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MuffinFlavored

For anybody else:

Why is strncpy insecure?

https://stackoverflow.com/questions/869883/why-is-strncpy-in...

> strncpy() doesn't require NUL termination, and is therefore susceptible to a variety of exploits.

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It also fills all remaining bytes (if any) with nuls. ‘N’ in its name is not the same ‘n’ as in snprintf etc. It could be named fldcpy, as it works with 0-padded fixed-width fields rather than 0-terminated strings.

Yes. Also as mentioned in the post, strlcpy (when it's available) is safer.

But still not safe, thanks to the return value. Imagine the case where you are using strlcpy because you can't be sure if the source buffer is properly null terminated.

If your buffer isn't a NUL-terminated, then don't call a function that is only defined for NUL-terminated buffers. It's as simple as that.

I'm baffled by how some people claim strlcpy() is 'broken' or 'not safe' because it doesn't handle non-NUL-terminated inputs; the exact same thing applies to just about any function in the C standard library that takes strings as input. Are functions like strchr(), fopen(), printf(), strstr(), setenv() 'broken' as well?

> If your buffer isn't a NUL-terminated, then don't call a function that is only defined for NUL-terminated buffers.

It's not that people want to pass strncpy source buffers that lack NUL termination, it's that strncpy in certain situations will not NUL terminate its results.

https://begriffs.com/posts/2019-01-19-inside-c-standard-lib....

> some people claim strlcpy() is 'broken'

Speaking of strlcpy, it thankfully doesn't have the problem that strncpy does. However strlcpy is not in the C standard or in POSIX, so can't be used portably. In C99 snprintf is a better choice.

Yes, strncpy() not NUL-terminating it's output is nasty and well-known. But the comment I was responding to was claiming strlcpy() being unsafe.

Yes, because cutting off the trailing NULL is a threat vector every C programmer needs to be conscious of whenever they're dealing with strings.

But it's also a problem because maybe the programmer is using strlcpy to grab the first five lines of a 10TB memory mapped file. If you're not thinking about the implications of that return value it can be a real surprise.

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