CODEWITHSUNDEEP
ccompiler-constructionmingw
NAME__
NAME__

Reputation: 635

C Weird compiler behavior

I am having some rather odd behavior with the MingW compiler for C.
I have a function, called getNextWord, shown here:

void getNextWord(FILE * pFile)
{
    char * w = (char *)malloc(MAX_WORD_SIZE * sizeof(char *));
    char * q = w;
    char c;
    while(c != ' ')
    {
        c = fgetc(pFile);
        if(c != ' ')
        {
            *(w++) = c;
        }
    }
    *(w++) = '\0';
    printf("%s\n",q);
    free(w);
}

For some reason, when I try to call this function twice in a row to print two words, it does not work. Weirdly enough however, if I put anything at all between the function calls, including printf, or fseek, then the function works as you would expect it.

This works:

int main(int argc, char** argv) {
    FILE * pFile = fopen("doc.txt", "r");
    getNextWord(pFile);
    printf("a");
    getNextWord(pFile);
    fclose(pFile);
    return (EXIT_SUCCESS);
}

This does not work:

int main(int argc, char** argv) {
    FILE * pFile = fopen("doc.txt", "r");
    getNextWord(pFile);
    getNextWord(pFile);
    fclose(pFile);
    return (EXIT_SUCCESS);
}

Does anyone have any ideas why this is occurring?

EDIT: Thanks for the help, this is the fixed function:

void getNextWord(FILE * pFile)
{
    char * w = (char *)malloc(MAX_WORD_SIZE * sizeof(char));
    char * q = w;
    char c;
    do
    {
        c = fgetc(pFile);
        if(c != ' ')
        {
            *(w++) = c;
        }
    } while(c != ' ');
    *(w++) = '\0';
    printf("%s\n",q);
    free(q);
}

Sorry for all the errors there was in the original post, I'm new to C.

Upvotes: 0

Views: 66

Answers (2)

Mat
Mat

Reputation: 206659

You're freeing the wrong pointer, leading to undefined behavior.

You need to free(q), not w, since w no longer points to what malloc returned.

Upvotes: 2

Martin R
Martin R

Reputation: 539675

The local variable char c; in getNextWord() is not initialized, therefore the initial test in

while(c != ' ')

is undefined behaviour. You should initialize e.g. as

char c = 0;

Upvotes: 2

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