CODEWITHSUNDEEP
cgcc
user3035418
user3035418

Reputation: 1

Putting error or warning message (extern variable)

My a.c file:

int sum(int a, int b) {
    return a + b;       
}

My b.c file:

#include<stdio.h>                                                                                                                       
extern int sum(int, int);
int main() {
    printf ("%d", sum(2, 3));
    return 0;
}

gcc a.c b.c -o output, working fine.

Let say tomorrow, I change the definition of "a.c file" sum function by increasing the argument to three. Like this, 

int sum(int a, int b, int c) {
    return a + b + c;       
}

But forget to change the usage in b.c file (means I'm still using with two variable)

gcc a.c b.c -o output (doesn't give compilation error or warning mssg, printf gives wrong answer, obviously)

Now consider I'm working in huge set of c file and I cannot make new header file, because it will create unnecessary dependency problem which may take huge time to resolve.

What is the best way to throw error or even warning message in case the extern original definition is changed in terms of argument ?

Regards

Upvotes: 0

Views: 202

Answers (3)

dotslashb
dotslashb

Reputation: 74

The best thing to do is try to avoid "extern" and include the header file for sum(). Using header files and prototyping your functions will help the compiler catch these issues.

test.c:

#include <stdio.h>
#include "math.h"

int main(void)
{
    printf("%d", sum(2, 3));
    return 0;
}

math.h:

int sum(int a, int b, int c)
{
    return (a + b + c);
}

output:

~]$ gcc test.c -o test
test.c: In function ‘main’:
test.c:6:5: error: too few arguments to function ‘sum’
     printf("%d", sum(2, 3));
     ^
In file included from test.c:2:0:
math.h:1:5: note: declared here
 int sum(int a, int b, int c)

Upvotes: 0

mahendiran.b
mahendiran.b

Reputation: 1323

Normally editors like (SourceInsight,Sublime) have the options to browse the symbols. By using this option you can easily find function calls and prototype.

Compiler never generate warnings or error for your problem.Self contained header files are best option to avoid this situation.

Upvotes: 1

David Heffernan
David Heffernan

Reputation: 613441

What is the best way to throw error or even warning message in case the extern original definition is changed in terms of argument?

Neither compiler nor linker will object to that. You'll just find out at runtime (if you are lucky) when your program stops working.

If this was C++ then name mangling would allow the linker to reject such mal-formed programs. However, for C the linker only needs to find a symbol with the right name. It has no means of checking the signature.

Using header files is the accepted way to get the compiler to make sure you do things right. Repeating function declarations over and over throughout your program is usually a very bad idea. Whatever downsides you perceive to using header files pale into insignificance when compared to your proposed approach.

If you simply won't use header files, then you'll just have to always be right!

Upvotes: 2

Related Questions