malloc function is not working properly in C program. the program crashes

Question

I am beginner in C and this is my first program using malloc() function. I think there might be some problem in using this function.I wanted to use an array(cyclelength) where a solution for a range of number(user input) will be put. so the array size depends on user so i used malloc(). but the program is crashing. Here is my code:

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int x,y,num,count,p,k;
    for(;;){
        printf("enter first integer. must be between 1 and 100000\n");
        scanf("%d", &x);
        printf("enter second integer. must be between 1 and 100000. must not equal the first integer.\n");
        scanf("%d", &y);
        if(x>=1 && x<100000 && y>=1 && y<100000 && x!=y){
            break;
        }
        else{
            printf("try the whole process again\n");
        }
    }
    if (x<y){
        int j;
        j=y;
        y=x;
        x=j;
    } //making x always greater than y
    int *cyclelength=malloc(5000*sizeof(int));
    if (NULL==cyclelength){
        printf("process aborted");
    }
    else{
        /*solution part for the range of number. and solution for each number  put into cyclelength.*/
        num=y;
        while(num<=x){
            p=1;
            k=num;
            while(k!=1){
                if(k%2==0)
                    k=k/2;
                else
                    k=3*k+1;
                p+=1;
                }
            count=0;
            cyclelength[count]=p;
            num+=1;
            count+=1;
        }
        free(cyclelength);
        cyclelength=NULL;
    }
    int c=0;
    int max=cyclelength[c];
    for(;c<x-y;c+=1){
        if(max<cyclelength[c+1]){
            max=cyclelength[c+1];
        }
    }
    printf("%d,%d,%d",x,y,max);
    return 0;
}

You should post the output of the crash with the question. It will help you get a better answer faster. — cytinus
– cytinus, Commented Jul 8, 2012 at 17:45

Keith Thompson · Accepted Answer · 2012-07-08 17:46:38Z

4

You're calling free(cyclelength) and then accessing the memory it points to (or, rather, the memory that it pointed to).

(And your error handling could stand some improvement; you print "process aborted", but then continue processing.)

answered Jul 8, 2012 at 17:46

Keith Thompson

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Comments

Daniel Fischer · Accepted Answer · 2012-07-08 17:47:05Z

4

You are using cyclelength after you freed it and set it to NULL:

        free(cyclelength);
        cyclelength=NULL;
    }
    int c=0;
    int max=cyclelength[c];
    for(;c<x-y;c+=1){
        if(max<cyclelength[c+1]){
            max=cyclelength[c+1];
        }

That's undefined behaviour and likely to crash.

answered Jul 8, 2012 at 17:47

Daniel Fischer

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Comments

Mahesh Meniya · Accepted Answer · 2012-07-08 17:50:54Z

0

You are using cyclelength after dealloating it so your program is crash.
Try this :

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int x,y,num,count,p,k;
    for(;;){
        printf("enter first integer. must be between 1 and 100000\n");
        scanf("%d", &x);
        printf("enter second integer. must be between 1 and 100000. must not equal the first integer.\n");
        scanf("%d", &y);
        if(x>=1 && x<100000 && y>=1 && y<100000 && x!=y){
            break;
        }
        else{
            printf("try the whole process again\n");
        }
    }
    if (x<y){
        int j;
        j=y;
        y=x;
        x=j;
    } //making x always greater than y
    int *cyclelength=(int *)malloc(5000*sizeof(int));
    if (NULL==cyclelength){
        printf("process aborted");
    }
    else{
        /*solution part for the range of number. and solution for each number  put into cyclelength.*/
        num=y;
        while(num<=x){
            p=1;
            k=num;
            while(k!=1){
                if(k%2==0)
                    k=k/2;
                else
                    k=3*k+1;
                p+=1;
                }
            count=0;
            cyclelength[count]=p;
            num+=1;
            count+=1;
        }        
        // don't assign null to cyclelength
        //cyclelength=NULL;
    }
    int c=0;
    int max=cyclelength[c];
    for(;c<x-y;c+=1){
        if(max<cyclelength[c+1]){
            max=cyclelength[c+1];
        }
    }
    printf("%d,%d,%d",x,y,max);
    // free here
    free(cyclelength);
    return 0;
}

answered Jul 8, 2012 at 17:50

Mahesh Meniya

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2 Comments

Nasif Imtiaz Ohi Over a year ago

well.. thanx everyone... i have got my problem about malloc function.. there were another problem in this code regarding declaring count variable.. that was a silly mistake. i am sorry for that.

Mahesh Meniya Over a year ago

You should also change this line int *cyclelength=malloc(5000*sizeof(int)); to int *cyclelength=(int *)malloc(5000*sizeof(int));

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