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Old 04-28-2023, 06:55 PM   #1
errigour
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c programming function variable ...


Hey can someone tell me how to put the function variable ... to use? honestly I want max function to be able to take any number of variables and tell me the maximum number. is that possible? and why do i have to put a variable before ...? it wouldn't let me just do long long int max(...). Would i have to use assembly for something like that and if so could someone show me a sample assembly program that would compile for 64 bit.

Code:
#include <stdio.h>

long long int max(int argc, ...)
{

}

int
main (int argc, char *argv[])
{
    max(27, 45, 30, 98, 52);
}

Last edited by errigour; 04-28-2023 at 07:03 PM.
 
Old 04-28-2023, 07:06 PM   #2
dugan
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This example looks pretty good to me:

https://www.tutorialspoint.com/c_sta...o_va_start.htm

And if you want something more boring:

https://www.gnu.org/software/libc/ma...Functions.html
 
Old 04-28-2023, 07:31 PM   #3
errigour
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what if i wanted to make my own va_start and stuff without using those functions is it possible?
 
Old 04-28-2023, 07:41 PM   #4
errigour
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like im trying to make a max function to use in c but i think i have to use assembly for it cause that gnu webpage says it cant be done with c. and I don't even want the first argument argc either. just want a function that will take any long long integers and tell me the max one.
 
Old 04-28-2023, 07:41 PM   #5
errigour
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Ive seen it done in other programs so i know it can be done.
 
Old 04-28-2023, 07:53 PM   #6
errigour
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nm im just gonna mark this as solved cause i got it to work with va list i was just trying to write my own library.
 
Old 04-28-2023, 10:10 PM   #7
ntubski
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Quote:
Originally Posted by errigour View Post
what if i wanted to make my own va_start and stuff without using those functions is it possible?
You can't write your own va_start in pure C, because it depends on the compiler's/platform's calling convention.
 
Old 04-28-2023, 11:34 PM   #8
NevemTeve
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In C, you cannot tell how many arguments were passed to the variadic function. On the other hand, in Assembly, you cannot tell how many arguments were passed to the variadic function.
 
Old 04-29-2023, 09:14 AM   #9
pan64
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https://stackoverflow.com/questions/...argc-char-argv
you can pass argc and argv directly to your max function, do not need to use va_list and friends if you don't want to.
But you need both of them (argv and argc), the pointer to the input list and the number of args. Otherwise you will not be able to handle them.
 
Old 04-29-2023, 10:14 AM   #10
teckk
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Quote:
I want max function to be able to take any number of variables and tell me the maximum number. is that possible
Ok, I'll play.

Code:
#include <stdio.h>
#include <limits.h>
#include <stdarg.h>

int max(int args, ...) {
    int i, max, cur;
    va_list valist;
    va_start(valist, args);    
    max = INT_MIN;
    
    for(i=0; i<args; i++) {
        cur = va_arg(valist, int);
        if(max < cur)
            max = cur;
    }
    
    va_end(valist);
    return max;
}

int min(int args, ...) {
    int i, min, cur;
    va_list valist;
    va_start(valist, args);    
    min = INT_MAX;
    
    for(i=0; i<args; i++) {
        cur = va_arg(valist, int);
        if(min > cur)
            min = cur;
    }
    
    va_end(valist);
    return min;
}
        
        
int main() 
{   
    printf("Maximum of three numbers: (10, 30, 20) = %d\n", max(3, 10, 30, 20));
    printf("Maximum of five numbers: (5, -45, 4, 60, 100) = %d\n", max(5, 5, -45, 4, 60, 100));
    
    printf("Minimum of three numbers: (5, 10, 20) = %d\n", min(3, 5, 10, 20));
    printf("Minimum of five numbers: (12, -5, 0, 10, 50) = %d\n", min(5, 12, -5, 0, 10, 50));

    printf("%s%s", "Etc", "\n");
    
    return 0;
}

//gcc mytest.c -o mytest
 
Old 04-29-2023, 12:24 PM   #11
Racho
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I came up with this:

Code:
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <limits.h>

long long int llin_max(int num_args, ...) {
	va_list valist;
	int i=0;
	long long int max, new_num;
	va_start(valist, num_args);
	max = LLONG_MIN;

	for (i=0; i < num_args; i++) {
		new_num = va_arg(valist, long long int);
		max = max > new_num ? max : new_num;
	}
	va_end(valist);

	return max;
}

int countparams(char *str) {
	int i=0, j=0, len = strlen(str);
	if (len == 0)
		return 0;
	for (i=0, j=1; i <  len; i++)
		if (str[i] == ',') j++;
	return j;
}

