C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Simple fwrite after getc macro question..          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Andrew Kibler Guest Posted: Thu Oct 27, 2005 4:56 pm    Post subject: Simple fwrite after getc macro question.. Two sections of code, in the first one fwrite works, in the second one it doesn't (ms VC++) both are identical except in the working one fseek is used twice to set the file pointer, once just before the fwrite. WHY??? Works: fseek(fp, itemloc, SEEK_SET); tmp = ReadLongBE(fp); if (*itemsize == tmp){ printf("n%Xn",ftell(fp)); fseek(fp, itemloc + 4, SEEK_SET); //added to make fwrite work. r += fwrite(itemdata, 1, *itemsize, fp); printf("n%Xn",ftell(fp)); Just goes through the motions, but doesn't write (even returns proper values!!) fseek(fp, itemloc, SEEK_SET); tmp = ReadLongBE(fp); if (*itemsize == tmp){ printf("n%Xn",ftell(fp)); //no fseek, but ftell is correct. r += fwrite(itemdata, 1, *itemsize, fp); //but this doesn't actually write!! printf("n%Xn",ftell(fp)); //but the file pointer is advanced properly BOTH return the same values for ftell before and after the write. The second one doesn't wrrite, but acts like it does during the debug. The ReadLongBE (big endian) macro: #define ReadLongBE(Stream) ((getc(Stream)<<24) + (getc(Stream)<<16) + (getc(Stream)<< + (getc(Stream))) Thanks! Back to top Paul Jarc Guest Posted: Thu Oct 27, 2005 5:30 pm    Post subject: Re: Simple fwrite after getc macro question.. Andrew Kibler <00 (AT) cwru (DOT) edu> wrote: Quote: fseek(fp, itemloc, SEEK_SET); tmp = ReadLongBE(fp); if (*itemsize == tmp){ printf("n%Xn",ftell(fp)); fseek(fp, itemloc + 4, SEEK_SET); //added to make fwrite work. r += fwrite(itemdata, 1, *itemsize, fp); When reading and then writing on the same stream, the C standard requires an fseek/fsetpos/rewind in between, unless the read reached EOF. Writing and then reading requires fflush/fseek/fsetpos/rewind in between. (See C99+TC2 (WG14/N1124) 7.19.5.3p6.) Any given implementation may or may not work without the fseek; yours apparently doesn't. The usual idiom, which you might like to wrap up in a macro or function, is fseek(fp, ftell(fp), SEEK_SET). Or, if you want to check for errors and be able to handle file positions that don't fit in long: int reset_fp(FILE* fp) { fpos_t pos; if (fgetpos(fp, &pos)!=0) return -1; if (fsetpos(fs, &pos)!=0) return -1; return 0; } paul Back to top red floyd Guest Posted: Thu Oct 27, 2005 5:31 pm    Post subject: Re: Simple fwrite after getc macro question.. Andrew Kibler wrote: Quote: [fwrite after fread question redacted] Thanks! I don't have a copy of the C standard (only the C++ standard) available, but I believe that's defined as required. That is, after you fread, you *must* fseek before you fwrite. Unfortunately, that's one of the "included by reference to C standard" areas. Anyone got the C standard? Back to top TIT Guest Posted: Thu Oct 27, 2005 5:34 pm    Post subject: Re: Simple fwrite after getc macro question.. Andrew Kibler sade: Quote: Two sections of code, in the first one fwrite works, in the second one it doesn't (ms VC++) both are identical except in the working one fseek is used twice to set the file pointer, once just before the fwrite. WHY??? Let me quote the C99 Standard 7.19.5.3/6: "When a file is opened with update mode [...], both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.[...]" TIT Back to top Andrew Kibler Guest Posted: Thu Oct 27, 2005 5:43 pm    Post subject: Re: Simple fwrite after getc macro question.. Quote: When reading and then writing on the same stream, the C standard requires an fseek/fsetpos/rewind in between, unless the read reached EOF. Thanks for your fast reply. I was using http://www.cplusplus.com/ref/cstdio/fwrite.html as my resource, and they don't have that requirement documented Strange that you have to do that.. after all, that's the point of the file pointer, no? Having to flush the write buffer before a read makes sense, though. Strange that fwrite would return the correct value, too. Oh well. 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