tips

Miscellaneous Tips for Hacking Libasync

Material in this section is largely derived from questions asked by students taking 6.824 and other associated course material.

Strings

The str class implements constant, garbage-collected (reference counted) variable-length strings. The constructor str("xxxx") creates a new str object initialized from the C string "xxxx". The method cstr() returns a pointer co char that holds the contents of the str object. Equality with == is defined on str objects, and returns true iff the two strings contain the same bytes. str objects have an explicit length, so they can contain nulls.

The cstr() method will not be "confused" by nulls per se but you probably don't want to call strlen on it or pass it to printf. cstr returns a pointer to the memory where the actual data is stored use the len() method to figure out how long the data is.

You can not modify a str. Don't modify the result of str.cstr(), since cstr() returns a pointer to the str's internal storage. (The compiler should warn you if you try to do this.)

strbuf is a string buffer and can also hold nulls. You can append to a string buffer, for example

    str b, c;
    strbuf a;
    a << b << c;       // Concatenates two strings
    a << "goodbye!\n"; // Appends a char *.

However, be careful when using string vs opaque in XDR defined structs! XDR strings translate into rpc_str in :sfslite which subclass str but implicitly assume that it can use null as a string terminator (and that the contents can be printed, e.g. in ASRV_TRACE).

You can use xdr2str and str2xdr to marshal and unmarshal structures defined in .x files (and compiled by rpcc) into strings.

Strbufs

You can reuse strbuf's by clearing them as follows:

    strbuf b;
    b << "hello!";
    str hi = b;
    b.tosuio ()->clear ();
    b << "goodbye!";
    str bye = b;
    warn << "hi=" << hi << "; bye=" << b << "\n";

This code will print out:

hi=hello!; bye=goodbye!

Arrays and Lists

Arrays can be indexed directly; variable length items (e.g. int<>) can be treated much like the vec class --- you can use x.push_back() to create a new object and then work with x.back() or you can call x.setsize () to set it to a certain length.