Why Linker does not throw, multiple function declaration error?

Question

Following are three files,

/* HANDLERS.H */
#define TRUE 1
#define FALSE 0

/* Functions  declarations - included in both UI.C & tiffhandler.c */
int canHandle(char *);
int drawImage(char *);
int saveFile(char *);

/* tiffhandler.c */
#include "HANDLERS.H"

int canHandle(char *fileName){
    return TRUE;
}

int drawImage(char *fileName){
    return TRUE;
}

int saveFile(char *fileName){
    return TRUE; 
}

/* UI.C */
#include "HANDLERS.H"
int main(void){
}

that are compiled as,

>gcc HANDLERS.H tiffhandler.c UI.C

My question,

HANDLERS.H is included in both UI.C & tiffhandler.c. So, function declarations are included in both.

During linking phase, before tiffhandler.o & UI.o are linked, Why linker does not complain, by saying, multiple function declarations for each function(say canHandle)?

Robᵩ · Accepted Answer · 2016-11-03 15:35:49Z

5

Because multiple function declarations are perfectly legal. Only multiple function definitions are disallowed.

It is okay to repeat the declaration:

int canHandle(char *);
int canHandle(char *);

But the following is incorrect since it repeats a definition:

int canHandle(char *fileName){
    return TRUE;
}

int canHandle(char *fileName){
    return TRUE;
}

The prohibition against multiple definitions of the same function is enforced by both the compiler (inside a single translation unit) and the linker (across multiple translation units).

answered Nov 3, 2016 at 15:35

Robᵩ

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Comments

Fantastic Mr Fox · Accepted Answer · 2016-11-03 15:36:24Z

1

The linker will only fail if you have multiple function definitions in a compilation unit. So even the following is ok in a file:

void func(void);
void func(void);
void func(void);

answered Nov 3, 2016 at 15:36

Fantastic Mr Fox

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