Understanding variable parameters of C language

The prototype of printf () is int printf (const char *fmt, ...); The back of three. Represents a variable for the C language.

So what is a variation? what function does it have?

Arguments are parameters that are uncertain and can be arbitrarily changed as needed.

Let's start with a function that has a fixed argument list:

int swap (int a,int b)

{

int C;

C=a;

A=b;

B=c;

return A;

}

This is a random function, although because it is all local variables so it is not possible to exchange parameters, but the form of this function is very good.

Another example is a variable parameter list function:

void names (int a, ...)

{

......

}

Anyway, it's just a definition, and the contents are omitted. As you can see, the next parameter can be any parameter, but the local variable shows that the values of all parameters are copied to the contiguous memory in the stack at the time of invocation, so there must be an ordinary variable addressing the type and address of the subsequent mutable parameter.

That is, when you use the C-language variable parameter, you need to change the parameter, that is ... Place at the last parameter, and at least one normal parameter before the argument.

Here's an example:

#include <stdarg.h>

#include <stdio.h>

void func1 (const char * str1, ...)

{

Char *pp;

pp = ((char*) &str1) + sizeof (STR1);

printf ("%d\n", * (int*) pp);

pp = pp + sizeof (int);

printf ("%d\n", * (int*) pp);

pp = pp + sizeof (int);

printf ("%s\n", * ((char**) pp));

}

int main ()

{

Func1 ("%d%d%s\n", 4, 5, "Hello World");

return 0;

}

This function is actually wrong because there is a memory alignment problem, but it is sufficient to recognize and understand the mutable parameters.

The const char * str1 here actually points to a string to print out.

Now let's get to the point and discuss it in detail later.

Understanding variable parameters of C language