Re: Bug#287824: libc6: fseek() fails and returns -1 without setting errno
At Thu, 30 Dec 2004 12:42:18 +0100,
Johan Walles wrote:
> The output of the following program is:
> johan@foo:~/src/test$ ./a.out
> fseek: Unknown error 12345
>
> Since I set errno to 12345 before making the (failing) fseek() call,
> fseek() itself apparently fails without setting errno.
>
> Here's the strace output between opening the file and failing:
> open("/proc/self/mem", O_RDONLY) = 3
> fstat64(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
> mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7fe8000
> _llseek(3, 18446744072635807744, [3526037055030344891], SEEK_SET) = -1073743872
...
> write(4, "fseek: Unknown error 12345\n", 27fseek: Unknown error 12345
> ) = 27
...
> int main(int argc, char *argv[])
> {
> int variable = 42;
> int readvalue = 0;
> FILE *mem = fopen("/proc/self/mem", "r");
>
> errno = 12345;
> if (fseek(mem, (size_t)&variable, SEEK_SET) != 0) {
> perror("fseek");
> exit(EXIT_FAILURE);
> }
In first, your program has some bugs.
1. Use fseeko instead of fseek.
2. You want to use variable instead of &variable.
3. (size_t) is invalid. When fseeko is used, (off_t) should be used.
4. compile option -D_FILE_OFFSET_BITS=64 helps you for a large file
like /proc/self/mem.
BTW, this issue is very special case. If you pass such &variable ==
0xbffff800, offset of _llseek becomes 18446744072635807744 ==
0xffffffffbffff800. But kernel mem_lseek() in proc.c simply sets
file->f_pos and it does not return any errors when offset is even
(long long)-1 on 32bit architecture. In this case, file->f_pos always
becomes minus values. Return value from lseek() and fseek() functions
are negative, but from POSIX or ANSI point of view, those are
interpreted as error condition.
I think mem_lseek() (thus lseek for /proc/self/mem) is special case
because sometimes memory is existed in all 32 or 64 bit space.
It's not compatible with POSIX's lseek/fseek interface. So I think
even if it shows error, it's not bug of either glibc or kernel.
I send my interpretation to debian-kernel to make sure. Debian-kernel
guys, if you don't agree with my thought, please let me know.
Otherwise, I'll close this bug.
Regards,
-- gotom
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