CODEWITHSUNDEEP
cdebugging
Akash Rupela
Akash Rupela

Reputation: 159

Need help debugging C code

Just learnt C this week. My task is to take a large big integer input from the user, store it into a struct Integer, and make a function to print out the appropriate struct integer to the standard output. The program works as such, but as soon as it gives the output, it stops responding. I dont get any direct error in the compiler and cannot figure out what is wrong. Any other advice/tips to improve upon programming style would also be really appreciated :)

// Header Files Go Here
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// Function Declarations Go Here
struct integer * convert_integer(char * stringInt);
void printer(struct integer * p);

struct integer {
    int * arr;
    int length;
};

// Main Program
int main() {
    char * x;
    x = (char *) malloc(sizeof(char) * 10000);
    printf("Enter a small string\n");
    scanf("%s",x);
    int j = 0;
    struct integer * book1;
    book1 = convert_integer(x);
    printer(book1);
    return 0;
}

// Function Definitions Go Here  
struct integer * convert_integer(char * stringInt) {
    struct integer * x = malloc(sizeof(int) * 100);
    int j = 0;
    while (stringInt[j] != '\0') {
        if (stringInt[j] < 48 || stringInt[j] >= 57) {
            printf("Invalid input. Enter a number ");
            return;
        }
        x->arr[j] = stringInt[j] - 48;
        j++;
    }
    x->length = j;
    printf("\n the length is %d\n", x->length);
    return x;
}

void printer(struct integer * p) {
    int j = 0;
    while (j < p->length) {
        printf("%d", p->arr[j]);
        j++;
    }
}

Upvotes: 0

Views: 131

Answers (4)

Jacob Pollack
Jacob Pollack

Reputation: 3751

I am adding this answer because NPE was not clear enough.

The are a few errors in this program but I would say the pressing one is in the function convert_integer. You are doing the following:

struct integer* x= malloc(sizeof(int) * 100);

... but that is incorrect. You are requesting too many bytes of memory for x considering its data type (nothing really wrong with that), however you are not requesting blocks of memory for arr. It needs to be as follows:

struct integer *x = malloc( sizeof( struct integer ) );

x->arr = malloc( sizeof( int ) * c );

... where c is some constant (in your case 100?). Make sure that when you free this structure you first free the arr and then free the structure, or else you will have a memory leak.

Something else I noticed you are not doing, always check the result of a system call. You are not checking to see if malloc returned an invalid memory block.

Upvotes: 2

unxnut
unxnut

Reputation: 8829

You need to add a prototype for printer on top. The if statement validating the number should be

if(stringInt[j]<48 || stringInt[j]>57)  // > instead of >=

It will make more sense to write it as

if ( stringInt[j] < '0' || stringInt[j] > '9' )

If you know about isdigit function, you can use that as well.

You are allocating memory for a string with 10000 characters. That seems like an overkill. You can allocate memory for 256 characters and that should be plenty. You also should deallocate memory before executing return using the statement free (x);

You need to allocate space for arr inside the convert_integer function. Your allocations should be:

struct integer* x= malloc(sizeof(integer)); x->arr = malloc ( sizeof(int) * 256 );

Upvotes: 0

Abrixas2
Abrixas2

Reputation: 3295

Your code is failing at the line 

x->arr[j]=stringInt[j]-48;

The reason is the way you allocate your memory slice: 

struct integer* x= malloc(sizeof(int) * 100);

This allocates a slice of n*100 bytes and stores a pointer to that slice in x. What you want to do is the following: 

struct integer *x = malloc(sizeof(*x));
x->arr = malloc(sizeof(int)*100);

That allocates the memory needed to store the integer structure x and it allocates memory to store 100 integers and assigns it to x->arr. With your code, x->arr is not initialized and thus has random values in it (more precisely: The values are those that were stored at that memory location before).

After finished using the allocated memory, you should also free it: 

free(x->arr);
free(x);

Upvotes: 0

NPE
NPE

Reputation: 500157

Here:

struct integer* x= malloc(sizeof(int) * 100);;

you are allocating memory for the structure (i.e. the pointer and the int), but not for the array.

The moment you try to assign to x->arr[j], you have undefined behaviour.

Upvotes: 0

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