Problem with lseek call.

Applications Programming
1

The following code:

    int fd;

    if (fd = open("mem", O_RDONLY) == -1)
        return 1;

    if (lseek(fd, 0, SEEK_SET) == -1) {
        char *buf = malloc(512);
        buf = strerror(errno);
        printf("%s\n", buf);
        return 1;
    }

always returns with "illegal seek" error. "mem" is executable with correct permissions. Can someone point me
in the right direction? :wink:

2

Try perror("error message prefix") , much simpler.

C is grouping your open statement like this:

if (fd = (open("mem", O_RDONLY) == -1) )

So fd becomes 1 if the file fails to open, or 0 if the file is there. It must be failing, since if it opened, == -1 it would evaluate false and quit.

By unfortunate coincidence, 1 is a valid file descriptor -- standard output. It's probably a terminal or pipe, which of course can't seek.

So either parenthesize defensively, or put the assignment outside the if-statement (usually preferred).

1 Like
3

Thank you! I will be more careful with parentheses next time.

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