Control Flow & Functions

In a nutshell

if/else, switch, for, while, do/while, break, continue — all present in Java, all present in C with nearly identical syntax. Two C-specific wrinkles matter this week. One, the C90 for loop requires the counter to be declared above the loop (Lab 1 compiles with -std=c90 -pedantic). Two, missing braces around a multi-statement if body is not a style issue; it is the exact bug pattern that produced Apple’s “goto fail” CVE and forged SSL certificate acceptance on every iPhone for two years.

Functions work like Java methods with no class around them. Every argument is copied (pass-by-value), and a function cannot directly modify a caller’s variable. That limitation is what pointers solve next week.

Why it matters

The for-counter rule and switch fall-through both land directly on Lab 1. The missing-braces discipline is the rule that shows up in every professional code review and every static analyzer. Functions and pass-by-value are the foundation for understanding pointers next week: the reason scanf needs &x is exactly because a function cannot otherwise write to the caller’s variable.

Key takeaways

• Always use braces on if/else/for/while bodies, even for single-statement bodies. CVE-2014-1266 is why.
• C90 for loop declares the counter at the top of the enclosing block: int i; ... for (i = 0; i < n; i++) { ... }.
• switch falls through. Missing break means the next case’s body runs too. This is a bug you will hit once and then never again. Stacked labels (case 'F': case 'f':) are the intentional, Lab 1-friendly use.
• continue in for runs the step clause. continue in while does not; if the counter update is in the body after continue, you get an infinite loop.
• Functions pass arguments by value. A copy goes into the parameter. The caller’s variable is not modified. To modify the caller, return a new value, or (next week) pass a pointer.
• Prototype before use. If main calls add but add is defined below, declare int add(int a, int b); above main or get an implicit-declaration warning.

Lessons in this topic

Practice and deep dives

Practice this topic: C Control Flow or C Functions drills, or browse the practice gallery.

For the concrete “goto fail” mechanism and the other four Lab 1 CWEs, see the memory-safety deep dive. For how stack frames hold the parameters and locals of each call (foundation for recursion and for next week’s pointer work), see the machine model deep dive.

What comes next

Pointers — the & operator you have been using with scanf, generalized. Once you have pointers, you can write functions that modify the caller’s variables, walk arrays through arithmetic, and allocate memory on the heap.