It also helps readability if the ! is before a function name that doesn't follow the right naming convention for it. One of my pet peeves in C is "if (!strcmp(a, b))". "!strcmp" I read as "not string compare" and I would expect it to mean that the strings don't compare when it means the exact opposite. This is true of anything following the "0 means success, anything else is an error condition" error handling scheme. So I use "if (strcmp(a, b) == 0)" instead because the "==" makes look at what value it's being compared to specifically and I make fewer assumptions.