Understanding macro functions in C that define variables

Question

From my understanding, preprocessor directives are used to modify source code before its compiled. I'm writing some code of an assignment, but my macro causes errors. In my assignment I have to write several files with main() with identical variables. I want to use macros like this.

#define SETUP (i)\
     char* memory = (char*) malloc(sizeof(char)*(i));\
     char* memory2 = (char*) malloc(sizeof(char)*(i))
#define CLOSE ()\
     free(memory);\
     free(memory2)

int main () {
     int i = 7;
     SETUP(i);

/****************************/
Do stuff with memory and memory2
/*****************************/

     CLOSE();
}

this way I can make changes to the beginning and end of each file's main() without having to change each one. Can someone tell me what I don't understand?

Welcome to Stack Overflow! In your `main` function, do you have a declaration for `i` ***before*** the invocation of `SETUP(i)`? — Adrian Mole, Nov 29 '19 at 10:12
`(char*) memory = ...` looks pretty invalid. Don't really remember if such a declarator is valid, but I'd start with that. — StoryTeller - Unslander Monica, Nov 29 '19 at 10:14
The macros themselves are ok but you never declare variables `memory` and `memory2` anywhere. Start by writing the correct code without any macros. — Lundin, Nov 29 '19 at 10:15
`(char *) memory` is not an _lvalue_ so cannot be assigned to. Perhaps you meant `char * memory`? — Ian Abbott, Nov 29 '19 at 10:20
You keep editing the code, which invalidates the comments and answers. Is it working now? — Ian Abbott, Nov 29 '19 at 10:24
no, it's not. The code I wrote was just an example, I added my actual code — yogurt, Nov 29 '19 at 10:27
One other thing - if your code uses `malloc()` and `free()`, it is necessary to `#include ` - without that, the casts on `malloc()` mask a programming error. Then, unless your C compiler is actually a C++ compiler, the `(char *)` casts of `malloc()` are not required. — Peter, Nov 29 '19 at 11:12
I recommend that you read and understand [the question on why not to cast the return value of `malloc()` and family in C](/q/605845). And note that `sizeof` is specified in units of `char`, so `sizeof (char)` must necessarily be 1, making that a redundant multiplication. Also, look into the `do {...} while (0)` idiom for multi-statement macros - that may well save your bacon when you put cleanup in the error-handling). — Toby Speight, Nov 29 '19 at 14:32

Ian Abbott · Answer 1 · 2019-11-29T10:33:59.903

4

In the macro definitions, you have added a space between the macro name and opening parenthesis:

#define SETUP (args_needed)\

That defines an ordinary macro rather than a macro that takes parameters. Therefore args_needed is not a macro parameter and is not defined.

Remove the space between the macro name and opening parenthesis:

#define SETUP(args_needed)\

It is OK to insert a space when invoking the macro:

SETUP (10);

but that is considered poor coding style by many.

edited Nov 29 '19 at 10:33

answered Nov 29 '19 at 10:29

Ian Abbott

15,083
19
33

thanks so much, that fixed it. I didn't know white space mattered haha – yogurt Nov 29 '19 at 10:32

score 0 · Answer 2 · answered Nov 29 '19 at 10:14

0

You didn't declare i anywhere. SETUP(i); just passes i for the macro parameter i, so the code that the macro expands to contains an undeclared i. You probably want something like SETUP(10); instead.

Also this here

(char*) memory

Isn't how you declare a variable. This is a typecast, and it fails because memory is undefined. Remove the parentheses.

answered Nov 29 '19 at 10:14

Blaze

16,736
2
25
44

sorry, I added my actual code. I was sloppy when I wrote an example – yogurt Nov 29 '19 at 10:19

Understanding macro functions in C that define variables

2 Answers2