gcc-commands.txt

--- https://www.rapidtables.com/code/linux/gcc.html ---
See above website for gcc syntax, options. It can also generate commands when we specify what to do

dynamic link library (DLL) is for windows and shared library is for UNIX-based os
windows -> .dll 
linux -> .so  (shared object)
naming convention for shared libraries -> 'lib<actual_name>.so', we can also mention the version like this 'lib<actual_name>.so.1.0'
commands to create shared libraries ->
1. 'g++ -Wall -fPIC -c *.cpp'
2. 'g++ -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 *.o'
   -fPIC      => means "Position Independent Code", a requirement for shared libraries in Linux
   -shared    => makes the object file created shareable by different executables
   -Wl        => passes a comma separated list of arguments to the linker
   -soname    => means "shared object name" to use
   -o <my.so> => generates the so file with name my.so
$LD_LIBRARY_PATH  -> is the path of system shared libraries

nm is the tool useful for inspecting the symols inside a .so file, run 'nm file.so' this will print all symbols in that file
   T => text section symbol
   U => undefined symbol
   I => indirect symbol
to directly find all undefined symbols run -> 'nm -u file.so'

Article for shared libraries -> https://gist.github.com/gubatron/32f82053596c24b6bec6


static libraries are formed by combining many onject (.o) files together and having .a extension
'ar -cvq stat_lib.a test1.o test2.o test3.o' -> this will create stat_lib.a static library

--- Linking static and dll libs to create an executable (example) ---
'g++ -o hellobeatles hellobeatles.o -L../johnpaul -L../georgeringo -ljohnpaul -lgeorgeringo'
      [or]
'g++ -o hellobeatles hellobeatles.o ../johnpaul/libjohnpaul.a ../georgeringo/libgeorgeringo.so'

Flags or options for g++/gcc
-Wall -> show all the warnings
-w -> to inhibit all the warnings
-g -> provides debugging feature for your program. You will need this when you want to use gdb or valgrind
--std=c++<##> -> uses version <##> of C++ when compiling, like --std=c++17
-o <file_name> -> compiles and links files into an executable named <filename>. The default filename is a.out
'gcc file1.c file2.c -o exefile' -> compiles file1 and file2 and forms the executable exefile
-c -> compiles and assembles files but doesn’t link them. This is useful when building large projects to separate file compilation 
      and minimize what is re-compiled.

-------------------------- compiling with libraries (-l & -L) ---------------------------------------------
-l links with a library file
-L looks in a directory for library files
'gcc -static file.c -lmath -o execfile'      -> for static linking the library file libmath.a use this cmd
'gcc file.c -lmath -o execfile'              -> for for linking a shared library like libmath.so use this cmd
'gcc file.c -L<dir_name> -lmath -o execfile' -> it will look in dir_name for user-defined library files to link to them

-------------------------- Defining macros (-D) ---------------------------------------------
suppose our program contains a macro like LINUX_OS, so to make this macro true we use following cmd
'gcc -D LINUX_OS file.c -o execfile'

-------------------------- Including directories (-I) ---------------------------------------------
assume there is a header file like proj/src/myHeader.h
now our c or cpp code is in some other dir and we have included above .h file in .c file like this #include "myHeader.h"
when we compile our code it will give error like - myHeader.h: no such file or directory
So, to include proj/src dir for our compilation we use -I option like following
'gcc -Iproj/src file.c -o execfile'

-------------------------- Optimizations (-O<level>) ---------------------------------------------
legends used here -> execution-time(et), code-size(cs), memory-usage(mu), compile-time(ct)

-O0       -> et, cs is more and mu, ct is less
-O1 or -O -> et, cs is less and mu, ct is more
-O2       -> cs becomes little less & et is much more less but mu and ct are more
-O3       -> same as O2 but et is much more reduced and ct is increased even further
-Os       -> cs is much reduced but ct is increased
-Ofast    -> same as O3 but math calculations are not accurate