exc_bad_access in C program

Question

Okay, I know there is another question about "exc_bad_access" here, but that seems to deal with Objective-C and iPhone dev., while mine is only regular C. I am new to C and am almost done with my first program until this error showed up. I have been trying to figure it out for a couple of days and am going insane. Any help is appreciated. buggy function:

void edit (int i){
    char* z;
    char* y;
    char compare1[] = "on bobbin\b\b\b\b";
    char compare2[] = "not on bobbin";
    char compare3[] = "have\b\b\b\b\b\b";
    char compare4[] = "don't have";
    char wrapedit[] = "wrapped";
    char haveedit[] = "have";
    char editing[9];

    FILE *wrappedlist = fopen("../../wrapped", "r+");
    FILE *havelist = fopen("../../havelist", "r+");

    fseek(wrappedlist, i*14, SEEK_SET);
    fseek(havelist, i*11, SEEK_SET);

    printf("Edit? (y=yes, n=no)");
    fgets(z, 2, stdin);

    if ((*z=='y') && (strncmp(haveslist[i], compare4, (size_t)1) == 0)) {
        printf("Switch \"don't have\" to \"have\"? (y=yes, n=no)");
        fgets(y, 2, stdin);

        if (*y=='y') {
            fputs(compare3, havelist);
            fclose(wrappedlist);
            fclose(havelist);
            return;
        }
        else if(*y=='n'){
            fclose(wrappedlist);
            fclose(havelist);
            return;
        }
        printf("Invalid input.");
        return;
    }

    else if ((*z=='y') && (strncmp(haveslist[i], compare3, (size_t)1) == 0)) {
        fpurge(stdout);
        printf("Edit \"wrapped\" or \"have\"?");
        fpurge(stdin);
        fgets(editing, 9, stdin);

        len = strlen(editing);
        editing[len-1]='\0';

        if (strcmp(editing, wrapedit)==0) {

        if (strncmp(wrapped[i], compare1, (size_t)1)==0) {

            printf("Switch \"on bobbin\" to \"not on bobbin\"? (y=yes, n=no)");
            fgets(y, 2, stdin);
            if (*y=='y') {

                fputs(compare2, wrappedlist);
                fclose(wrappedlist);
                fclose(havelist);
                return;
            }
            else if(*y=='n'){
                fclose(wrappedlist);
                fclose(havelist);
                return;
            }
        }
            else if(strncmp(wrapped[i], compare2, (size_t)1)==0){

                fpurge(stdout);
                printf("Switch \"not on bobbin\" to \"on bobbin\"? (y=yes, n=no)");
                fgets(y, 2, stdin);
                if (*y=='y') {

                    fwrite(compare1, (size_t)strlen(compare1), 1, wrappedlist);
                    fclose(wrappedlist);
                    fclose(havelist);
                    return;
                }
                else if(*y=='n'){
                    fclose(wrappedlist);
                    fclose(havelist);
                    return;
        }
                fpurge(stdout);
                printf("Invalid input.");
        }
            fpurge(stdout);
            printf("You don't want to edit wrapped apparently.");
            fclose(wrappedlist);
            fclose(havelist);
            return;
        }
        else if(strcmp(editing, haveedit)==0){

            if (strncmp(haveslist[i], compare3, 1) == 0){

                printf("Switch \"have\" to \"don't have\"? (y=yes, n=no)");
                fgets(y, 2, stdin);
                if (*y=='y') {
                    fputs(compare4, havelist);
                    fclose(wrappedlist);
                    fclose(havelist);
                    return;
                }
                else if(*y=='n'){
                    fclose(wrappedlist);
                    fclose(havelist);
                    return;
                }
                printf("Invalid input.");
            }
            else if(strncmp(haveslist[i], compare4, 1)==0){

                printf("Switch \"don't have\" to \"have\"? (y=yes, n=no)");
                fgets(y, 2, stdin);
                if (*y=='y') {
                    fputs(compare3, havelist);
                    fclose(wrappedlist);
                    fclose(havelist);
                    return;
                }
                else if(*y=='n'){
                    fclose(wrappedlist);
                    fclose(havelist);
                    return;             
            }
        }
            printf("Invalid input.");
        }
            printf("Not editing.");

        fclose(wrappedlist);
        fclose(havelist);

        return;
    }
    else if(*z=='n'){
        fclose(wrappedlist);
        fclose(havelist);
        return;
    }

    printf("Invalid entry");
    fclose(havelist);
    fclose(wrappedlist);
    return;
}

I can input a char to the fgets after the "Edit?" prompt, but then I get the exc_bad_access error. Please help, thanks.code:

Answer 1

It happens because your pointers point to no memory.

char* z; // pointer points nowhere
/* snip */
fgets(z, 2, stdin); // writing to pointer that points nowhere: boom!

This tries to put into z the two next characters from stdin. However, z points to nowhere useful. You need it to point to existing memory: declaring a pointer alone isn't enough to get memory next to it.

You probably want a buffer of 2 characters for it:

char z[2];

With this code, z will be a pointer to enough memory for 2 characters. (In C, an array can be passed wherever a pointer is expected.)

I didn't look too far into your code, but you'll have the same problem for y.

Answer 2

You haven't allocated any memory for z or y and they're left uninitialized so fgets is writing to random addresses.