CODEWITHSUNDEEP
clinuxgcc
doubleE
doubleE

Reputation: 1063

GCC linked library for compile

Why do we have to tell gcc which library to link against when that information is already in source file in form of #include?

For example, if I have a code which uses threads and has:

#include <pthread.h>

I still have to compile it with -pthread option in gcc:

gcc -pthread test.c

If I don't give -pthread option it will give errors finding thread function definitions.

I am using this version:

gcc --version
gcc (Ubuntu 4.8.4-2ubuntu1~14.04) 4.8.4

Upvotes: 3

Views: 1162

Answers (4)

old_timer
old_timer

Reputation: 71506

there are/were compilers that you told it where the lib directory was and it simply scanned all the files hoping to find a match. then there are compilers that are the other extreme where you have to tell it everything to link in. the key here is include simply tells the compiler to look for some definitions or even simpler to include some external file into this file. this does not necessarily have any connection to a library or object, there are many includes that are not tied to such things and it is a bad assumption. next the linker is a different step and usually a different program from the compiler, so not only does the include not have a one to one relationship with an object or library, the linker is not the compiler.

Upvotes: 1

dlmeetei
dlmeetei

Reputation: 10371

They are two separate things. .h files holds the declarations, sometimes the inline function also. As we all know, every functions should have an implementation/definition to work. These implementations are kept seperately. -lpthread, for example is the library which holds the implementation of the functions declared in headers in binary form.

Separating the implementation is what people want when you don't want to share your commercial code with others

So, 

gcc -pthread test.c

tell gcc to look for definitions declared in pthread.h in the libpthread. -pthread is expanded to libpthread by linker automatically

Upvotes: 2

Mad Physicist
Mad Physicist

Reputation: 114230

This may be one of the most common things that trip up beginners to C.

In C there are two different steps to building a program, compilation and linking. For the purposes of your question, these steps connect your code to two different types of files, headers and libraries.

The #include <pthread.h> directive in your C code is handled by the compiler. The compiler (actually preprocessor) literally pastes in the contents of pthread.h into your code before turning your C file into an object file.

pthread.h is a header file, not a library. It contains a list of the functions that you can expect to find in the library, what arguments they take and what they return. A header can exist without a library and vice-versa. The header is a text file, often found in /usr/include on Unix-derived systems. You can open it just like any C file to read the contents.

The command line gcc -lpthread test.c does both compilation and linking. In the old days, you would first do something like cc test.c, then ld -lpthread test.o. As you can see, -lpthread is actually an option to the linker.

The linker does not know anything about text files like C code or headers. It only works with compiled object files and existing libraries. The -l flag tells it which libraries to look in to find the functions you are using.

Upvotes: 4

Zbynek Vyskovsky - kvr000
Zbynek Vyskovsky - kvr000

Reputation: 18825

The name of the header has nothing to do with the name of the library. Here it's really just by the accident. Most often there are many headers provided by the library.

Especially in C++ there is usually one header per class and the library usually provides classes implementations from the same namespace. In C the headers are organized that they contain some common subset of functions - math.h contains mathematical operations, stdio.h provides IO functions etc.

Upvotes: 3

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