undefined reference Error in compiling *.cpp with gcc

[bozloo]

Fedora linux system.

i woudl like to complie simple sample cpp file.

files are main.cpp a.cpp b.cpp.

when i compile this files. undefined reference error is occured.
i dont know reason why...

that is link error i thought.

thx alot.

----------------------------------------------
main.cpp
#include <stdio.h>
#include "a.h"
#include "b.h"

int main(void)
{
    int x = 1;
    int y = 3;
    int z = 0;

    float a = 3.0f;
    float b = 2.0f;
    float c = 0.0f;

    printf("entered main\n");

    z = a_sum(x, y);

    c = b_sum(a, b);

    printf("z=%d, c=%f\n", z, c);
}

a.cpp
int a_sum(int x, int y)
{
    return x+y;
}

b.cpp
float b_sum(float x, float y)
{
    return x + y;
}

a.h
extern "C" {
    extern int a_sum(int a, int b);
}

b.h
extern "C"
{
    extern float b_sum(float x, float y);
}

---------------------
gcc -D_REENTRANT -I/home/bozlo/test1 -lpthread -L. -o test.out
/home/bozlo/test1/a.o /home/bozlo/test1/b.o /home/bozlo/test1/main.o

/home/bozlo/test1/main.o(.text+0x63): In function `main':
main.cpp: undefined reference to `a_sum'
/home/bozlo/test1/main.o(.text+0x77):main.cpp: undefined reference to
`b_sum'
/home/bozlo/test1/main.o(.eh_frame+0x11): undefined reference to
`__gxx_personality_v0'
collect2: ld returned 1 exit status