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.

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.

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