CODEWITHSUNDEEP
cpointersmultidimensional-arraymallocfunction-call
tiamat
tiamat

Reputation: 13

2-Dimensional array with malloc, function call to allocate

I'm working on a project and I'm kinda stuck at a problem. I'm trying to read a file that contains a number in the first line that gives the number of rows, and then following the matrix of integers, separated by spaces.

I want to make a pointer in the main, then call the function with the pointer as parameter, the function should read the first number in the txt file, then create a 2d array with malloc, then read the matrix in the textfile and return. but either i get it so the function can allocate and read the matrix, but then i have something wrong with the pointer when calling the function so i cant use the allocated and read stuff in the main, or im getting errors when trying to call by reference and allocate stuff in the function then.

void readjobs(FILE* fp, int ***array, int linesToRead, int facilityCount) {
    int ch = 0;
    int rows = 0;
    while ((ch = fgetc(fp)) != '\n')
    {
        rows = ch - 48;
        //rows = rows * 10 + (ch - 48);
    }

    if (rows > linesToRead) rows = linesToRead;

    *array = (int**)malloc(rows * sizeof(int*));
    for (int i = 0; i < rows; i++) {
        /* size_y is the height */
        *array[i] = (int*)malloc(facilityCount * sizeof(int));
    }
    int i = 0, j = 0;
    while ((ch = fgetc(fp)) != EOF)
    {
        if (ch == '\n')
        {
            i++;
            printf("\n");
        }
        else if (ch == ' ')
        {
            j++;
            printf("%i ", *array[i][j]);
        }
        else    //wenn es ne nummer ist
        {
            *array[i][j] = (*array[i][j] * 10) + (ch - 48);
        }
    }
}

int main(int argc, char** argv) {

    int facilities_count = -1;
    int jobs_count = -1;
    int **jobs = NULL;
    FILE *fp;   //Zeiger für Datei
    fp = fopen("jobs.txt", "r");    //Dateizugriff, Datei als read 

    if (fp == NULL) {   //falls die Datei nicht geoeffnet werden kann
        printf("Datei konnte nicht geoeffnet werden!!\n");
    }
    else {  //Datei konnte geoeffnet werden
        printf("Datei ist lesbar\n");

        readjobs(fp, &jobs, 6, 6);

        if (jobs == NULL)printf("nullpionter");
        else {
            for (int i = 0; i < 6; i++) {
                printf("\n");
                for (int j = 0; j < 6; j++) {
                    printf("%x ", jobs[i][j]);
                }
            }
        }
        fclose(fp); //Dateizugriff wieder freigeben
    }

    MPI_Finalize();
    getchar();
    return 0;
}

Textfile example: 

6
3 2 2 1 5 4
1 1 3 4 2 0
1 2 3 4 5 1
3 4 2 0 1 5
1 0 5 2 3 4
4 0 1 3 5 2

the first number "6" in this case is how many lines, and the rest is the matrix to be read

Upvotes: 1

Views: 252

Answers (1)

klutt
klutt

Reputation: 31389

Your main problem

I compiled your code with debugging on:

$ cc -g mat.c -Wall -Wextra

Ran it in debugger:

$ gdb a.out
(gdb) run
Starting program: /tmp/a.out 
Datei ist lesbar

Program received signal SIGSEGV, Segmentation fault.
0x00005555555548ff in readjobs (fp=0x555555756010, array=0x7fffffffe740, linesToRead=6, facilityCount=6)
    at mat.c:18
18          *array[i] = (int*)malloc(facilityCount * sizeof(int));

Ok, so it crashes at this line: 

*array[i] = (int*)malloc(facilityCount * sizeof(int));

Doing so is always a good idea to find out what the problem is. What could be the cause? Unfortunately it is not trivial in this case. Unless you really understand pointers. *array[i] is not what you want. You want (*array)[i].

Remember that x[i] is just a shortcut for *(x+i). Furthermore, [] has higher priority than *. So *x[i] = *(x[i]) = *(*(x+i)), but what you want is (*x)[i] = *((*x) + i) which clearly is not the same thing.

Other stuff

I would definitely extract the creation of the matrix, like this:

int ** createMatrix(int rows, int columns) {
    printf("%d %d\n", rows, columns);
    int **ret;
    ret = malloc(rows * sizeof *ret);
    if(!ret) { perror("Error: "); exit(EXIT_FAILURE); }
    for(int i=0; i<columns; i++) {
        ret[i] = malloc(columns * sizeof (*ret)[0]);
        if(!ret[i]) { perror("Error: "); exit(EXIT_FAILURE); }
    }
    return ret;
}

and then in your code:

*array = createMatrix(rows, facilityCount);
// Don't trust my code. Check the pointer.
if(!array) { perror("Error: "); exit(EXIT_FAILURE); }

Remember to ALWAYS check if malloc succeeded.

And your way of reading the numbers is very weird. If you are a beginner who came up with this method on your own, that's actually pretty impressive, but it's not a good method. Read about fscanf and getline.

Upvotes: 2

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