2009-03-14

Things I didn't know in C

I've been creating an interpreter in C99 for the last six weeks. This is the largest project I've done in C, as normally I get paid for writing C++. I've also read the C99 spec, as I was taught C in '91, and things change.

Here are a couple of things I've discovered which I didn't know before:

You can use field names in struct initialisers, such as
( foo_t ) { .bar = 3, .baz = 8 }
which give you an automatic struct with the named fields filled in. Unfortunately, astyle makes a pig's ear of formatting it.

Unlike all the other printf family, the return value of the snprintf function is not the number of bytes written, but the number of bytes which would have been written had there been space in the buffer. That caused a few bugs in some string building code I'd written before I RTFM. I'd assumed the maximum value returned would be the buffer length, which meant I was assuming this is safe:

size_t max = sizeof(buf);
size_t offs = snprintf ( buf, max, "foo" );
offs += snprintf ( buf + offs, max - offs, "foo" );

But when offs > max, max - offs is a large value.

I also have given up using splint as a checker, as that was exactly the sort of error I was hoping to get detected. The amount of annotation necessary to get splint to work with my allocation scheme was too painful to justify.

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