CODEWITHSUNDEEP
perl
Jim Salter
Jim Salter

Reputation: 141

Perl - can't flush STDOUT or STDERR

Perl 5.14 from stock Ubuntu Precise repos. Trying to write a simple wrapper to monitor progress on copying from one stream to another:

use IO::Handle;
while ($bufsize = read (SOURCE, $buffer, 1048576)) {
    STDERR->printflush ("Transferred $xferred of $sendsize bytes\n");
    $xferred += $bufsize;
    print TARGET $buffer;
}

This does not perform as expected (writing a line each time the 1M buffer is read). I end up seeing the first line (with a blank value of $xferred), and then nothing until everything flushes on the 7th and 8th lines (on an 8MB transfer). Been pounding my brains out on this for hours - I've read the perldocs, I've read the classic "Suffering from Buffering" article, I've tried everything from select and $|++ to IO::Handle to binmode (STDERR, "::unix") to you name it. I've also tried flushing TARGET with each line using IO::Handle (TARGET->flush). No dice.

Has anybody else ever encountered this? I don't have any ideas left. Sleeping one second "fixes" the problem, but obviously I don't want to sleep a second every time I read a buffer just so my progress will output on the screen!

FWIW, the problem is exactly the same whether I'm outputting to STDERR or STDOUT.

Upvotes: 3

Views: 1977

Answers (1)

sds
sds

Reputation: 60014

Perl read calls fread(3), not read(2).

This means that it goes through libc and may be using an internal buffer larger than yours; i.e., it gets all the data there is to be received and then quickly throws it at you in 1MB increments.

If this conjecture is correct, the solution might be to use sysread, which calls read(2), instead of read.

Upvotes: 5

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