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[2005-11-03 19:16 UTC] askalski at gmail dot com
Description:
------------
The documentation for stream_set_write_buffer() says that fwrite()s are buffered at 8K by default. In reality, it does not buffer at all. Any attempt to call stream_set_write_buffer() on a regular file handle results in failure.
Reproduce code:
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$fh = fopen("asdf", "w");
// write some data using the default buffering
for ($i = 0; $i < 5; $i++)
fwrite($fh, "test");
// demonstrate that stream_set_write_buffer fails
$n = stream_set_write_buffer($fh, 8192);
if ($n != 0)
echo "stream_set_write_buffer failed\n";
// write some more data to demonstrate the failure
for ($i = 0; $i < 5; $i++)
fwrite($fh, "again");
fclose($fh);
Expected result:
----------------
PHP should buffer its I/O, and strace should log a single write(). stream_set_write_buffer() should not fail on regular files.
Actual result:
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Output:
stream_set_write_buffer failed
Abridged strace output:
open("/tmp/php5-200511031730/asdf", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
fstat64(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
lseek(3, 0, SEEK_CUR) = 0
write(3, "test", 4) = 4
write(3, "test", 4) = 4
write(3, "test", 4) = 4
write(3, "test", 4) = 4
write(3, "test", 4) = 4
write(1, "stream_set_write_buffer failed\n", 31) = 31
write(3, "again", 5) = 5
write(3, "again", 5) = 5
write(3, "again", 5) = 5
write(3, "again", 5) = 5
write(3, "again", 5) = 5
close(3) = 0
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Last updated: Mon Nov 03 11:00:02 2025 UTC |
> instead it uses the system's file io buffering function > to set the buffer. Assuming you're talking about setvbuf() and not kernel level write-behind caching, this statement is false. The file is opened by _php_stream_fopen(): fd = open(realpath, open_flags, 0666); (It is never subsequently wrapped with fdopen()) At this point, the stream is not buffered at all. When I later call stream_set_write_buffer(), it hits php_stdiop_set_option(): case PHP_STREAM_OPTION_WRITE_BUFFER: if (data->file == NULL) { return -1; } /* setvbuf happens down here */ Because there is no FILE* handle (data->file), setvbuf() is never called for the stream. One of either the documentation (http://us3.php.net/stream_set_write_buffer) or the PHP source is wrong.