just stop working when compile...

This is a discussion on just stop working when compile... within the C Programming forums, part of the General Programming Boards category; this code is about prime numbers ... and compile print prime numbers but then shows message stop working.. i used ...

  1. #1
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    just stop working when compile...

    this code is about prime numbers ... and compile print prime numbers but then shows message stop working.. i used dynamic memory allocation but cant find where is problem... thanks in advance...

    Code:
    #include<stdio.h>
    #include<stdlib.h>
    
    int main() {
        int *p=NULL,N,v,b,i=0;   
    
        printf("Enter number of elements array:");
        scanf("%d",&N);
        
        p=(int *)malloc(N*sizeof(int)); 
        
        if(p==NULL) {
           printf("Out of memory");
           exit(1);
        }
    
        while(i<=N) { 
           *(p+i)=0;
           i++;
        }
        
        for(b=2;b<=N;b++) {
           v=b;
           if(*(p+v)==0) {
    	   printf("%d ", b);
    	      for (i=2;v<=N;i++) {
    	         v=b*i;               /*  2  3  4  5  6  7  8  9  10  11 ... n  */
    	         *(p+v)=1;	   	      	      }                                                
           }		       
        }				 
        
        free(p); 
        
        printf("\n\nEnd");   /* i put this here just to see if compiler will go to this point,compile will not print this,he just shows message "stop working" before he go to this line */ 
        
        getch();
        return 0;
    }

  2. #2
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    i erased one line
    free(p);

    and now there is no message...and all works fine and he prints the last line...
    why...we should always use free after dynamic memory allocation...is not that right...

    i am now really confused...?

  3. #3
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    You allocated N elements so valid indices would be 0 to N-1. inclusive.
    You are having heap overrun.

  4. #4
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    i dont get it... please explain...

  5. #5
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    From your code....

    Code:
      p=(int *)malloc(N*sizeof(int)); 
     
    ...
       
      while(i<=N) { 
         *(p+i)=0;
           i++;
        }
    
    ...
       for(b=2;b<=N;b++) {
    
    ...
     for (i=2;v<=N;i++) {
    The variable p points to an array of n elements, numbered 0 to N-1. That is to say that when N = 5 you have an array of elements numbered from 0 to 4. Your loops terminate on <= condtions thus terminating at 5 which is out of bounds and attempts to access unallocated memory.

  6. #6
    and the hat of wrongness Salem's Avatar
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    Well you allocate N, but you also have <= N in your loops.

    You're stepping off the end of your allocation.

    The point that you notice that the damage has been done is when you call free.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  7. #7
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    i think i understand what zou are saying...

    but if i put
    ...
    for(b=2;b<=N-1;b++) {

    ...
    for (i=2;v<=N-1;i++) {


    it still is not working...he still shows message stop working....

  8. #8
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    Quote Originally Posted by Salem View Post
    Well you allocate N, but you also have <= N in your loops.

    You're stepping off the end of your allocation.

    The point that you notice that the damage has been done is when you call free.
    Hi Salem,
    This drove me nuts when I started in C. Coming from pascal where it was legitimate to declare and first and last number for array indexes ( array x[11..43]; ) and not being keenly aware of how many elements there were, I found myself constantly confusing the "number of elements" ( int x[10]; ) with the "numbers of the elements"... It took quite a while to adjust to the 0 to n-1 thing.

    I'm thinking that's the same mistake our friend is making.

  9. #9
    and the hat of wrongness Salem's Avatar
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    And your while loop?

    Post your latest code
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  10. #10
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    Quote Originally Posted by notcodemonkey View Post
    i think i understand what zou are saying...

    but if i put
    ...
    for(b=2;b<=N-1;b++) {

    ...
    for (i=2;v<=N-1;i++) {


    it still is not working...he still shows message stop working....
    Try just using the v < N comparison which is pretty much bog standard in C. If it's getting blown up it's probably at your while statement... it just doesn't report it till later.

  11. #11
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    its just one difference ...i saw that calloc inicialize all elements to 0 so i put that...and what you said about loop to N-1..i put it and its still problem...
    Code:
    #include<stdio.h>
    #include<stdlib.h>
    
    int main() {
        int *p=NULL,N,v,b,i=0;   
    
        printf("Enter number of elements array:");
        scanf("%d",&N);
        
        p=(int *)calloc(N,sizeof(int)); // inicijalizira sve na 1 za razliku od malloc
        
        if(p==NULL) {
           printf("Out of memory");
           exit(1);
        }
        
        for(b=2;b<=N-1;b++) {
           v=b;
           if(*(p+v)==0) {
    	   printf("%d ", b);
    	      for (i=2;v<=N-1;i++) {
    	         v=b*i;               /*  2  3  4  5  6  7  8  9  10  11 ... n  */
    	         *(p+v)=1;	   	      	      }                                                
           }		       
        }				 
        
        free(p);
        
        printf("\n\nEnd");   /* this i put here just to see if compiler will go to this point,compile will not print this,he just shows message "stop working" before he go to this line */ 
        
        getch();
        return 0;
    }

  12. #12
    and the hat of wrongness Salem's Avatar
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    > for (i=2;v<=N-1;i++)
    Maybe test i, rather than v ?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  13. #13
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    CommonTater i try using that too..
    but its still not working to the end...

  14. #14
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    v is important in this loop so i think that i should not move this...

  15. #15
    and the hat of wrongness Salem's Avatar
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    Fine, print the value of v and i as you go, and see how large i gets before you end the loop.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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