Member Avatar for flageolet

Hi

I have a sequence that I want to put into a seperate function:
The variables I want to take into that function and back to the main function are declared globally.

declaring the function and the testing variable:

void sanction ();
int test[10];

my main function:

int main(int argc, char *argv[])
{
 
test[10] = 10;
printf ("in main: 10 = %d\n", test[10]);     
sanction ();    
printf ("in main after adding: 10 = %d\n", test[10]);  
 
return (0);
}

the separate function:

void sanction( void )
{
printf ("in sanction: 10 = %d\n", test[10]); 
test[10] = test[10] + 1;
}

output:
in main: 10 = 10
in sanction: 10 = 10
in main after adding: 10 = 11

So everything works like it should, but why?
I thought the void ment no variable-values are taken to the separate function and none are returned.
I just want to be sure that when I use this with the actual code, I'm not making any mistakes.

thx

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Dani AI

Generated

Short answer: the function can change test because test is a global. void in sanction(void) only controls the function signature (no parameters) or, when used as the return type, that the function returns no value. The behavior you saw when writing to test[10] is not reliable — indexing past the last element is undefined behavior and can appear to “work” on one run or crash on another. See the language notes on void and functions for the precise rules: and C language: functions.

Prefer explicit, bounds-checked code instead of relying on globals or out-of-range writes. One safe pattern is to pass the array and its length to the function and check the index before writing:

#include <stdio.h>
#include <stddef.h>

void increment_index(int *arr, size_t len, size_t idx)
{
    if (idx >= len) {
        fprintf(stderr, "index out of range\n");
        return;
    }
    arr[idx] += 1;
}

During development enable strict compiler checks and run sanitizers so you catch these errors early. Compile with -std=c11 -Wall -Wextra -pedantic and use AddressSanitizer (-fsanitize=address,undefined) or run under Valgrind to detect out-of-bounds writes and other undefined behavior: GCC Warning Options, AddressSanitizer docs, Valgrind quick start.

Related notes from the thread: correctly flagged the out-of-bounds index; was right that void can indicate “no arguments” (or “no return” when used as the return type); gave good advice about avoiding globals; and ’s point about testing is valid — just make the tests safe and use tools that will catch undefined behavior.

Recommended Answers

All 5 Replies

Member Avatar for Ancient Dragon

>>I thought the void ment no variable-values are taken to the separate function and none are returned.

It does mean that -- and the function is working as expected. But the void does NOT mean the function can't change global data.

>>test[10] = test[10] + 1;
array test only has 10 elements, numbered 0, 1, 2, 3, ... and 9. There is no 10th element number as you are attempting to use, so it is writing outside the bounds of the array.

Member Avatar for Lazaro Claiborn

Yes, to implicitly set the functions argument list to none, you use void. you use void to say there is no return type, or return therefore.

Good luck, LamaBot

Member Avatar for flageolet


>>test[10] = test[10] + 1;
array test only has 10 elements, numbered 0, 1, 2, 3, ... and 9. There is no 10th element number as you are attempting to use, so it is writing outside the bounds of the array.

Dammit, I actually know that. LOL

Thx guys

Member Avatar for ssharish2005

And more using global variables is not a good idea, because of the above reasons. The value of the variable can be changes outside the function.

Why use array only for using a single element in an array. You could have just declared a single int variable.

ssharish2005

Member Avatar for WaltP

And more using global variables is not a good idea, because of the above reasons. The value of the variable can be changes outside the function.

Why use array only for using a single element in an array. You could have just declared a single int variable.

ssharish2005

Maybe because he's testing... ;)

During development a good programmer always writes test code and uses the same structures he's going to use in the finished program to make sure the concepts work.

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