Advanced formula 46 of C language: function and macro analysis

Functions and macros

  Macros are directly replaced and expanded by preprocessors , The compiler does not know the existence of macros ;
Functions are entities compiled directly by the compiler , The calling behavior is determined by the compiler ;
Using macros multiple times will increase the size of the final executable program ;
The function is executed by jumping , Only one function body exists in memory ;
Macros are more efficient than functions , Because it's a direct expansion , No call overhead ;
An activity record is created when the function is called , Not as efficient as macro ;

Functions and macros , Examples are as follows

#include <stdio.h>

#define RESET(p, len)          \
    while( len > 0 )           \
        ((char*)p)[--len] = 0

void reset(void* p, int len)
{
    while( len > 0 ) 
        ((char*)p)[--len] = 0;
}

int main()
{
    int array[] = {1, 2, 3, 4, 5};
    int len = sizeof(array);
    int i = 0;
    
    for(i=0; i<5; i++)
    {
        printf("array[%d] = %d\n", i, array[i]);
    }
    
    return 0;
}

Macros are slightly more efficient than functions , But its side effects are huge ;
Macros are text substitutions , Parameter type cannot be checked ;
Functions that can be completed with functions never need macros ;
Recursive definitions cannot appear in macro definitions ;

Side effects of macros , Examples are as follows

#include <stdio.h>

#define _ADD_(a, b) a + b
#define _MUL_(a, b) a * b
#define _MIN_(a, b) ((a) < (b) ? (a) : (b))

int main()
{
    int i = 1;
    int j = 10;
    
    printf("%d\n", _MUL_(_ADD_(1, 2), _ADD_(3, 4)));
    printf("%d\n", _MIN_(i++, j));
    
    return 0;
}

The magic of macro

Used to generate some regular code ;
Packaging function , Add type information ;

The magic of macro , Examples are as follows

#include <stdio.h>
#include <malloc.h>

#define MALLOC(type, x)   (type*)malloc(sizeof(type)*x)
#define FREE(p)           (free(p), p=NULL)

#define LOG_INT(i)        printf("%s = %d\n", #i, i)
#define LOG_CHAR(c)       printf("%s = %c\n", #c, c)
#define LOG_FLOAT(f)      printf("%s = %f\n", #f, f)
#define LOG_POINTER(p)    printf("%s = %p\n", #p, p)
#define LOG_STRING(s)     printf("%s = %s\n", #s, s)

#define FOREACH(i, n)     while(1) { int i = 0, l = n; for(i=0; i < l; i++)
#define BEGIN             {
#define END               } break; } 

int main()
{
    int* pi = MALLOC(int, 5);
    char* str = "D.T.Software";
    
    LOG_STRING(str);
    
    LOG_POINTER(pi);
    
    FOREACH(k, 5)
    BEGIN
        pi[k] = k + 1;
    END
    
    FOREACH(n, 5)
    BEGIN
        int value = pi[n];
        LOG_INT(value);
    END
    
    FREE(pi);
    
    LOG_POINTER(pi);
    
    return 0;
}

Summary

Macros and functions are not competitors ;
Macros can accept any type of parameter , Efficient , Error prone ;
The arguments to the function must be of fixed type , Slightly inefficient , It's not easy to make mistakes. ;
Macros can perform functions that functions cannot ;