C- Interaction between pointers, arrays, and structs

Question

I've been reading up on pointers and arrays in C in an effort to learn how to implement something VLA-ish as a member of a struct. (Specifically, I need an array or array-like object with length different between instances of the struct but defined at compile-time for any particular struct instance.)

It seems to me as though something like this:

typedef struct example{
    unsigned char length;
    char *data;
}example;

int buildExample(example e, unsigned char l, char * const d){
    e.length = l;
    e.data = d;
    return 0;
    //not safe I know, but that fact isn't relevant to the question.
}


main(){
    example e;
    unsigend char a = 5;
    char b[] = {1, 2, 3, 4, 5};
    buildExample(e, a, b);
    int i = 0;
    while(i < e.length){
        printf("%d  %d\n", i, e.b[i]);
        i++;
    }
    printf("%d  %d\n", i, e.b[i]);
}

should result in something like this output:

0  1
1  2
2  3
3  4
4  5
5  some unknown value or a segfault, not sure which

and the various pointers and memory cells and such should go something like this:

before call to buildExample:

example e.data                 b               address 0, 1, 2...
|null pointer|         |ptr to address 0|      |  1  |  2  |  3  |  4  |  5  |

after call to buildExample:

example e.data                  address 0, 1, 2...
|ptr to address 0|              |  1  |  2  |  3  |  4  |  5  |

But instead I just get a segfault. If I swap out e.data for b, I get the 'unknown value' outcome (32765, 32766 or 32767, for whatever reason), and if I swap out e.data for a new int* c, defined and set equal to b in main, I get the same result as b, which implies to me that e.data is not in fact being set to b by buildExample.

Why not?

In addition to cdont's answer, this will still be run-time, not compile time as you say you want to achieve. — kdopen
– kdopen, Commented Feb 6, 2016 at 2:04
Thank you for the information! My comment on compile time was to indicate that the number was available at compile time, more than that I wanted it to be set at compile time, and so compile time solutions would work for me. — P...
– P..., Commented Feb 6, 2016 at 4:10

cdonts · Accepted Answer · 2016-02-06 02:02:21Z

2

When calling buildExample() you're actually passing a copy of the e struct created in main(), so any changes will be lost when the function returns. Use a pointer instead.

int buildExample(example *e, unsigned char l, char * const d){
    e->length = l;
    e->data = d;
    return 0;
    //not safe I know, but that fact isn't relevant to the question.
}

And when calling:

buildExample(&e, a, b);

However you have other errors such as unsigend char a = 5; (should be unsigned not "unsigend") and trying to access an element named b (should be data).

buildExample(&e, a, b);
while(i < e.length){
    printf("%d  %d\n", i, e.data[i]);
    i++;
}
printf("%d  %d\n", i, e.data[i]);

Hope it helps!

answered Feb 6, 2016 at 2:02

cdonts

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1 Comment

P... Over a year ago

Thank you! That was a very important piece of information! The errors weren't actually in my code, I put them in by accident when copying over and simplifying it, so that's my error.

user3629249 · Accepted Answer · 2016-02-06 17:42:32Z

This posted code presents a few problems.

1) the return type from main() is always int not a blank

2) the function: buildExample() modifies the e parameter (on the stack) but does not have any effect on the e variable back in the caller Should pass a pointer to the struct and change the assignments to e->length and e->data

3) use unsigned not unsigend`

4) the use of meaningless variable and parameter names makes the code unnecessarily difficult to understand

5) in the calls to printf() the example struct has no member b

Strongly suggest actually compiling the posted code before posting a question.

typedef struct example
{
    unsigned char dataCount;
    char *pData;
} example;

void buildExample(example *pExample, unsigned char dataCount, char * const pData)
{
    pExample->dataCount = dataCount;
    pExample->pData = pData;
}


int main( void )
{
    example myExample;
    unsigned char dataCount = 5;
    char data[] = {1, 2, 3, 4, 5};

    buildExample(&myExample, dataCount, data);

    for( int i=0; i<myExample.count; i++)
    {
        printf("%d  %d\n", i, myExample.pData[i]);
    }
    printf("%d  %d\n", i, myExample.pData[i]);
}

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