How does gcc link static library? - c++

Question

I have a simple program use static library for control pci device. I have got some examples. But i want to make myself some examples but i can't link static libraries.
I have 3 file: led.cpp main.cpp main.h
gcc -c led.cpp -I../utils -I../driver -o led.o
gcc -c main.cpp -I../utils -I../driver -o main.o
it' s ok. Succesfuly creating main.o and led.o object files.
But when linking state, its broken. 24dsi20c500k_utils.a and 24dsi20c500k_dsl.a static libraries.
gcc led.o main.o ../utils/24dsi20c500k_utils.a ../docsrc/24dsi20c500k_dsl.a -o led
Output is shown:
led.o: In function `led_tests(int)':
led.cpp:(.text+0x18): undefined reference to `gsc_label(char const*)'
led.cpp:(.text+0x31): undefined reference to `dsi20c500k_initialize(int, int)'
led.cpp:(.text+0x39): undefined reference to `gsc_label_level_inc()'
led.cpp:(.text+0x5b): undefined reference to `dsi20c500k_led(int, int, int, int*)'
led.cpp:(.text+0x81): undefined reference to `dsi20c500k_led(int, int, int, int*)'
led.cpp:(.text+0xa2): undefined reference to `gsc_label_level_dec()'
led.cpp:(.text+0xb1): undefined reference to `dsi20c500k_initialize(int, int)'
main.o: In function `_perform_tests(int)':
main.cpp:(.text+0x3a1): undefined reference to `gsc_label(char const*)'
main.cpp:(.text+0x3c6): undefined reference to `gsc_id_driver(int, char const*)'
main.o: In function `main':
main.cpp:(.text+0x42c): undefined reference to `gsc_label_init(int)'
main.cpp:(.text+0x49a): undefined reference to `gsc_id_host()'
main.cpp:(.text+0x4a4): undefined reference to `gsc_count_boards(char const*)'
main.cpp:(.text+0x4b6): undefined reference to `gsc_select_1_board(int, int*)'
main.cpp:(.text+0x4d7): undefined reference to `gsc_label(char const*)'
main.cpp:(.text+0x500): undefined reference to `gsc_dev_open(unsigned int, char const*)'
main.cpp:(.text+0x54f): undefined reference to `gsc_dev_close(unsigned int, int)'
collect2: ld returned 1 exit status
make: *** [led] Error 1
if i rename cpp files to c files compilation succesfull. What is the problem?

Answer 1

gcc -c main.cpp will compile your code as C++ code.
What likely happens then, is that your main.cpp includes other header files, that are not meant to be compiled as C++. This means that when including these header files in a C++ program , gcc will assume all the stuff the header files declares are C++.
And if they're not, you get linker errors if you try to link to C code that the compiler assumed was C++.
You can remedy this by stating that those header files are C. e.g. in your main.cpp you'd do
extern "C" {
#include "some_c_lib.h"
}

Answer 2

You probably didn't use extern "C".
if i rename cpp files to c files compilation succesfull. What is the
problem?