Need to generate arbitrary-length error strings and provide a new API needed to fetch those arbitrary-length error strings

[guyharris]

libpcap has traditionally generated error message strings by formatting into a fixed-length buffer of size PCAP_ERRBUF_SIZE (or PCAP_ERRBUF_SIZE + 1).

This causes -Wformat-truncation warnings to be generated by GCC, Clang, and maybe other compilers.

We should, instead, generate dynamically-allocated error messages using pcapint_asprintf(), have a new API routine to fetch the arbitrary-length message (callers of pcap_geterr() may expect the string it returns to fit in PCAP_ERRBUF_SIZE bytes, even though it's not documented as doing that), and have pcap_geterr() and pcap_perror() truncate that to fit in PCAP_ERRBUF_SIZE and not to truncate in the middle of a UTF-8 sequence.

That routine will be defined to return UTF-8 error messages; callers who want some other character encoding, for any reason, must do the translation themselves.

That's straightforward for messages fetched from the pcap_t - have a char * variable that points to the last error message generated, initialize it to NULL when a pcap_t is allocated, and have routines that are passed a pointer to a char * (similar to pcapint_asprintf()) and that free whatever that char * points to before overwriting the char * with a pointer to the new message, and change pcapint_fmt_errmsg_for_errno() and pcapint_fmt_errmsg_for_win32_err() to work similarly.

It's a bit more work for routines such as pcap_create() that are passed pointers to fixed-length buffers; that will require replacements for those routines.

See #1543, #1029, and places in the code for which there aren't issues but there are DIAG_OFF_FORMAT_TRUNCATION calls to suppress the warnings.

[infrastation]

A potential means to address the problem could be introducing an option to control the error buffer size and defaulting it (and setting the minimum valid value) to PCAP_ERRBUF_SIZE:

char myerrbuf[10240] = "";
pcap_setopt(p, PCAP_OPT_ERRBUFSIZE, sizeof(myerrbuf));

Then every function that takes a handle and an error string buffer (e.g. pcap_setnonblock()) or just a handle (e.g. pcap_activate()) would use the buffer size configured for the handle, and every function that takes an error string buffer only (e.g. pcap_create()) would use PCAP_ERRBUF_SIZE. Would that work well?