How to Use Snprintf

https://bernsteinbear.com/blog/snprintf/

by surprisetalk • 11 months ago

52 22 11 months ago

hdjrudni• 11 months ago

This sentence is confusing:

I have size_with_nul because snprintf man pages say
The functions snprintf() and vsnprintf() write at most size bytes (including the terminating null byte (‘\0’)) to str.

If 'size' includes the null byte, why do we have to add 1?

king_geedorah• 11 months ago

If the output was truncated due to this limit, then the return value is the number of characters (excluding the terminating null byte) which would have been written to the final string if enough space had been available.

The initial call with size 0 tells you the necessary length of the buffer for the string you want, but does not include the null byte.

orbisvicis• 11 months ago

For clarity, all snprintf calls "return the number of bytes that would be written to s had n been sufficiently large excluding the terminating null byte" [1].

1. https://man7.org/linux/man-pages/man3/fprintf.3p.html

chrsw• 11 months ago

The first sentence is confusing for me.

st_goliath• 11 months ago

There are `asprintf` and `vasprintf` (takes a va_list argument). Those allocate a sufficiently sized buffer that can be released with `free`.

Yes, it's a GNU extension, but it's also supported by various BSDs [1][2][3], and yes, Musl has it too. It's present in pretty much any sane C library.

[1] https://man.openbsd.org/man3/printf.3

[2] https://man.netbsd.org/vasprintf.3

[3] https://man.freebsd.org/cgi/man.cgi?query=vasprintf&sektion=...

rwmj• 11 months ago

And combine it with __attribute__((cleanup)) to get the string automatically freed at the end of your function (if that's the right thing to do). Looks like cleanup with be standardized finally in the next C2x.

tom_• 11 months ago

Another tip: don't use normal asprintf as-is, but write your own very similar helper!

1. have it free the passed-in buffer, so that you can reuse the same pointer

2. have it do step 1 after the formatting, so the old value can be a format argument

3. when getting the size of the full expansion, don't format to NULL, but do it to a temp buffer (a few KB in size) - then if the expansion is small enough, you can skip the second format into the actual buffer. Just malloc and memcpy. You know how many chars to memcpy, because that's the return value from snprintf

(Don't forget to check for errors and all that.)

wahern2• 11 months ago

And combine it with __attribute__((cleanup)) to get the string automatically freed at the end of your function (if that's the right thing to do). Looks like cleanup with be standardized finally in the next C2x.

The problem is that on error the buffer pointer value is undefined, so you can't just unconditionally call free on the pointer. There's at least one proposal for C2x that avoids adopting asprintf for this reason despite it already being added to POSIX.

This undefined'ness is a vestige of the original glibc implementation. The proper solution is to either require that the pointer value be preserved on error (thus preserving NULL if the caller initialized it) or require the implementation set it to NULL. IIRC, when added by the BSDs (1990s) and later Solaris they explicitly documented it to set the pointer to NULL. And it seems that late last year glibc finally adopted this behavior as well.[1]

[1] https://sourceware.org/git/?p=glibc.git;a=commit;h=cb4692ce1...

ygritte• 11 months ago

Came here to say exactly this.

The lost art of RTFM.

panbd• 11 months ago

I've actually learned a few little tricks reading the fucking gcc manual. If you're coding C (or C++) regularly, the manual is a good learning source and is well-written.

wahern2• 11 months ago

asprintf and vasprintf are part of POSIX, now.

bluGill2• 11 months ago

Thanks, first I've heard of them and they happen to solve a real problem I'm working on today. Always nice when you can learn something new...

lelanthran• 11 months ago

Thanks, first I've heard of them and they happen to solve a real problem I'm working on today. Always nice when you can learn something new...

You don't really need to, TBH. I pretty much always wrote a malloc-memory `sprintf` alternative if the system didn't have one. it's only a few lines of code, that'll take maybe 10m of your day the first time you realise `sprintf` on the platform doesn't exist.

Here is a sample from more recently: https://github.com/lelanthran/libds/blob/b5289f6437b30139d42...

RhysU• 11 months ago

Once I hacked up an snprintf wrapper that automated any required realloc calls:

https://github.com/RhysU/snprintf_realloc/blob/master/snprin...

Worth critically reviewing before using. It's been a while.

jeroenhd• 11 months ago

That little-known feature turns out to be crucial if you're not careful. printf returns how many bytes were printed. snprintf returns how many bytes would have been printed had the buffer been large enough. Useful for sizing your buffer, but making it a dangerous printf replacement if you don't know the difference.

Cisco and many of Ciscro's customers found out the hard way (during CitrixBleed, https://www.assetnote.io/resources/research/citrix-bleed-lea...), leaking random blocks of memory in the proprietary, C-based web server of their security appliance that gets compromised every now and then.

sumtechguy• 11 months ago

Always read the docs of your system. All of the xxprintf functions are not the same. They are sneaky and look the same on the surface. Even silly things like what the % items do can vary between platforms, or have different meanings, or be missing all together.

MintPaw• 11 months ago

This has annoyed me a lot, crazy you can't rely on the print functions being the same on different platforms. This is why I always add stb_sprintf and make an allocSprintf() that allocates the right sized buffer first.

How to Use Snprintf

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