Advanced formula 45 of C language: the secret of function parameters (Part 2)

Parameter stack order

The calculation order of function parameters depends on the compiler , So how to determine the stacking order of function parameters ?

Calling convention

When a function call occurs
– Parameters are passed to the called function ;
– The return value is returned to the function caller ;
The calling convention describes how parameters are passed to the stack and how the stack is maintained
– Parameter passing order ;
– Call stack cleanup ;

  The calling convention is predefined and can be understood as the calling protocol
Call convention is usually used in library call and library development
- Stack from right to left : _stdcall , _cdecl, _thiscall
- Stack from left to right ︰ _pascal , _fastcall

#include <stdio.h>

float average(int array[], int size)
{
    int i = 0;
    float avr = 0;
    
    for(i=0; i<size; i++)
    {
        avr += array[i];
    }
    
    return avr / size;
}

int main()
{
    int array[] = {1, 2, 3, 4, 5};
    
    printf("%f\n", average(array, 5));
    
    return 0;
}

Variable parameters

C Functions with variable parameters can be defined in language
The implementation of variable parameter functions depends on stdarg.h The header file
- va_list- Parameter set
- va_arg- Take the specific parameter value
- va_start One identifies the beginning of parameter access
- va_end - Identifies the end of parameter access

#include <stdio.h>
#include <stdarg.h>

float average(int n, ...)
{
    va_list args;
    int i = 0;
    float sum = 0;
    
    va_start(args, n);
    
    for(i=0; i<n; i++)
    {
        sum += va_arg(args, int);
    }
    
    va_end(args);
    
    return sum / n;
}

int main()
{
    printf("%f\n", average(5, 1, 2, 3, 4, 5));
    printf("%f\n", average(4, 1, 2, 3, 4));
    
    return 0;
}

Limitation of variable parameters

Variable parameters must be accessed sequentially from beginning to end ;
At least one definite named parameter must exist in the parameter list ;
The variable parameter function cannot determine the number of parameters that actually exist ;
Variable argument function cannot determine the actual type of argument ;

Be careful :
va_arg If the wrong type is specified in , Then the result is unpredictable .

Summary

1、 The calling convention specifies the stacking order of function parameters and the cleaning method of the stack ;
2、 Variable parameters are C Language provides a function design skill ;
3、 Functions with variable parameters provide a more convenient way to call functions ;
4、 Variable parameters must be accessed sequentially , Cannot directly access the intermediate parameter value ;