Interesting tidbit I found

CERT says Thou-Shalt-Not use fseek/ftell to determine file sizes!

https://www.securecoding.cert.org/confluence/display/seccode/FIO19-C.+Do+not+use+fseek%28%29+and+ftell%28%29+to+compute+the+size+of+a+file

Turns out ISO doesn't require binary streams to support SEEK_END, and of course most people already know fseek is wonky on text streams. So that leaves nowhere that it's both safe AND portable to do this >_>

Apparently this is what may be causing the read/write issues with savegames for Doom64ex. Problem is this may affect other source ports as well such as Chocolate Doom.

So what are we supposed to use? INT 21 ?

CERT recommends fstat, which is really slow compared to fseek/ftell where they actually work.

Well, unless you have to somehow find out the size of several thousands files by opening them first, does this really matter? I guess it also has to do with how a particular OS/library actually populates the fstat object. In some development environments it's easy to get info about a file that you *didn't* open, or even receive a full directory listing without much effort, while in others you have to jump through hoops.

Still, it's a method we've all have used :-/

Their compliant solution sucks. You shouldn't have to use the kernel facilities like open() for something as basic as this, the whole point of fopen(), fread() etc is to abstract away those low-level facilities.

This is a recommendation I will happily ignore unless someone points me to a system where it really does not work.

Quasar said:

CERT recommends fstat, which is really slow compared to fseek/ftell where they actually work.

Really? On my system, at least according to strace, calling fseek calls fstat (as well as precaching a bit of the file.) So fseek must be at least as slow as fstat...

$ strace -o trace ./fseek ~/doom/W/epic2/epic2.wad

/home/rjy/doom/W/epic2/epic2.wad: 37243712

$ sed -ne '/open.*epic2.wad/,$p' trace

open("/home/rjy/doom/W/epic2/epic2.wad", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=37243712, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7755000
fstat64(3, {st_mode=S_IFREG|0644, st_size=37243712, ...}) = 0
_llseek(3, 37240832, [37240832], SEEK_SET) = 0
read(3, " s,\2\0\20\0\0SFLOOR05 \203,\2\0\20\0\0SFLOOR06"..., 2880) = 2880
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7754000
write(1, "/home/rjy/doom/W/epic2/epic2.wad"..., 43) = 43
close(3)                                = 0
munmap(0xb7755000, 4096)                = 0
exit_group(0)                           = ?

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char **argv)
{
  while (++argv, --argc)
  { 
    char *name = *argv;
    FILE *F = fopen(name, "rb");
    size_t length;
    
    fseek(F, 0, SEEK_END);
    length = ftell(F);

    printf("%s: %lu\n", name, length);
    
    fclose(F);
  } 
    
  return EXIT_SUCCESS;
}