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

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 "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) — David C. Rankin
– David C. Rankin, Commented Aug 6, 2019 at 3:31
@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
– Omid, Commented Aug 6, 2019 at 3:32
Omid my answer should work for you, as @DavidC.Rankin mentioned, you can't have a static VLA. — CoffeeTableEspresso
– CoffeeTableEspresso, Commented Aug 6, 2019 at 3:34

CoffeeTableEspresso · Accepted Answer · 2019-08-06 04:37:57Z

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 6, 2019 at 4:37

answered Aug 6, 2019 at 3:30

CoffeeTableEspresso

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12 Comments

Omid Over a year ago

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

CoffeeTableEspresso Over a year ago

I literally just compiled it on my machine

Antti Haapala Over a year ago

@Omid You have a substandard C compiler (MSVC) which does not support variable-length arrays. You need to use malloc in main too

Omid Over a year ago

@AnttiHaapala Thanks but I have no idea how to use malloc yet. Need to learn first and it seems very complicated to me.

CoffeeTableEspresso Over a year ago

@Omid you can't really do this without malloc especially if your compiler doesn't support VLAs properly.

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