C programming Seg fault help

This is a discussion on C programming Seg fault help within the C Programming forums, part of the General Programming Boards category; This is my program to read a file & print the lines out, since im not familiar with link list, ...

  1. #1
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    C programming Seg fault help

    This is my program to read a file & print the lines out,
    since im not familiar with link list, i got a seg fault from this.
    Anyone can help me out,thx?


    Code:
    #include<stdio.h>
    #include<stdlib.h>
    #include<string.h>
    
    typedef struct Node {
         char   line[1000];
         struct Node *next;
    } NodeT;
    
    int main(int argc, char **argv) {
       
    
       char line [1000];
       FILE *fp;
       NodeT *s, *head;
       head = s;
    
       if(argv[1] != NULL){   
          if((fp = fopen(argv[1],"r")) != NULL) {
             while (fgets(s->line, 1000, fp) != NULL) {
             printf("%s" ,line);
             s = s->next;
             }
             s->next = NULL;
          }
       }
       free(s);
       free(head);
    
       head = NULL;
       s = NULL;
    
    return 0;
    }

  2. #2
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    You need to malloc() memory for `s' - at the moment, `s' and `head' both point to some unknown location.

  3. #3
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    i did put them but forgot to post them ;P i free them at the end....but after i malloc those two, im still getting seg fault.

    NodeT *s = malloc(sizeof(NodeT* ));
    NodeT *head = malloc(sizeof(NodeT* ));

  4. #4
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    This is what i got by now
    Code:
    #include<stdio.h>
    #include<stdlib.h>
    #include<string.h>
    
    typedef struct Node {
         char   line[1000];
         struct Node *next;
    } NodeT;
    
    int main(int argc, char **argv) {
       
    	
       char line [1000];
       FILE *fp;
       NodeT *s = malloc(sizeof(NodeT* ));
       NodeT *head = malloc(sizeof(NodeT* ));
    
       if(argv[1] != NULL){   
          if((fp = fopen(argv[1],"r")) != NULL) {
             while (fgets(s->line, 1000, fp) != NULL) {
             printf("%s" ,line);
             s = s->next;
             }
             s->next = NULL;
          }
       }
       free(s);
       free(head);
    
       head = NULL;
       s = NULL;
    
    return 0;
    }

  5. #5
    and the hat of wrongness Salem's Avatar
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    Linked list - Wikipedia, the free encyclopedia
    You need a malloc for each line you read.

    Code:
    char buff[BUFSIZ];
    while ( buff, BUFSIZ, fp ) != NULL ) {
      // malloc a new node
      // copy buff
      // append node to the list
    }
    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.

  6. #6
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    Code:
       if(argv[1] != NULL){   
          if((fp = fopen(argv[1],"r")) != NULL) {
             while (fgets(line, 1000, fp) != NULL) {
    				s = malloc(sizeof(NodeT*));
    				strcpy(s->line,line);
    				s = s->next;
             }
             s->next = NULL;
          }
       }
    i guess assume that's what you want me to do.
    but still seg fault ;(
    am i doing something wrong in other line?

  7. #7
    and the hat of wrongness Salem's Avatar
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    Look at the wiki page.
    s = s->next is not going to append anything to the end of an existing list.
    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.

  8. #8
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    I'm not quite getting it, say s is a pointer that will go through every nodes in the list, should I do "s = s -> next" to move pointer s to the next node?

  9. #9
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    oh wait..the problem occur after the loop, thx a lot for ur help!

  10. #10
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    Quote Originally Posted by vince3214 View Post
    I'm not quite getting it, say s is a pointer that will go through every nodes in the list, should I do "s = s -> next" to move pointer s to the next node?
    Code:
    				s = malloc(sizeof(NodeT*));
    				strcpy(s->line,line);
    But after those two lines, what is the value of s->next? Hint: You have not set it yet, so it is undefined or indeterminate.

  11. #11
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    should I initialize ever s->next = NULL then give them value in the loop?

  12. #12
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    *every

  13. #13
    ATH0 quzah's Avatar
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    Why did you make a new post instead of hitting the big edit button?


    Quzah.
    Hope is the first step on the road to disappointment.

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