Laboratory Exercises, C++ Programming

The GNU Compiler Collection and C++ 5
named file.s. The -E option stops the driver after preprocessing and prints the result from this
first stage on standard output.
A1. Write a “Hello, world!” program in a file hello.cc, compile and test it.
A2. Generate preprocessed output in hello.ii, assembly code output in hello.s, and object code
in hello.o. Study the files:
• hello.ii: this is just to give you an impression of the size of the header.
You will find your program at the end of the file.
• hello.s: your code starts at the label main.
• hello.o: since this is a binary file there is not much to look at. The command nm
hello.o (study the manpage) prints the file’s symbol table (entry points in the
program and the names of the functions that are called by the program and must be
found by the linker).
2 Options and messages
There are many options to the g++ command that we didn’t mention in the previous section. In
the future, we will require that your source files compile correctly using the following command
line:
g++ -c -pipe -O2 -Wall -W -ansi -pedantic-errors -Wmissing-braces \
-Wparentheses -Wold-style-cast file.cc
Short explanations (you can read more about these and other options in the gcc and g++
manpages):
-c just produce object code, do not link
-pipe use pipes instead of temporary files for communication between
tools (faster)
-O2 optimize the object code on “level 2”
-Wall print most warnings
-W print extra warnings
-ansi follow standard C++ syntax rules
-pedantic-errors treat “serious” warnings as errors
-Wmissing-braces warn for missing braces {} in some cases
-Wparentheses warn for missing parentheses () in some cases
-Wold-style-cast warn for old-style casts, e.g., (int) instead of static cast
Do not disregard warning messages. Even though the compiler chooses to “only” issue warnings
instead of errors, your program is erroneous or at least questionable. Another option, -Werror,
turns all warnings into errors, thus forcing you to correct your program and remove all warnings
before object code is produced. However, this makes it impossible to test a “half-written” program
so we will normally not use it.
Some of the warning messages are produced by the optimizer, and will therefore not be
output if the -O2 flag is not used. But you must be aware that optimization takes time, and
on a slow machine you may wish to remove this flag during development to save compilation
time. Some platforms define higher optimization levels, -O3, -O4, etc. You should not use these
levels, unless you know very well what their implications are. Some of the optimizations that are
performed at these levels are very aggressive and may result in a faster but much larger program.