Thread: variable length array

this code snippet compiles and runs fine

Code:

#include <stdio.h>

int main(void) {
        int i, a[i];


        i=2;
        for(int n=0; n<=i; n++)
                a[n]=0;
        for(int n=0; n<=i; n++)
                printf("%i ", a[n]);
        printf("\n");


        return 0;
}

it prints out 3 "0", but when slightly changed

Code:

#include <stdio.h>

int main(void) {
        int i, a[i];


        i=200;
        for(int n=0; n<=i; n++)
                a[n]=0;
        for(int n=0; n<=i; n++)
                printf("%i ", a[n]);
        printf("\n");


        return 0;
}

it compiles fine but core dumps when run, how come? i'm using gcc version 4.4.7, and compiling like this

gcc -std=c99 test.c -o test

> int i, a[i];
The value in i is garbage when it comes to declaring the array.
If the value is negative (50% chance), or larger than your stack (99% of all positive numbers), you're screwed.

> i=200;
> for(int n=0; n<=i; n++)
Even if this were to magically create an array of 200 elements, your for loop is still a buffer overrun.

Originally Posted by brassbin

i'm using gcc version 4.4.7, and compiling like this

gcc -std=c99 test.c -o test

Compile like this instead (warnings are your friend):

Code:

gcc -std=gnu99 -Wall -Wextra -o test test.c

I use gnu99 with GCC because without it it sometimes gives me some weird problems. Just avoid using the GNU extensions if you want your code to be portable.

I think that Salem doesn't have to elaborate.
Take this code

Code:

int main(void)
{
     int a[5];

     ...

     return 0;
}

As one can see, the first line of main(), creates an array of type int, with a size equal to five.

Now, consider this code.

Code:

int main(void)
{
     int i, a[i];

     ...

     return 0;
}

As one can see, the first line of main(), declares a variable named "i", of type int and creates an array of type int, with a size equal to i.
..... Didn't I say enough?
Print the value of i? Do you know what this value is from before? I think not! It is undefined behavior!
"i" variable has a garbage value, since it is unintialized!
So, you create an array of ints, with size equal to a garbage value.

Originally Posted by tabstop

what kind of C++ compiler is recommending malloc instead of new?

One that shares a code base with a C compiler

Originally Posted by MutantJohn

C++ itself is unsafe (more 'unsafe' than C)

o_O
Never heard this argument before. They both seem equally unsafe to me, but then that's mainly because C++ is somewhat of a superset of C.

Originally Posted by MutantJohn

C++ has superior options than variable length arrays, namely std::vector

Well, "superior" depends on what you need. If you want to have the array size fixed at runtime, then std::vector won't do unless you also declare it const, in which case you cannot modify the elements. (Then again, you can always write a container that has the given constraints, with the implementation simplified thanks to std::vector.) As far as I know, VLAs in C could be implemented to use the stack (though it isn't guaranteed), whereas std::vector in C++ uses the free store by default.

Memory Model - Handbook | mbed