gcc cheat sheet: compile, warnings, debug, optimize, link and preprocess C code

The gcc commands you need most often, grouped by the job they do. Each option is described in the GCC manual: overall options, warnings, debugging, optimization and linking.

Compile and link

See compile multiple C files and which -std value to use.

Warnings

Debug, optimize, sanitize

• -g adds debug information so gdb shows source lines and variable names.
• -O0 (default) does no optimisation, -O1, -O2 and -O3 optimise more, -Os optimises for size. Use -O0 -g while debugging, since optimised code reorders lines.
• -fsanitize=address builds with AddressSanitizer, which stops on out-of-bounds access and use-after-free with a report. It is the quickest way to find the cause of a segmentation fault. -fsanitize=undefined catches undefined behaviour such as signed overflow.

Headers and libraries

Put -l options after the files that need them, because the linker reads its inputs in order.

Inspect the stages

cat > hello.c <<'EOF'
#include <stdio.h>
#define GREETING "hello"
int main(void) { puts(GREETING); return 0; }
EOF
gcc -Wall -Wextra -std=c11 hello.c -o hello && ./hello
gcc -E -P hello.c | tail -n 1
gcc -S -o - hello.c | grep -c "call.*puts"
gcc -c hello.c && ls hello.o

hello
int main(void) { puts("hello"); return 0; }
1
hello.o

-E stops after the preprocessor and prints the expanded source, -S stops after compilation and writes assembly, -c stops before linking. Reading the expanded output shows why a macro or an include behaves unexpectedly.

Common mistakes

• Forgetting -o and then running the wrong old a.out.
• Using -Wall as if it covered everything; add -Wextra.
• Benchmarking a binary built with -O0 or sanitizers.
• Placing -lm before the source file, which causes undefined reference to errors for math functions.

FAQ