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Comment by mgerdts

2 days ago

What is up with fin? Is it really just writing an int 0 in the memory right after some variable present in libc or similar?

        extern fin;

        if(getpw(0, pwbuf))
                goto badpw;
        (&fin)[1] = 0;

6 comments

mgerdts

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formerly_proven  1 day ago

I’m guessing v4 C didn’t have structs yet (v6 C does, but struct members are actually in the global namespace and are basically just sugar for offset and a type cast; member access even worked on literals. That’s why structs from early unix APIs have prefixed member names, like st_mode.

  • topspin  1 day ago

    > I’m guessing v4 C didn’t have structs yet

    There may have been a early C without structs (B had none,) but according to Ken Thompson, the addition of structs to C was an important change, and a reason why his third attempt rewrite UNIX from assembly to a portable language finally succeeded. Certainly by the time the recently recovered v4 tape was made, C had structs:

        ~/unix_v4$ cat usr/sys/proc.h
    struct proc {
            char    p_stat;
            char    p_flag;
            char    p_pri;
            char    p_sig;
            char    p_null;
            char    p_time;
            int     p_ttyp;
            int     p_pid;
            int     p_ppid;
            int     p_addr;
            int     p_size;
            int     p_wchan;
            int     *p_textp;
    } proc[NPROC];

    /* stat codes */
    #define SSLEEP  1
    #define SWAIT   2
    #define SRUN    3
    #define SIDL    4
    #define SZOMB   5

    /* flag codes */
    #define SLOAD   01
    #define SSYS    02
    #define SLOCK   04
    #define SSWAP   010

flatline  2 days ago

According to the chatbot, the first word of `fin` is the file descriptor, the second its state. "Reset stdin’s flags to a clean state".