Thread: Reallocation of memory (3d dynamic matrix)

Hello again!
Again problems with my project this time is the reallocation of a 3d dynamic matrix what is driving me mad... I'm trying to add a new column to the matrix and for this I'm trying to use the realloc routine in the cell pointer to pointer to pointer to char... the project is the same I explained here.
This is what I have so far:

Code:

/* add_column: add a column to table t */
void add_column(table *t){
    int i, j;

    // Increase columns count
    ++(t->cols);

    t->cell = (char ***) realloc(t->cell, t->rows * sizeof(char **));

    if(t->cell == NULL){
        printf("\n\t\tERROR: Out of memory\n");
        printf("\n\t\tPress any key to exit...\n");
        getchar();
        exit(EXIT_FAILURE);
    }

    for(i = 0; i < t->rows; ++i){
        t->cell[i] = (char **) realloc(t->cell[i], t->cols * sizeof(char *));
        if(t->cell[i] == NULL){
            printf("\n\t\tERROR: Out of memory\n");
            printf("\n\t\tPress any key to exit...\n");
            getchar();
            exit(EXIT_FAILURE);
        }
        for (j = 0; j < t->cols; j++){
            t->cell[i][j] = (char *)realloc(t->cell[i][j], NAME_MAX_LEN * sizeof(char));
            if(t->cell[i][j] == NULL){
                printf("\n\t\tERROR: Out of memory\n");
                printf("\n\t\tPress any key to exit...\n");
                getchar();
                exit(EXIT_FAILURE);
            }
        }
    }

    // Name & add new column
    printf("\nNew column's name:\n");
    printf("\nC%d: ", t->cols);
    fflush(stdin);
    gets(t->cell[0][t->cols]);

    for(i = 1; i < t->rows; ++i){
        printf("\nR%d C%d: ", i, t->cols);
        fflush(stdin);
        gets(t->cell[i][t->cols]);
    }
}

Remember I want to add a new column only, (I know the problems of using gets but I'll change that to fgets in a later stage...) the problem I'm having is sometimes the program crashes and I get a segmentation fault (again...)
So first I need to know if the reallocation of memory is done correctly? (Because I'm not used to reallocate 3dimensional arrays) Thank you very much in advance!

Sorry it was meant to be how it is now:

Code:

t->rows * sizeof(char  *)

and not

Code:

t->cols * sizeof(char *)

... I don't know why I changed that neither lol...

Originally Posted by tabstop

What you need to do is realloc each individual pointer to be one larger (since the rows are malloc'ed in a loop using (cols * sizeof pointer), and that will change).

But that's what this does doesn't it?:

Code:

for(i = 0; i < t->rows; ++i){
        t->cell[i] = (char **) realloc(t->cell[i], t->cols * sizeof(char *));
        if(t->cell[i] == NULL){
            printf("\n\t\tERROR: Out of memory\n");
            printf("\n\t\tPress any key to exit...\n");
            getchar();
            exit(EXIT_FAILURE);
        }
        for (j = 0; j < t->cols; j++){
            t->cell[i][j] = (char *)realloc(t->cell[i][j], NAME_MAX_LEN * sizeof(char));
            if(t->cell[i][j] == NULL){
                printf("\n\t\tERROR: Out of memory\n");
                printf("\n\t\tPress any key to exit...\n");
                getchar();
                exit(EXIT_FAILURE);
            }
        }
    }

I first incremented the columns count so it should be reallocating the memory for each row to one column more...
Should I remove the second loop and just reallocate the memory for the last column string?

You may want to take a look at C-FAQ 6.16 it goes through the explanation of dynamically allocating arrays, including, higher order ones very well.

Additionally, read here on why fflush(stdin) is wrong.

On a side note, you may want to take a step back and think about your program design. Under C99 you can dynamically allocate higher order arrays, however that doesn't mean you should. We don't get to many 3 star programmers around here.........

Ok my new code is this:

Code:

void add_column(table *t){
    int i;

    // Increase columns count
    ++(t->cols);

    for(i = 0; i < t->rows; ++i){
        t->cell[i] = (char **) realloc(t->cell[i], t->cols * sizeof(char *));
        if(t->cell[i] == NULL){
            printf("\n\t\tERROR: Out of memory\n");
            printf("\n\t\tPress any key to exit...\n");
            getchar();
            exit(EXIT_FAILURE);
        }
        t->cell[i][t->cols - 1] = (char *) malloc(NAME_MAX_LEN * sizeof(char));
        if(t->cell[i][t->cols - 1] == NULL){
            printf("\n\t\tERROR: Out of memory\n");
            printf("\n\t\tPress any key to exit...\n");
            getchar();
            exit(EXIT_FAILURE);
        }
    }

    // Name & add new column
    printf("\nNew column's name:\n");
    printf("\nC%d: ", t->cols - 1);
    fflush(stdin);
    gets(t->cell[0][t->cols - 1]);

    for(i = 1; i < t->rows; ++i){
        printf("\nR%d C%d: ", i, t->cols - 1);
        fflush(stdin);
        gets(t->cell[i][t->cols - 1]);
    }
}

It seems to work now! Will keep testing!

Yeah sorry I just changed that before you posted! lol
And now it works as expected sooo problem solved and thank you very much!! I will have to change the gets and the fflush thing now...
P.S. @tabstop do you think the design for this small project is alright for the purpose and knowing the reasoning I gave?