Cannot resolve the errors with returning an array using pointer from a function in C?

Question

I am trying to receive a number of float variables through a function and then pass them to the main and store them in another array in the main. The size of the array is specified by the user, so I am using a variable-length array. I have already searched and read similar questions about returning arrays from functions, but couldn't find a solution for the errors in my code.

For your convenience, I have summarized the original code and included only the lines related to this problem.

float *receiveFloatValues(int numOfValues);

int main(void)
{
    int numberOfValues;
    float *receivedValues;

    printf("\nHow many values? ");
    scanf("%d", &numberOfValues);

    receivedValues = receiveFloatValues(numberOfValues);

    float valuesArray[numberOfValues];

    for (int counter = 0; counter < numberOfValues; counter++)
    {
        receivedValues += counter;
        valuesArray[counter] = *receivedValues;
    }

    for (int counter = 0; counter < numberOfValues; counter++)
    {
        printf("\nvaluesArray[%d]: %.2f", counter, valuesArray[counter]);
    }
    return(0);
}

float *receiveFloatValues(int numOfValues)
{
    static float values[numOfValues];
    for (int counter = 0; counter < numOfValues; counter++)
    {
        printf("\nEnter value %d: ", counter + 1);
        scanf("%.2f", &values[counter]);
    }
    return(values);
}

My expected result is to display the list of float values received from user, but I get these error messages:

C2057: expected constant expression

C2133: unknown size

C2466: Cannot allocate an array of constant size 0

[C11 Standard - 5.1.2 Execution environments](http://port70.net/~nsz/c/c11/n1570.html#5.1.2) *"All objects with static storage duration shall be initialized (set to their initial values) before program startup."* (that cannot be satisfied with a VLA) See [C11 Standard - 6.2.4 Storage durations of objects(p3)](http://port70.net/~nsz/c/c11/n1570.html#6.2.4p3) — David C. Rankin, Aug 06 '19 at 03:31
@CoffeeTableEspresso I thought I should use `static` because otherwise I would be passing the address of a local variable to the main which is not usually considered proper. — Omid, Aug 06 '19 at 03:32
Omid my answer should work for you, as @DavidC.Rankin mentioned, you can't have a `static` VLA. — CoffeeTableEspresso, Aug 06 '19 at 03:34
Additionally [C11 Standard - 6.7.6.2 Array declarators(p2)](http://port70.net/~nsz/c/c11/n1570.html#6.7.6.2p2) *"If an identifier is declared to be an object with static or thread storage duration, it shall not have a variable length array type."* — David C. Rankin, Aug 06 '19 at 03:38
Passing the address of a local variable is *never* proper, not just usually — Antti Haapala -- Слава Україні, Aug 06 '19 at 03:45
@Omid if you really want to do this with a VLA, you could create the array in main, and pass it in to the function. Of course, based on your comments, I'm not sure how well your compiler even supports VLAs — CoffeeTableEspresso, Aug 06 '19 at 03:54

CoffeeTableEspresso · Accepted Answer · 2019-08-06T04:37:57.203

1

You can't have a static VLA, just use malloc and free, as shown here:

#include <stdio.h>
#include <stdlib.h>

float *receiveFloatValues(int numOfValues);

int main(void)
{
    int numberOfValues;
    float *receivedValues;

    printf("\nHow many values? ");
    scanf("%d", &numberOfValues);

    receivedValues = receiveFloatValues(numberOfValues);

    float *valuesArray = malloc(sizeof(float) * numberOfValues);

    for (int counter = 0; counter < numberOfValues; counter++)
    {
        valuesArray[counter] = receivedValues[counter];
    }

    for (int counter = 0; counter < numberOfValues; counter++)
    {
        printf("\nvaluesArray[%d]: %f", counter, valuesArray[counter]);
    }
    free(receivedValues);    // free memory here.
    free(valuesArray);
    return(0);
}

float *receiveFloatValues(int numOfValues)
{
    float *values = malloc(sizeof(float) * numOfValues);   // allocate memory here.
    for (int counter = 0; counter < numOfValues; counter++)
    {
        printf("\nEnter value %d: ", counter + 1);
        scanf("%f", &values[counter]);
    }
    return(values);
}

edited Aug 06 '19 at 04:37

answered Aug 06 '19 at 03:30

CoffeeTableEspresso

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This code still does not compile and gives these errors: C2057: expected constant expressio, C2133: 'valuesArray' : unknown size, C2466: cannot allocate an array of constant size 0 – Omid Aug 06 '19 at 03:38
I literally just compiled it on my machine – CoffeeTableEspresso Aug 06 '19 at 03:43
@Omid You have a substandard C compiler (MSVC) which does not support variable-length arrays. You need to use malloc in main too – Antti Haapala -- Слава Україні Aug 06 '19 at 03:43
@AnttiHaapala Thanks but I have no idea how to use `malloc` yet. Need to learn first and it seems very complicated to me. – Omid Aug 06 '19 at 03:49
@Omid you can't really do this without malloc especially if your compiler doesn't support VLAs properly. – CoffeeTableEspresso Aug 06 '19 at 03:52
Thanks for the new edit to the code. This code gives no compilation errors; however, as soon as I enter the first value and press ENTER, the code breaks! – Omid Aug 06 '19 at 04:18
yes, it's your format strings I'm guessing. – CoffeeTableEspresso Aug 06 '19 at 04:26
I used debugger to run the code step by step to find where the problem lies. This code receives only "Value 1" and does not receive any other value for the array. Then the code breaks at the first `free()` line. – Omid Aug 06 '19 at 04:32
that issue is totally unrelated to this question. – CoffeeTableEspresso Aug 06 '19 at 04:33
@Omid This edit fixes it though – CoffeeTableEspresso Aug 06 '19 at 04:37
Now it receives all the numbers but before printing them it breaks! – Omid Aug 06 '19 at 04:46
@Omid it works perfectly fine with my machine/compiler. And "it breaks" is not a useful error message. You should probably open a new question if you're still running into issues, this answer covers what was asked in your question – CoffeeTableEspresso Aug 06 '19 at 16:30

Cannot resolve the errors with returning an array using pointer from a function in C?

1 Answers1