#define lli_max(...) llin_max(countparams(#__VA_ARGS__), __VA_ARGS__);

int main() {
	long long int num=0;
	long long a= 5000;
	long long b= 500;
	num = lli_max(1,2,a, b, 300,4,5);
	printf("The greatest number is: %lli\n", num);
	return 0;
}
I tried passing an int argument to lli_max and it worked as intended but i think that it worked just because it's a 64 byte processor.

I think that an int as argument for lli_max it should fail on 32 byte processors.
Am I right?
In that case, will it compile without errors or warnings?
 
Old 04-29-2023, 12:49 PM   #12
NevemTeve
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On a 64-bit platform, if you pass an int (32-bit) to a variadic function, and it fetches it as long long (64-bit), it will be a single stack entry, but the result might be different to what you expect: amd64 for example leaves the upper 32 bits uninitialized.

On a 32-bit platform (e.g. x86), va_arg(ap, long long) fetches two entries from the stack.

PS: try this: lli_max((1,2),(3,4),(5,6)) - it should be equivalent with lli_max(2,4,6)

Last edited by NevemTeve; 04-30-2023 at 12:57 AM.
 
2 members found this post helpful.
Old 04-29-2023, 02:04 PM   #13
Racho
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Quote:
Originally Posted by NevemTeve View Post
PS: try this: lli_max((1,2),(3,4),(5,6)) - it should be equivalent with lli_max(2,4,6)
Hmmm.... I see.. not a common way to use a function though.

This kind of thing can be fixed in the countparams function but it's annoying.

Anyway I was wondering what would happen if someone typed an int variable by mistake. It would be hard to trace that bug.

I didn't know that AMD processors would leave the upper 32 bytes uninitialized ...

Thanks for your reply!

Last edited by Racho; 04-29-2023 at 02:19 PM.
 
Old 04-29-2023, 03:56 PM   #14
Racho
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Yet another problem with this function:

The compiler defaults any numeric parameter to int if the function definition doesn't specify the type. So the user has to write long long int before every number, as shown in the code. (So annoying!)


Code:
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <limits.h>

long long int llin_max(int num_args, ...) {
	va_list valist;
	int i=0;
	long long int max, new_num;
	va_start(valist, num_args);
	max = LLONG_MIN;

	for (i=0; i < num_args; i++) {
		new_num = va_arg(valist, long long int);
		max = max > new_num ? max : new_num;
	}
	va_end(valist);

	return max;
}

int countparams(char *str) {
	int i=0, j=0, parentesis=0, len = strlen(str);
	if (len == 0)
		return 0;
	for (i=0, j=1; i <  len; i++) {
		if (str[i] == ',' && !parentesis) {
			j++;
			continue;
		} 
		if (str[i] == '(') {
			parentesis ++;
			continue;
		}
		if (str[i] == ')')
			parentesis --;

	}
	return j;
}

#define lli_max(...) llin_max(countparams(#__VA_ARGS__), __VA_ARGS__);

int main() {
	long long int num=0;
	long long a = 5000;
	long long b = 77500;
	long long c = -583;
	/* The definition doesn't tell the compiler the type of va_arg arguments
	 * so it defauts any number to int */

	num = lli_max(a, b, c);	// works as intended, every argument is lli
	printf("The greatest number is: %lli\n", num);

	num = lli_max(-80,17,18, 32);	// Doesn't work: arguments are int
	printf("The greatest number is: %lli\n", num);

	num = lli_max((long long int) -80,(long long int) 17,(long long int) 18,
		       	(long long int) 32);	// Now works

	printf("The greatest number is: %lli\n", num);
	return 0;
}
Is there any other way to mess the number of parameters?
By the way, I don't know the meaning of a parameter like (3,4).
 
Old 04-30-2023, 01:04 AM   #15
NevemTeve
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https://en.m.wikipedia.org/wiki/Comma_operator
Counting the commas it slow and not universal solution. How about using an array?
Code:
{
    long long arr[] = { values };
    size_t narr= (sizeof arr)/(sizeof arr[0]);
    maxval= maxfun (narr, arr);
}
 
  


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