"Segmentation fault (core dumped)" Error while reading words from a text file and adding them to a list

Question

i have this code trying to read a word from a list of words stored in a .txt file

char *getWord(char *filename)  {
    char formatstr[15], *word;
    static FILE *input;
    static int firstTime = 1;
    if (firstTime) { 
        input = fopen(filename, "r");
        if (input == NULL) {
            printf("ERROR: Could not open file \"%s\"\n", filename);
            exit(1);
        }
        firstTime = 0;
    }
    word = (char*)malloc(sizeof(char)*WORDLEN);
    if (word == NULL) {
        printf("ERROR: Memory allocation error in getWord\n");
        exit(1);
    }
    sprintf(formatstr, "%%%ds", WORDLEN-1);
    fscanf(input, formatstr, word);
    if (feof(input)) {
        fclose(input);
        return NULL;
    }   
    return word;    
}

And then this code to try and store it into a linked list

struct num1* wdStr(int wdLength, char *filename)
{

    int d; // total words.
    char *wordC;

    struct num1 *head = NULL;
    struct num1 *temp;  

    d = edCount(filename);      

    for(int i = 0; i < d; i++) 
    {       
        wordC = getWord(filename);              

        if(strlen(wordC) == wdLength)
        {
            temp = (struct num1*)malloc(sizeof(struct num1));
                temp->val = wordC;
                temp->next = head;
                head = temp;
        }       
    }   
    //return head;
}

edCount just counts how many words are there.

So after running the program, always before the last word i get a "Segmentation fault (core dumped)" error. I know i am doing something wrong with the pointers but still i cant find what.

This is fairly easily solved with a debugger. Which OS are you running on? — dutt, Nov 28 '13 at 12:14
[Please don't cast the return value of `malloc()` in C](http://stackoverflow.com/a/605858/28169). — unwind, Nov 28 '13 at 12:15
you are returning local pointers from both of your functions. They are out of scope after returning from the function, so behaviour is undefined. — Dipto, Nov 28 '13 at 12:15
@Dipto: not a problem; the pointers are initialized via `malloc()`, so it isn't an issue. — Jonathan Leffler, Nov 28 '13 at 12:18

score 3 · Accepted Answer · answered Nov 28 '13 at 12:18

3

after running the program, always before the last word, I get a "Segmentation fault (core dumped)"

This is because your getWord returns NULL after the last word, but your main calls strlen on it unconditionally. This dereferences the NULL pointer, leading to a crash.

Note that your code produces a memory leak: your getWord implementation forgets to free the allocated word before returning NULL for the end of stream marker.

NOTE: Unless this is a specific learning exercise on statics, the use of FILE *input is highly questionable, because it makes your getWord non-reentrant for no good reason.

answered Nov 28 '13 at 12:18

Sergey Kalinichenko

714,442
84
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I know about the memory leak, but i just wanted to fix this issue first. Knowing that getWord gives NULL at eof i also tried to add this if(wordC ==NULL) return head; before if(strlen(wordC) == wdLength), but still i get the same error – ealiaj Nov 28 '13 at 12:25
@green_leaf Returning when `wordC` is `NULL` should have worked fine. The new crash is probably coming from some other place that calls your `wdStr` function. – Sergey Kalinichenko Nov 28 '13 at 12:33
indeed you where correct. And yes actually i have to use getWord and not alter it unfortunately, including the memory leak i suppose. – ealiaj Nov 28 '13 at 12:43

score 1 · Answer 2 · answered Nov 28 '13 at 12:19

1

getWord returns NULL for eof case , you dont seem to be handling it !

answered Nov 28 '13 at 12:19

Srikanth

447
2
8

"Segmentation fault (core dumped)" Error while reading words from a text file and adding them to a list

2 Answers2