CODEWITHSUNDEEP
cstringfunctionpointers
mrquiksilver
mrquiksilver

Reputation: 9

Returning a pointer of string arrays from a function

So i need help returning a pointer of string arrays from a function obtained from a file. The strings being no larger than 10.

Input file:

3
102
A3B
50

The first number being how many strings I need, the following numbers being what I need to store in the strings.

The function:

   char ** buildArray(){

    int x, i;

    fscanf(fp, "%d", &x);

    char answer[x][10];

    for(i=0; i<x; i++){

        fscanf(fp, "%s", answer[i]);

    }

    return answer;
}

I can get the values to be stored on the string array 'answer' properly, i just cant return it properly.

Main Function:

    int main() {

    char * answers;

    fp = fopen("data.txt", "r");

    if(fp == NULL){
        printf("Could not find file.\n");
        exit(EXIT_FAILURE);
    }

    answers = buildAnsArray();

    printf("%s", answer[1]); //used as a test to see if i passed the array of strings correctly

    fclose(fp);
    return 0;
}

in the main function when i try and print a value it just ends up crashing, or printing weird symbols

Upvotes: 0

Views: 80

Answers (3)

Degustaf
Degustaf

Reputation: 2670

As @Scott Hunter pointed out, when you exit the function buildArray, answer goes out of scope, and the memory it occupied is free to be used by another variable. If you want to allocate the memory within buildArray, then you will need to use malloc. A starting point would be:

char *answer;
answer = malloc(x * 10 * sizeof(*answer));

But, using malloc does require you to pay attention to memory and manage it appropriately. This means:

1) Checking that malloc was successful in allocating memory. If it fails it returns Null, so you can add

if(answer == NULL){
    //Error Code here
}

2) Freeing the memory when you are done. This should also be done safely. I would suggest the following just before fclose

if(answer != NULL)
{
    free(answer);
}

3) Performing your own handling the double indexing yourself. This means that answer[i] becomes either answer + i*10 or &(answer[i*10])

Also, not related to your question, but important to notice:

1) You have a type mismatch between your definition char *answer in main and returning a char** from buildArray. Turning on -Wall and -Wextra on your compiler should warn you about these types of things. And you should try to clean up all of the warnings that are generated. They tend to mean that you either made a subtle mistake, or are using a bad coding practice that you should get out of the habit of, before it creates a major debugging headache.

2) You appear to be using fp as a global variable. You should be trying to avoid global variables as much as possible. There may be times when they are necessary, but you should think about that long and hard before comitting to it.

3) You (correctly) checked that fopen was successful. But, you didn't check that fscanf was successful. You should get in this habit as a failed read from file won't automatically generate a runtime error. You will just get strange results when it uses whatever bits were already in memory for your logic later on.

Upvotes: 1

BLUEPIXY
BLUEPIXY

Reputation: 40145

sample code

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

#define STRING_SIZE 10
#define S_(x) #x
#define S(x) S_(x)


char (*buildAnsArray(FILE *fp))[STRING_SIZE+1]{
    int x, i;
    char (*answer)[STRING_SIZE+1];

    fscanf(fp, "%d", &x);
    answer = malloc(x * sizeof(*answer));

    for(i=0; i<x; i++){
        fscanf(fp, "%" S(STRING_SIZE) "s", answer[i]);//fscanf(fp, "%10s", answer[i]);
    }

    return answer;
}

int main(void) {
    char (*answers)[STRING_SIZE+1];

    FILE *fp = fopen("data.txt", "r");//error proc omit

    answers = buildAnsArray(fp);

    printf("%s\n", answers[1]);//A3B

    fclose(fp);
    free(answers);
    return 0;
}

Upvotes: 0

Scott Hunter
Scott Hunter

Reputation: 49803

Assuming this is some flavor of C, you are trying to return a pointer to a variable, answer, which was automatically allocated inside a function, which means it gets automatically deallocated when you exit that function. So either create answer outside of this function and pass it in to be filled, or allocate the space for it explicitly so it can live once the function returns.

Upvotes: 1

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