Question

I’m having a hard time figuring out why lseek doesn’t work properly. Basically all I need to do is take input from the standard input, store it into a file named “log.txt” and let the program stop as the “STOP” word is encountered.

My problem is as I try to run the program I get an lseek error: illegal seek and I don’t know why. I thought the issue was the call to lseek but if I substitute lseek(fd, 0, SEEK_END) with lseek(fd, atol(0), SEEK_END) I get a segmentation fault.

I’m just so confused, I don’t know how to proceed in order to fix this, I hope you could give your advice.

int fd;
char buffer[SIZE];
int n, cur;

if (fd = open("log.txt", O_RDWR, S_IRWXU|S_IRWXG|S_IRWXO) < 0)
{
    perror("Error: open");
    return 1;
}

if ((cur = lseek(fd, 0, SEEK_END)) < 0)
{
    printf("Offset corrente: %d\n", (int) lseek(fd, SIZE, SEEK_CUR));
    perror("Error: lseek");
    return 2;
}

while ((n = read(fd, buffer, SIZE)) > 0 && (strncmp(buffer, "STOP", 4) != 0))
{
    write(fd, buffer, SIZE);
}

Answer 1

if (fd = open("log.txt", O_RDWR, S_IRWXU|S_IRWXG|S_IRWXO) < 0)

should be

if ((fd = open("log.txt", O_RDWR, S_IRWXU|S_IRWXG|S_IRWXO)) < 0)

otherwise it will be parsed as

if (fd = (open("log.txt", O_RDWR, S_IRWXU|S_IRWXG|S_IRWXO) < 0))

(because < has a higher operator precedence than =); if the open() succeeds, it will return a value that is >= 0, so the expression open(...) < 0 will be false, so fd will be set to 0.

File descriptor 0 represents the standard input for your process. Unless you’ve redirected it, this will be a terminal device, for which seeking is illegal.