C programming - questions about libraries needing to be linked in when compiling

ratman1 · 10-09-2012, 04:06 AM

I am learning C (still very much a beginner) on Linux using the GCC compiler. I have noticed that some libraries, such as the library used with the "math.h" header, need to be linked in manually when included. I have been linking in the libraries using various flags of the form

Code:

-l[library-name]

such as -lm for the above-mentioned math library.

However, after switching from the command line and/or Geany to Code::Blocks, I noticed that Code::Blocks uses g++ to compile the programs instead of the gcc that I am used to (even though the project is definitely specified as C). Also, Code::Blocks does not require the libraries to be manually linked in when compiling - libraries such as the math library just work.

I have two questions:

Firstly, is it "bad" to compile C programs with the g++ compiler? So far it seems to work, but after all, C++ is not C and I am quite sure that the g++ compiler is meant for C++.

Secondly, is it the g++ compiler that is doing the automatic linking of the libraries in Code::Blocks?

Thanks,
James

ntubski · 10-09-2012, 07:53 AM

Quote:

Firstly, is it "bad" to compile C programs with the g++ compiler? So far it seems to work, but after all, C++ is not C and I am quite sure that the g++ compiler is meant for C++.

You shouldn't compile (that is, create .o files from .c files) C programs because it will treat your C source as C++. Linking (that is, create executables from .o files) is fine though. At worst you'll have some unneeded code linked in.

Quote:

Secondly, is it the g++ compiler that is doing the automatic linking of the libraries in Code::Blocks?

You can see what gcc/g++ does by passing the -v option:

Code:

% gcc -v link.o -o link
Using built-in specs.
...
 /usr/lib/gcc/x86_64-linux-gnu/4.7/collect2 ... link.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s ...

Code:

% g++ -v link.o -o link
Using built-in specs.
...
 /usr/lib/gcc/x86_64-linux-gnu/4.7/collect2 ... link.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc ...

So you can see that g++ always links with the math library.