diff options
Diffstat (limited to 'winsup/cygwin/how-fhandlers-work.txt')
| -rw-r--r-- | winsup/cygwin/how-fhandlers-work.txt | 75 |
1 files changed, 0 insertions, 75 deletions
diff --git a/winsup/cygwin/how-fhandlers-work.txt b/winsup/cygwin/how-fhandlers-work.txt deleted file mode 100644 index cef46fedc..000000000 --- a/winsup/cygwin/how-fhandlers-work.txt +++ /dev/null @@ -1,75 +0,0 @@ -Copyright 2001 Red Hat Inc., Robert Collins - -fhandlers are the core mechanism by which cygwin provides a file descripter (fd) -interface to things such as a random number generated, winsock sockets, raw disk -devices, the clipboard, the console and so on. Under unix access to all such -devices is via a combination of IOCTL's and open/close/read/write calls. Some -special functions do exist - such as bind () and listen () for sockets, but -these consistently operate on fd's. Under Win32 there are disparate interfaces -that have little in common with each other. See for example Direct Sound and -the Clipboard. - -The fhandler class provides all open,read,write,close, ioctl and fork()/exec() -functionality for the fd interface. The base class operates on win32 backed -files. The various derived classes utilise win32 primitives to provide their -specific functionality. - -When a file is opened - not necesarily via open() a fd is assigned to it. The fd -includes a pointer to the actual fhandler that operates this specific file. All -file-oriented system calls then operate off this basic structure. - -For example, lets examine lseek (). - -extern "C" off_t -_lseek (int fd, off_t pos, int dir) -{ - off_t res; - sigframe thisframe (mainthread); - - if (dir != SEEK_SET && dir != SEEK_CUR && dir != SEEK_END) - { - set_errno (EINVAL); - res = -1; - } - else if (cygheap->fdtab.not_open (fd)) - { - set_errno (EBADF); - res = -1; - } - else - { - res = cygheap->fdtab[fd]->lseek (pos, dir); - } - syscall_printf ("%d = lseek (%d, %d, %d)", res, fd, pos, dir); - - return res; -} - -The sigframe thisframe (mainthread); is signal related - see -"how_signals_work.txt". - -The if, else if, else tests (in order) -* the validity of the dir parameter, -* is the fd being passed actually open? (cannot seek on a closed fd) -* call the lseek virtual function in the associated fhandler. - -So as you can see, there is no code that attempts to understand the nature of -the fhandler. - -fhandlers that make cross-function-call use of win32 objects that are not -inheritable cross-process need to implement fixup-after-fork and recreate those -objects. HANDLES can be inherited, but memory mapped regions (for example) -cannot. - -For an example step-by-step to create a new fhandler, see -../doc/fhandler-tut.txt - -Note: In most case, it is safe to assume that using new/delete (or -malloc/free) in an fhandler is dangerous and to be avoided. The reason -for this is that memory allocated to fhandlers is copied between execed -processes in the cygwin heap. Memory allocated in new/delete is only -copied to forked processes. So use cmalloc/cfree. - -Obviously it is possible to use new/delete in some situations but if you're -seeing strange core dumps with usages like cat < /my/newfhandler then the -above may well be the culprit. |