undefined reference error even after linking c files

Question

I have a main.cpp file and a read.c file. I am calling a function read_int from main function which is defined in read.c file. Also i have declared the function in read.h file and i have included the file in main.cpp. But when i try to compile I am getting "undefined reference to read_int" error.

the commands used for compiling are:

gcc -c ./input/read.c -o ./bin/input/read.o

gcc ./main.cpp ./bin/input/read.o -o ./bin/main.out

the main.cpp file :

#include<stdio.h>
#include "./input/read.h"
int main() {
    int *vectorA = (int *) malloc(sizeof(int)*VECTOR_A_SIZE);
    int *vectorB, *matrixA, *matrixB;
    int *flags = (int *)malloc(sizeof(int)*4);
    flags[0] = 1;
    read_int(vectorA, vectorB, matrixA, matrixB, flags);
}

the read.c file is

 #include<stdio.h>

 int read_int(int *vectorA, int *vectorB, int *matrixA, int *matrixB, int *flags) {
 int flag_count = 4;
 int err = 0;
 FILE *fp = fopen("input_int.txt","R");
 int i,j;
 if(flags[0]) {
     for(i = 0; i < VECTOR_A_SIZE; i++) {
         fscanf(fp, "%d", (vectorA + i));
     }
 }
 fclose(fp);
 return 0;
}

the read.h file is

int read_int(int *vectorA, int *vectorB, int *matrixA, int *matrixB, int *flags);

might want to read up on mixing C and C++ code - eg. : http://www.parashift.com/c++-faq/mixing-c-and-cpp.html — Sander De Dycker, Sep 04 '15 at 08:03

score 3 · Accepted Answer · answered Sep 04 '15 at 08:06

This is because when you include the header in your C++ file, the name "read_int" is "mangled".
The C++ compiler decorates the name with type information in order to enable overloading.

You need to tell the C++ compiler that it's actually a C function.
This is done with the linkage specifier extern "C".

Put this in "read.h":

#ifdef __cplusplus
extern "C" {
#endif 

int read_int(int *vectorA, int *vectorB, int *matrixA, int *matrixB, int *flags);

#ifdef __cplusplus
}
#endif

And remember to include "read.h" in your C file as well, or everything will break mysteriously if you change the function's protype in one place and forget about the other.

thanks. that solved the issue – Johns Paul Sep 04 '15 at 08:08 — Johns Paul, Sep 04 '15 at 08:08

Vercetti · Answer 2 · 2015-09-04T08:06:46.417

0

Shouldn't it be

#include "read.h"

in the top of read.c?

And read.h should contain

#ifndef READ_H
#define READ_H
…
#endif

edited Sep 04 '15 at 08:06

answered Sep 04 '15 at 08:04

Vercetti

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the read.h file is in a different directory. Thats why i have ./input/read.h – Johns Paul Sep 04 '15 at 08:05
I've spoken about read.c file, not main.cpp. By the way, why not main.c? – Vercetti Sep 04 '15 at 08:09
No i dont need to include read.h in read.c. This is because only main.cpp needs the declaration of read_int in read.h – Johns Paul Sep 04 '15 at 08:11

undefined reference error even after linking c files

2 Answers2