I read ftp source code in busybox, But I can't under what's meaning: "%s %s\r\n" 3

static int ftpcmd(const char *s1, const char *s2)
{
    if (s1) {
        fprintf(control_stream, (s2 ? "%s %s\r\n" : "%s %s\r\n" 3),
                        s1, s2);
        fflush(control_stream);
    }

    do {
        strcpy(buf, "EOF"); /* for ftp_die */
        if (fgets(buf, BUFSZ - 2, control_stream) == NULL) {
            ftp_die(NULL);
        }
    } while (!isdigit(buf[0]) || buf[3] != ' ');

    buf[3] = '\0';
    n = xatou(buf);
    buf[3] = ' ';
    return n;
}

CodePudding user response：

A string literal in an expression decays to a pointer to its first character.

Adding 3 to it results in a pointer to its fourth character.

In this case fprintf() receives the format string "%s\r\n".

The statement:

    fprintf(control_stream, (s2 ? "%s %s\r\n" : "%s %s\r\n" 3), s1, s2);

is quite "hacky" and create confusion, as we see here.

It is better expressed as:

    if (s2) {
        fprintf(control_stream, "%s %s\r\n", s1, s2);
    } else {
        fprintf(control_stream, "%s\r\n", s1);
    }

CodePudding user response：

Lets say we have something like this:

const char format_string[] = "%s %s\r\n";
const char *ptr = &format_string[3];

The pointer ptr will be somewhat equivalent to "%s %s\r\n" 3.

When using ptr as a string it will be "%s\r\n", i.e. it will skip the first %s and the following space.