C language > Beginner questions
Unexpected filepos after skipping a pascal string
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jw:
Hello everybody,
suppose I have a pascal type string "RecLen" (binary 06 00 00 00 52 65 63 4C 65 6E) and I want to skip it. I read the prepended length and use it on fseek like so:
--- Code: ---fseek(stdin, readu32(), SEEK_CUR)
--- End code ---
which works in part of my code. But sometimes it only skips "RecLe" and subsequent reads return nonsense.
I checked fseek is always returning zero (i.e. success) and readu32 of course is called before fseek (I checked the disassembly). I experimented with fflush(stdin) and ftell(stdin), because it may be a buffering problem or some optimization with the file position indicator (though I switched it off for bug hunting). I do not see a mistake, but it may be obvious to you. Please give me a hint.
Thank you for your time and for considering the problem.
PS: Just in case...
--- Code: ---uint32_t readu32(void)
{
uint32_t ret;
if (fread(&ret, sizeof(uint32_t), 1, stdin) < 1) {
fputs("read error", stderr);
exit(-1);
}
return ret;
}
--- End code ---
TimoVJL:
fseek() isn't reliable with stdin, better to read a file instead.
sometimes piping is difficult to do.
Bitbeisser:
--- Quote from: TimoVJL on January 05, 2021, 05:22:43 PM ---fseek() isn't reliable with stdin, better to read a file instead.
sometimes piping is difficult to do.
--- End quote ---
Yeah, I don't think that fseek() behavior in case of stdin is
properly defined, just like you can't get a file size of it. This is one of those cases where the *ix philosophy of "everything is a file" in C shows its idiocy...
Ralf
jw:
That is disappointing.
Before seeing your reply, I had double-checked with WCRT and thought to have found a bug in Pelles CRT. This would have been right off my first contribution. :D
Oh well, I just want to skip forward a few byte, so I replaced fseek with loops of fgetc. Works for me(tm).
TimoVJL:
With setbuf or setvbuf a user buffer can be usable to some purboses.
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