Re: [PATCH v3 2/4] util: Introduce read_file() and read_file_long() function

On Wed, 29 Oct 2025 11:43:00 +1100 David Gibson wrote:

On Wed, Oct 29, 2025 at 12:12:48AM +0100, Stefano Brivio wrote:

...

On Wed, 15 Oct 2025 15:46:12 +1100 David Gibson wrote:

...

On Wed, Oct 15, 2025 at 11:50:53AM +0800, Yumei Huang wrote:

...

On Wed, Oct 15, 2025 at 7:28 AM David Gibson wrote:

...

On Tue, Oct 14, 2025 at 03:38:34PM +0800, Yumei Huang wrote:

...

Signed-off-by: Yumei Huang [snip] + * @path: Path to the sysctl file + * @fallback: Default value if file can't be read + * + * Return: Parameter value, fallback on failure +*/ +long read_file_long(const char *path, long fallback) +{ + char buf[32];

Rather than just using a semi-arbitrary 32 here, I'd suggest defining a new constant similar to UINT16_STRLEN. Except that's trickier for a type that doesn't have a known fixed width. Pity the C library doesn't have constants for these AFAICT.

I will just define a UINTMAX_STRLEN with (sizeof("2147483647")).

That's not quite right. - It should be INTMAX_STRLEN (signed), UINTMAX would be for the unsigned version - That assumes intmax_t is 32-bit which is probably not the case (it will be 64-bit, maybe even 128-bit on modern systems) - For signed cases, it's the minimum (negative) value that gives the longest possible string (for 32-bit, "-2147483648")

By the way, while it doesn't cover intmax_t explicitly, I think this is a pretty good resource as it covers most architectures supported by the Linux kernel (hence, most architectures we support):

https://wiki.debian.org/ArchitectureSpecificsMemo#Summary

Oh, nice, that is a very handy resource.

...

and judging from intmax_t(3type) I'd say that the sizeof(long double) column tells you how big intmax_t is.

...

Well, at least, that's the page I use to know which architectures I can use to check things when I suspect a type portability bug.

That's because 'long double' should always be the biggest "native" data type, that is, excluding __int128 or vectorised / SIMD types such as __m256i.

This part isn't true, alas. Theoretically speaking there's not necessarily any relation between the largest native integer type and the largest native float type.

Oops, yes, I misread intmax_t(3type), that's *integer* only (of course, the name says it). So probably it has to match sizeof(long long)?

But more importantly, it's not true in practice: according to the table sizeof(long double) is 16 for amd64, but sizeof(intmax_t) is 8 empirically.

I think sizeof(long long) is more likely to match sizeof(intmax_t), but I don't love relying on it.

Right... well, about relying on it, without a change in the C11 standard, can it ever differ? I don't think so. We could have a look at C17 / C23 and if long long is still the largest integer type, we know we're fine for quite a few years / pretty much forever. By the way, just as a reminder (also to self): we don't actually need this here. -- Stefano