gcc Command: Compile C in CI

gcc preprocesses, compiles, assembles, and links C (and other) source.

gcc is the default C toolchain on most Linux runners. In CI the recurring failures are missing header paths and missing libraries, both of which usually trace back to an uninstalled -dev package on the runner image.

Common flags

-c - compile/assemble only, do not link (produces .o)
-o NAME - name the output file
-I DIR - add a header search directory
-L DIR / -lNAME - add a library dir / link libNAME
-Wall -Wextra - enable warnings; -Werror makes them fatal
-O2 / -O0 -g - optimize / debug build with symbols
-std=c11 - select the C language standard

Example in CI

Compile and link a small C program with warnings and optimization:

shell

gcc -std=c11 -Wall -Wextra -O2 -Iinclude src/main.c -o app -lm

Common errors in CI

fatal error: foo.h: No such file or directory - install the -dev package or add -I
undefined reference to func - missing library; add -lNAME after the sources
cannot find -lfoo - the .so/.a is not in any -L directory
error: unknown type name - missing include or wrong -std

Key takeaways

-I adds header paths; -L/-l add and link libraries (order matters).
"No such file or directory" on a header means a missing -dev package.
"undefined reference" is a link-time missing-library error, not a compile error.