------- Additional Comments From joseph at codesourcery dot com 2005-06-10 02:00 ------- Subject: Re: GCC should combine adjacent stdio calls
On Fri, 10 Jun 2005, ghazi at gcc dot gnu dot org wrote: > Case (b) involves fmemopen, and I assume you refer to a case where you open > memory for writing, printf to the resulting FILE*, and pass a pointer to the > memory area back into printf. This can only lead to disaster as you clobber > the same memory you are reading from. Since fmemopen is a gnu extension, it > can do whatever it wants, but I suspect you're entering unspecified territory > here for C programs. > > Case (c) with mmap again looks like you're reading and writing to the same > place, but the results depend on how buffering and disk syncing interact. > Again, what guarantees from the C standards do we have here on what the > results > should look like? Since IIRC mmap isn't part of C, there are no guarantees. We support programs which use functions other than the standard C ones - naturally as the compiler for the GNU system we support extensions in the GNU libraries. The example I gave used %.5s specifically so that it would only look at bytes with known values (if the programmer knows something about the possible values of the integer written) and not at bytes being modified by the printf. POSIX defines how streams and file descriptors for the same file can be used together and for an unbuffered stream it appears to me that the results from writing to it with stdio (within parts of the file that already existed; not after its end) should be immediately available in the mapped region. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21982
