[Asterisk-Dev] [RFC] strncpy -> ast_copy_string

[Kevin P. Fleming]

The recent UTF-8 discussion brought to light something that I had not 
realized before... strncpy() zero-fills the destination buffer, which is 
waste of time (and cache pollution). In Asterisk, we are nearly always 
copying strings into buffers that are much larger than required, so this 
extra zero-writing is just wasted CPU cycles.

I've committed an ast_copy_string() function that works the same way as 
strncpy() except for three differences:

- there is no return value (of the nearly 1,300 uses of strncpy in CVS 
HEAD, only two use the return value)
- it guarantees that the output buffer will be null-terminated
- it does not null-pad the output buffer

I tested a simple search/replace conversion of chan_sip with no ill 
effects, and the resulting .so file was slightly (a few hundred bytes) 
smaller. No modules in Asterisk have actually been converted yet, though.

I do understand that GCC has a highly-optimized strncpy implementation 
for each architecture it runs on, and I don't doubt that an open-coded 
version will not be optimized as well. However, we'd be trading a 
slightly slower implementation against not writing many, many billions 
of excess zero bytes over the lifetime of an Asterisk instance.

Thoughts?