Re: possible bug in fseek of buffered files shared between processes (example uses stdin)

[christos%astron.com@localhost (Christos Zoulas)]

In article <a91717c9-3b97-2756-5b73-81d5dee4e7%SDF.ORG@localhost>,
RVP  <rvp%SDF.ORG@localhost> wrote:
>On Tue, 2 Nov 2021, Carlo Arenas wrote:
>
>> while troubleshooting a test failure for pcre2[0] in OpenBSD, tracked
>> down the problem to code that was inherited from NetBSD for their
>> fseek implementation in libc.
>>
>
>This isn't actually an fseek issue. Glibc and musl do an explicit lseek()
>when closing a file if there is unread data in the buffer:
>
>$ cc -o h head.c
>$ (strace -o x.log ./h 1; echo .; ./h 2; echo .; cat) < head.c
>#include <stdio.h>
>.
>#include <stdlib.h>
>
>.
>int
>main(int argc, char* argv[])
>{
>         char line[BUFSIZ];
>         int n;
>
>         if (argc != 2)
>                 return EXIT_FAILURE;
>
>         n = atoi(argv[1]);
>         while (n-- && fgets(line, sizeof line, stdin))
>                 printf("%s", line);
>         return EXIT_SUCCESS;
>}
>$ tail x.log
>munmap(0x7f0f0e89d000, 78789)           = 0
>fstat(0, {st_mode=S_IFREG|0600, st_size=258, ...}) = 0
>brk(NULL)                               = 0x5564cc789000
>brk(0x5564cc7aa000)                     = 0x5564cc7aa000
>read(0, "#include <stdio.h>\n#include <std"..., 4096) = 258
>fstat(1, {st_mode=S_IFCHR|0600, st_rdev=makedev(136, 2), ...}) = 0
>write(1, "#include <stdio.h>\n", 19)    = 19
>lseek(0, -239, SEEK_CUR)                = 19
>exit_group(0)                           = ?
>+++ exited with 0 +++
>$
>
>This looks like a useful behaviour to add to the libc close routines.

I agree!

christos