Thread: Having trouble with lseek and file pointer

I have a file formatted something like this:

Code:

HERO NAME: <hero_name>
EQUIPMENT 1: <equip_1>
EQUIPMENT 2: <equip_2>
EQUIPMENT 3: <equip_3>
EQUIPMENT 4: <equip_4>
...

I want to read that file, skip the "HERO NAME: " part and just get the name; same with the equipment. But I'm having trouble with lseek and the file pointer. It seems every time the file is read, the pointer is at a different place?

Code:

while ((dirEntry = readdir(dir)) != NULL) {
        if (strcmp(dirEntry->d_name, ".") == 0 || strcmp(dirEntry->d_name, "..") == 0) {
            // do nothing
        } else {
            sprintf(filename, "%s/%s", dirName, dirEntry->d_name);
            fileDesc = open(filename, O_RDONLY);

            if (fileDesc < 0) {
                perror("Error: ");
                exit(EXIT_FAILURE);
            }
            
            // skip the text "ROOM NAME: "
            lseek(fileDesc, 11, SEEK_CUR);
            
            int j = 0, offset = 0;
            char content[MAX_FILECONTENT_LENGTH];

            // read until newline is encountered
            while ((nRead = read(fileDesc, content, 1)) > 0) {
                if (content[j] == '\n') {
                    content[j] = '\0';
                }
               
                j++;
            }
        }
    }

I just cannot accurately grab the name that I want.

Originally Posted by jjwooyoung

I want to read that file, skip the "HERO NAME: " part and just get the name; same with the equipment. But I'm having trouble with lseek and the file pointer. It seems every time the file is read, the pointer is at a different place?

It looks like you are overwriting your content, i.e., instead of:

Code:

while ((nRead = read(fileDesc, content, 1)) > 0) {

you should have written:

Code:

while ((nRead = read(fileDesc, &content[j], 1)) > 0) {

Actually, I would restructure the whole loop as it presently doesn't account for the loop ending before the newline is reached, and relies too much on the content length actually being less than MAX_FILECONTENT_LENGTH. For example:

Code:

// read until newline is encountered
while (j < MAX_FILECONTENT_LENGTH - 1 && (nRead = read(fileDesc, &content[j], 1)) > 0 && content[j] != '\n) {
    j++;
}
content[j] = '\0';

I don't think your lseek trick is going to work for equipment though, since you presumably have to handle skipping both "EQUIPMENT 1: " and "EQUIPMENT 10: ". But it should be easy to read up to ": " and discard whatever came before that on each line.

You should also close the file when done. It would be better if you moved the entire opening/reading/closing of each file into a separate function. This way, you can more easily test it separately.

EDIT:
We don't get to see the declaration of filename, so maybe it is a non-problem, but you otherwise have a buffer overflow vulnerability in your use of sprintf to construct the filename. You should use snprintf, or (double) check that the combined lengths of dirName, "/", and dirEntry->d_name do not exceed the maximum length for filename before calling sprintf.

Using seek on a text file is fraught with all sorts of difficulties, not least of which is the variable length of each line.

Try something like this, if all your tags like "HERO NAME" are well formed.

Code:

while ( fgets( buff, sizeof(buff), fp ) ) {
  if ( sscanf(buff, "HERO NAME: %s", hero ) == 1 ) {
  }
}