Reading line by line using fread and fseek

Question

im trying to read a .txt file, line by line, where each line is 19 characters long, this is my code (I want to only use fseek and fread to understand pointers rightly):

int main(int argc, char *argv[]){
    FILE *file = fopen(argv[1], "r");
    if(file == NULL){
        perror("fopen");
        return 0;
    }
    int i = 0;
    int j = 0;
    char L[20];
    while(1){
        if(feof(file)){
            break;
        }
        fseek(file, i*20, SEEK_SET);
        fread(L,19,sizeof(char),file);
        printf("%s\n", L);
        i++;
        }
    fclose(file);
    return 0;   
 }

What I think should happen with this is that I move the pointer to the file to the start of each line in a loop, then move one line and so. The source .txt file is:

123456789:123456789
perro:guau         
gato:miau          
                   
vaca:muuu          
                   
lobo:auuu

while the output I get from my program is:

123456789:123456789�V
perro:guau         �V
gato:miau          �V
                   �V
vaca:muuu          �V
                   �V
lobo:auuu          �V
                   �V
                   �V

I tried changing the 20s and 19s since I dont know if im including the end of string character in the right way, but got even worse results, any help would be apreciated.

Where do you make sure `L` is 0-terminated before treating it like a string? — Shawn, Nov 07 '20 at 04:09
Check `fread()` return value, not [`feof()`](https://stackoverflow.com/q/5431941/2410359). — chux - Reinstate Monica, Nov 07 '20 at 05:04
If you are reading a line of 19-chars -- what are you doing with the `'\n'` at the end of each line? Or is that included in your total? According to `"123456789:123456789"` it isn't... (pointers and `fread()` are not related concepts) — David C. Rankin, Nov 07 '20 at 05:20

chux - Reinstate Monica · Accepted Answer · 2020-11-07T05:18:22.067

OP's code has undefined behavior, (UB) as "%s" expects a string. Yet L[] lacks a null character. To print a character array, use "%.*s"
Check fread() return value, not feof().
Use return value of fread() to know how much was read.
Review argument order for fread(void * restrict ptr, size_t size, size_t nmemb, FILE * restrict stream);

Sample:

size_t sz = sizeof L / sizeof L[0];
size_t n;
while((n = fread(L,sizeof L[0], sz, file)) > 0) {
  printf("<%.*s>\n", (int) n, L);
  i++;
  fseek(file, i*20, SEEK_SET);
}

fread() is not the best tool to read a line. Research fgets() instead.

"each line is 19 characters long," --> C defines a line as "each line consisting of zero or more characters plus a terminating new-line character." From a C-point-of-view, your lines include a new-line and so are 20 characters total.

Reading line by line using fread and fseek

1 Answers1