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Date: Mon, 29 Jun 2015 15:45:02 +0100
From: Justin Cormack <>
Subject: Re: fseek EOVERFLOW

On 29 June 2015 at 14:19, Alexander Monakov <> wrote:
> Hello,
> if I run the following test:
> cat <<EOF >test-fseek.c
> #include <stdio.h>
> #include <string.h>
> #include <errno.h>
> int main()
> {
>   FILE *f = fopen("/dev/zero", "r");
>   int r = fseek(f, -1, SEEK_SET);
>   printf("%d %s\n", r, strerror(errno));
>   return 0;
> }
> I observe the following results:
> - on musl, the argument (-1) is sign-extended for syscall, and no failure is
>   reported;
> - on glibc, the argument is sign-extended for syscall (and a syscall is made),
>   but return value 'r' is set to -1 to indicate an error, but errno is not
>   set.
> It's not entirely obvious to me if (and how) the implementation should
> diagnose this, but in light of the fact that a syscall is made with a huge
> 64-bit value as offset, the following seems to apply on 32-bit platforms:
>     [CX] For fseek(), the resulting file offset would be a value which cannot
>     be represented correctly in an object of type long.
> (I hit this issue due to using fseek with size_t offsets and SEEK_SET: on
> 32-bit, my offsets in range 2G-4G were sign-extended, leading to failure with
> unclear diagnostics)

The sign extension is correct - the argument to fseek is off_t (and so
is the return value, it is not an int), and off_t is always 64 bit on
Musl. For glibc it depends if it is compiled with LARGEFILE_SOURCE.

So the sign extension is nothing to do with libc, it is your code.

Negative offsets are allowed by Posix, and Linux does seem ok with
them; you need to reset errno before lseek. I dont have a 32 bit Musl
machine at the minute, I should install one, but on 64 bit I get 0
returned correctly.


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