Hello
I wanted to write a small program which reads exactly n bytes from a port (and calculates an md5 hash on them, like "head -c n | md5sum").
It turned out that even with buffering: #f, read-subu8vector reads through buffers, as strace shows (it reads in 1024 byte chunks, not in the lengths I'm giving to read-subu8vector).
The reason why I want it to not read past the given number of bytes is: - I wanted to checksum a cdrom (by reading from /dev/cdrom), and reading past the length of the cdrom image is giving read errors on my OS (linux on ppc). - I might need it for reading from pipes and not block (I've not checked whether gambit would block the thread or not), or worse, read from pipes used for interprocess-locking (even if gambit doesn't block when trying too many bytes from a pipe, it will break the idea of using pipes for locking).
(define-macro (qq . lis) (list 'quasiquote lis))
(define (test bytes path) (call-with-input-file (qq path: ,path ;;char-encoding: #f -> STRING or port settings expected char-encoding: ascii buffering: #f ) (lambda (port) (let ((buf (make-u8vector bytes))) (let ((res (read-subu8vector buf 0 bytes port))) res)))))
(test 20 "./cj-md5sum-cdrom.scm" )
20
open("/home/chris/schemedevelopment/gambit/mod/./cj-md5sum-cdrom.scm", O_RDONLY|O_NONBLOCK) = 4 ioctl(4, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbf911cc8) = -1 ENOTTY (Inappropriate ioctl for device) fstat64(4, {st_mode=S_IFREG|0664, st_size=2258, ...}) = 0 read(4, "; cj Sun, 08 Jan 2006 01:29:03 +"..., 1024) = 1024 close(4) = 0 write(3, "2", 1) = 1 write(3, "0", 1) = 1 write(3, "\n", 1) = 1 write(3, ">", 1) = 1 write(3, " ", 1) = 1
In this strace output a second issue can be seen as well: output to the console is done in single bytes (maybe chars). This makes output of a gambit process to emacs (when running under run-scheme) slow (much so on my laptop, where there seems to be an issue with frequent system calls between programs). Either outputting objects as a whole instead of single characters (the way unbuffered output in e.g. perl works), or buffering on line boundaries, would be an improvement.
Finally, I can't understand why I'm getting this:
(test 10 "/dev/cdrom")
*** ERROR IN (console)@7.1 -- Unknown error (call-with-input-file '(path: "/dev/cdrom" char-encoding: ascii buffering: #f) '#<procedure #3>) 1>
head -c 10 /dev/cdrom works just fine, as the same system user. It might help if the error message would include the OS level message.
strace output: open("/dev/cdrom", O_RDONLY|O_NONBLOCK) = 4 ioctl(4, TCGETS, 0x7ffff148) = -1 EINVAL (Invalid argument) fstat(4, {st_mode=S_IFBLK|0660, st_rdev=makedev(22, 0), ...}) = 0 close(4) = 0
Thanks Christian.