fd being passed actually open? (cannot seek on a closed fd)
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* call the lseek virtual function in the associated fhandler.
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So as you can see, there is no code that attempts to understand the nature of
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the fhandler.
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fhandlers that make cross-function-call use of win32 objects that are not
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inheritable cross-process need to implement fixup-after-fork and recreate those
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objects. HANDLES can be inherited, but memory mapped regions (for example)
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cannot.
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For an example step-by-step to create a new fhandler, see
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../doc/fhandler-tut.txt
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Note: In most case, it is safe to assume that using new/delete (or
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malloc/free) in an fhandler is dangerous and to be avoided. The reason
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for this is that memory allocated to fhandlers is copied between execed
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processes in the cygwin heap. Memory allocated in new/delete is only
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copied to forked processes. So use cmalloc/cfree.
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Obviously it is possible to use new/delete in some situations but if you're
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seeing strange core dumps with usages like cat < /my/newfhandler then the
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above may well be the culprit.
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