Strange behavior after fseek() has worked

Question

I've came across a strange behavior. While debugging, when a while-loop loops at first time: after going though /* "data-url" */ and /* "data-author" */ parts of code I have the next result in Debugging windows -> Watches:

(I'm using Code::Blocks IDE, Ubuntu 13.04)

The length of dataUrl_tempString is 8 bytes, the length of dataAuthor_tempString is 11 bytes, the length of dataName_tempString is 9 bytes...

But after going through /* data-name */ part of code I have the the result that confuses me:

Now they are not of size 8, 11 and 9 bytes!

What is the matter?

Could you help me finding the reason of such behavior?

---

Here is the code of that function:

int SubString_Search(char *fnameNew, char *strUrl, char *strAuthor, char *strName) {

    FILE *fp;
    FILE *ofp_erase;
    FILE *ofp;
    char ch_buf;
    int count = 0;

    char dataUrl[8] = "";
    char dataAuthor[11] = "";
    char dataName[9] = "";
    char *dataUrl_tempString = &dataUrl[0];
    char *dataAuthor_tempString = &dataAuthor[0];
    char *dataName_tempString = &dataName[0];


    if( (fp = fopen("output_temp.txt", "r")) == NULL) {
        printf("File could not be opened.\n");
        return (-1);
    }
    else {
        /* Erasing 'NEW' file if exists */
        ofp_erase = fopen(fnameNew, "w");
        fclose(ofp_erase);
    }



    ofp = fopen(fnameNew, "a");
    rewind(fp);

    while(!feof(fp)) {

        /* "data-url" */
        fread(dataUrl_tempString, 8, sizeof(char), fp);
        if(memcmp(dataUrl_tempString, strUrl) == 0) {
            fseek(fp, 2, SEEK_CUR);     // going up to required place to copy a string
            while( (ch_buf = getc(fp)) != '"') {
                fputc(ch_buf, ofp);
            }
            fputc('\n', ofp);
        }
        fseek(fp, -8, SEEK_CUR);


        /* "data-author" */
        fread(dataAuthor_tempString, 11, sizeof(char), fp);
        if(memcmp(dataAuthor_tempString, strAuthor) == 0) {
            fseek(fp, 2, SEEK_CUR);     // going up to required place to copy a string
            while( (ch_buf = getc(fp)) != '"') {
                fputc(ch_buf, ofp);
            }
            fputc(' ', ofp);
            fputc('-', ofp);
            fputc(' ', ofp);
        }
        fseek(fp, -11, SEEK_CUR);


        /* "data-name" */
        fread(dataName_tempString, 9, sizeof(char), fp);
        if(memcmp(dataName_tempString, strName) == 0) {
            fseek(fp, 2, SEEK_CUR);     // going up to required place to copy a string
            while( (ch_buf = getc(fp)) != '"') {
                fputc(ch_buf, ofp);
            }
            //fputc() not needed
        }
        fseek(fp, -8, SEEK_CUR); // jumping over 1 symbol from the beginning: `-8` instead of `-9`...


        count++;
        if(count == 5)
            break;
    }

    rewind(fp);
    fclose(fp);
    fclose(ofp);

    return 0;
}

Answer 1

You might like to change the call to

int strcmp(const char *s1, const char *s2);

to become calls to

int memcmp(const void *s1, const void *s2, size_t n);

This shall fix the issue, as long as you do not use other members of the str*() family of function on those (non 0-terminated) char arrays.

Note: However memcmp() always compares the number of characters passed as 3rd parameter (n). This might not be what you want.

---

Update:

Alternativly (as mixure of both calls above) there also is:

int strncmp(const char *s1, const char *s2, size_t n);

Which compares up until it finds a 0-terminator in either s1 or s2 and to a maximum of n characters.

Answer 2

A string needs to have space for a '\0' termination - you only allocated 8 bytes for a string with 8 characters (which therefore needs 9 bytes minimum). Depending on what follows in memory, you will get unpredictable results.