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Macros vs Functions

Macros are pre-processed which means that all the macros would be processed before your program compiles. However, functions are not preprocessed but compiled.

See the following example of Macro:

#include<stdio.h>
#define NUMBER 10
int main()
{
     printf("%d", NUMBER);
     return 0;
}

Output:



10

See the following example of Function:

#include<stdio.h>
int number()
{
    return 10;
}
int main()
{
    printf("%d", number());
    return 0;
}

Output:

10

Now compile them using the command:

gcc –E file_name.c

This will give you the executable code as shown in the figure:



b12

This shows that the macros are preprocessed while functions are not.


In macros, no type checking(incompatible operand, etc.) is done and thus use of micros can lead to errors/side-effects in some cases. However, this is not the case with functions. Also, macros do not check for compilation error (if any). Consider the following two codes:

Macros:

#include<stdio.h>
#define CUBE(b) b*b*b
int main()
{
     printf("%d", CUBE(1+2));
     return 0;
}

Output: Unexpected output

7

Functions:

#include<stdio.h>
int cube(int a)
{
     return a*a*a;
}
int main()
{
    printf("%d", cube(1+2));
    return 0;
}

Output: As expected



27
  • Macros are usually one liner. However, they can consist of more than one line, Click here to see the usage. There are no such constraints in functions.
  • The speed at which macros and functions differs. Macros are typically faster than functions as they don’t involve actual function call overhead.

Conclusion:
Macros are no longer recommended as they cause following issues. There is a better way in modern compilers that is inline functions and const variable. Below are disadvantages of macros:

a) There is no type checking

b) Difficult to debug as they cause simple replacement.

c) Macro don’t have namespace, so a macro in one section of code can affect other section.

d) Macros can cause side effects as shown in above CUBE() example.

Macro Function
Macro is Preprocessed Function is Compiled
No Type Checking is done in Macro Type Checking is Done in Function
Using Macro increases the code length Using Function keeps the code length unaffected
Use of macro can lead to side effect at later stages Functions do not lead to any side effect in any case
Speed of Execution using Macro is Faster Speed of Execution using Function is Slower
Before Compilation, macro name is replaced by macro value During function call, transfer of control takes place
Macros are useful when small code is repeated many times Functions are useful when large code is to be written
Macro does not check any Compile-Time Errors Function checks Compile-Time Errors

See following for more details on macros:
Interesting facts about Macros and Preprocessors

 

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



This article is attributed to GeeksforGeeks.org

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