• RawNotes
• gcc

Author

Christopher Marshall (christopherlmarshall@yahoo.com)

Raw Notes on gcc

# this creates a.o and b.o from a.c and b.c
gcc -c a.c b.c

# this links a.o and b.o into prog
gcc -o prog a.o b.o

#gcc options
-Idir   prepend 'dir' to list of dirs to find include files in
-Ldir   prepend 'dir' to list of dirs to find libraries in
-lfoo   link against libfoo.so or libfoo.a
-static link against static libraries only

# variables involved in finding headers:
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
# CPATH
#    used for all languages.  directories specified here are
#    serached after any paths given with -I on the command line
# C_INCLUDE_PATH
#    used just for C.  directories specified here are searched
#    after any paths given with -isystem on the command line
# CPLUS_INCLUDE_PATH
#    used just for C++.  directories specified here are searched
#    after any paths given with -isystem on the command line
# OBJC_INCLUDE_PATH
#    used just for objc.  directories specified here are searched
#    after any paths given with -isystem on the command line

# variables involved in finding libraries:
/etc/ld.so.conf
LIBRARY_PATH
LD_LIBRARY_PATH
LD_PRELOAD
# the dirs listed in /etc/ld.so.conf are searched by ldconfig when it sets links.
# the dirs listed in LIBRARY_PATH are searched by gcc when linking and using -lfoo args.
# the dirs listed in LD_LIBRARY_PATH are searched by the program loader as it
#    follows references in the executable to dynamic libraries
# the dirs listed in LD_PRELOAD are like LD_LIBRARY_PATH, only they are guaranteed to be
#    searched first.

# ar commands
# create libname.a from foo.o, bar.o, and baz.o
ar rcs libname.a foo.o bar.o baz.o
# create the library one file at a time
ar rcs libname.a foo.o
ar rcs libname.a bar.o
ar rcs libname.a baz.o

# list the contents of a static library
# how do you do this for a shared library?
ar t libname.a

# list the shared libraries an executable is linked against
ldd program

# creating a shared library
# libfoo.so.5.3.12
# lib is built from f1.c, f2.c, and f3.c and linked against libc and libm
# the soname of the library is libfoo.so.5 and the actual library file is libfoo.so.5.3.12
# note how we create a link from the soname to the lib file at the end
gcc -fPIC -c f1.c
gcc -fPIC -c f2.c
gcc -fPIC -c f3.c
gcc -shared -Wl,-soname,libfoo.so.5 -o libfoo.so.5.3.12 f1.o f2.o f3.o -lc -lm
ln -sf libfoo.so.5.3.12 libfoo.so.5
ln -sf libfoo.so.5 libfoo.so
# here we compile and link a program against libfoo, assuming libfoo is in the current directory and
# not system wide. Notice how we have to add the current directory to the LD_LIBRARY_PATH when loading the program.
gcc -c prg.c
gcc -L"." -lfoo -o prg prg.o
LD_LIBRARY_PATH=$(pwd) ; ./prg
# here is how it would work if it were system-wide.
cp libfoo.so.5.3.12 /usr/lib
ldconfig
gcc -lfoo -o prg prg.o

# dlopen(), dlerrer(), dlsym(), dlclose()