Why -pthread working but -lpthread does not? - hyperlink

Question

I have successfully linked my simple gtest test with command
g++ -o build/test1 build/test1.o -pthread -lgtest -lgtest_main
but i'm curios about options -pthread. Why it is not working with -lpthread as
it working with -lgtest.
And why -gtest doest not working but -pthread without "l" is working...

Answer 1

I have successfully linked my simple gtest test with command g++ -o
build/test1 build/test1.o -pthread -lgtest -lgtest_main
but i'm curios about options -pthread. Why it is not working with
-lpthread as it working with -lgtest.
The order of library link options on the command line is significant. I take it that you tried simply changing -pthread to -lpthread in place, to get this:
g++ -o build/test1 build/test1.o -lpthread -lgtest -lgtest_main
But if the gtest or gtest_main library uses any pthreads functions, then -lpthread needs to appear later in the library list than those (libraries can be listed more than once if necessary):
g++ -o build/test1 build/test1.o -lgtest -lgtest_main -lpthread
Although that's not the appropriate way to build pthreads code with g++ (as described in your other answer), it typically does work.
And why -gtest doest not working but -pthread without "l" is
working...
-pthread is a specific compilation option recognized by the GCC suite of compilers. -lgtest is a combination of the general -l option with gtest as an option argument. One can also write the analogous -lpthread option, which, in principle, has a somewhat different meaning than -pthread.

Answer 2

In gcc, the -pthread flag is an indication to the compiler that it should set up things to allow for threaded code. I believe (but I'm not absolutely sure) that one of the things it does is add -lpthread so that the linker will use the relevant libraries when searching for unresolved symbols.
However, it also does other things, like set -D_REENTRANT to specify the use of re-entrant code.
In other words, -lpthread may not be enough on its own, since it only specifies that the threading library should be searched. I tend to use both to be certain that it does the right thing - yes, I am paranoid :-)