Cherished GCC flags


A (work in progress) compilation of useful GCC configuration, focused on C.

-W warnings.

  • -Wall is a must.
  • -Werror to treat warnings as errors.

-l instructs the linker it has some work to do (i.e. link a library please). -lm is shorthand for -libm and in turn -libm.a is a static library is selected.

-L hint to the linker where to locate libraries. -L/opt/gdbm-1.10/src in combination with -lgdbm tells the linker exactly where to find the gdbm libraries. Multiple -L can be specified. Alternatively use the LIBRARY_PATH environment variable like so:

$ LIBRARY_PATH=.:/opt/gdbm-1.8.3/lib:/net/lib
$ export LIBRARY_PATH

-I hint to compiler where to locate headers. -I/opt/gdbm-1.10/src. Multiple -I include may be specified. Alternatively use the C_INCLUDE_PATH environment variable like so:

$ C_INCLUDE_PATH=.:/opt/gdbm-1.8.3/include:/net/include
$ export C_INCLUDE_PATH

-g debug symbols. Now you’ll get variable and function names, yummy.

-std=gnu11 by default GCC uses the C89 standard. C11 FTW.

-O3 optimisation level 3. This is heavy duty. If you find debugging tough (e.g. variables have been optimised out etc), try dropping it back to level zero -O0.

Examples:

Link with explicit static library:

$ gcc -Wall -I/opt/gdbm-1.8.3/include dbmain.c /opt/gdbm-1.8.3/lib/libgdbm.a

Link with explicit shared library:

$ gcc -Wall -I/opt/gdbm-1.8.3/include dbmain.c /opt/gdbm-1.8.3/lib/libgdbm.so

Provide header and library hint paths:

$ gcc -Wall -I/tmp/gdbm-1.10/src -L/tmp/gdbm-1.10/src dbmain.c -lgdbm

A simple makefile:

P=program_name
OBJECTS=
CC=gcc
CFLAGS= -Wall -g -std=gnu11 -O3
LDLIBS=

$(P): $(OBJECTS)

clean:
  rm -f main main.o hello_fn.o

POSIX standard make uses the following recipe:

$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $*.c

When GNU make decides you have an executable to build from object files:

$(CC) $(LDFLAGS) first.o second.o $(LDLIBS)