The C Standard: C89 Through C23

"C" isn't one fixed thing — it's an evolving ISO standard, and a compiler flag picks which version's rules apply:

• C89/C90 ("ANSI C") — the original standardized version; variables had to be declared at the top of a block.
• C99 — added // comments, variable declarations anywhere in a block, and <stdbool.h>.
• C11 — added multithreading support (<threads.h>) and anonymous structs/unions.
• C17 — a bugfix-only release with no new features; this course's baseline, and still the most universally-supported target today.
• C23 — the newest standard, adding features like nullptr and improvements to enum and preprocessor support.

gcc -std=c17 program.c -o program

In practice, most real-world C code still targets C99 or C11 for maximum compatibility across compilers and platforms.