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Asked: 2026-05-11T13:48:01+00:00

I seem to be forgetting my C++ … I’m trying to declare some functions

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I seem to be forgetting my C++ …

I’m trying to declare some functions in C in separate sources, and including the appropriate .h when necessary. It compiles OK; but the problem is during linking, where the linker complains about functions already being defined.

I even tried defining the functions as extern, in a (vain) attempt to just declare the functions and let the implementation come true only on the .c.

This is an abridged description of my code:

common.h

#include <stdio.h>

module1.h

#include 'common.h' #ifndef MODULE1_H_ #define MODULE1_H_ int init(int option); int open(char* db, char* username, char* password); int get(int handler, int date[2], int time[2], int* data, int& rowsize, int& numrows); int put(int handler, int* data, int& datasize, int& numrows); int close(int handler); int finalize(); #endif /* MODULE1_H_ */

module2.h

#include 'common.h' #ifndef MODULE2_H_ #define MODULE2_H_ int get1(int handler, int date, int time, int *data, int& datasize, int& rowsize); int put1(int handler, int* data, int datasize); #endif /*MODULE2_H_*/

module1.cpp

#include 'module1.h' int init(int option) { ... } int finalize() { ... } int get(int handler, int date[2], int time[2], int* data, int& rowsize, int& numrows) {     .... } ...

module2.cpp

#include 'module1.h' #include 'module2.h' int get1(int handler, int date, int time, int* data, int rowsize) {   int daterange[2]={date,date};   int timerange[2]={time,time};   int rsize, numrows, result;   result=get(handler, daterange,timerange, data, rsize, numrows);   rowsize=rsize;    if(numrows!=1) printf('Uh oh...\n');   return result; } ...

Compilation & linkage:

g++ -o module1.o -c module1.cpp g++ -o module2.o -c module2.cpp g++ -fPIC -shared -o library.so module1.o module2.o

As I said, it compiles OK. The problem is during linkage, where the linker ‘sees’ that there are two implemented functions from module1.h: one from the inclusion of module1.h in module1.cpp; and the other from the inclusion of module1.h together with module2.h in module2.cpp.

I know the functions are supposed to be declared, but I’m obviously doing a faux-pas. Could someone please point out where it is? Thank you in advance.

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1 Answer

  1. Thank you all for your replies and comments. I figured out the problem (it turned out to be a very stupid thing) and am close to solving it (hopefully).

    It turns out it comes from another include file (cfortran.h) which implements a layer for using C function calls in Fortran (and vice-versa). It’s very useful and I’ve been using it with success up until now, but I was ‘blinded’ by the errors; that include’s documentation states that care should be taken when using it in C++ context (not in C) but this is the first instance where that warning actually produces effects.

    Thank you once more for your help.

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