What is the classic = vs == bug in C, and how can you use it intentionally?

Writing = (assign) where you meant == (compare) silently assigns and then tests the result for non-zero — it compiles fine but does the wrong thing.

// Bug: This ASSIGNS b to a, then checks if result is non-zero
if (a = b) c = 0;

// This is equivalent to:
a = b;
if (a != 0) c = 0;

The problem: Compiles without error, does something completely different!

Intentional use (useful idiom):

// Assign and check in one step
if ((c = open("f.txt")) < 0)
    printf("Can't open file");

// Read until EOF
while ((c = getchar()) != EOF) {
    process(c);
}

Best practice when intentional:

1. Double parentheses: if ((a = b)) signals intent
2. Explicit comparison: if ((a = b) != 0)

Compiler help: Use -Wall -Werror to catch accidental assignments in conditions.