On 8-Jan-06, at 2:45 AM, Christian wrote:
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).
Fixed.
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.
On terminals I/O is unbuffered by default, so that output immediately appears on the terminal when it is produced without having to call force-output (for example when outputting dots to show progress, or outputting a prompt before reading input from the user). I know that this is a performance problem, but I think it is the right default ("least surprise" for novices which are not aware of buffering issues).
I was surprised to discover that there is no runtime option to override the default buffering setting, so I added this to the runtime options:
Usage: program [-:OPTION,OPTION...] ... where OPTION is one of: ... f[OPT...] set file options; see below for OPT t[OPT...] set terminal options; see below for OPT -[OPT...] set standard input and output options; see below for OPT where OPT is one of: a/1/2/4/8 character encoding (ASCII/LATIN1/UCS2/UCS4/UTF8) l/c/cl end-of-line encoding (LF/CR/CR-LF) u/n/f buffering (unbuffered/newline buffered/fully buffered) e/E [for terminals only] enable line-editing (on/off)
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
Fixed (block devices can now be read like normal files).
Keep those bug reports coming!
Marc