PHP写日志什么时候需要加锁？

先分析fwrite，直接找到PHP源代码：

static size_t _php_stream_write_buffer(php_stream *stream, const char *buf, size_t count TSRMLS_DC)
{vv
    size_t didwrite = 0, towrite, justwrote;

    /* if we have a seekable stream we need to ensure that data is written at the
     * current stream->position. This means invalidating the read buffer and then
     * performing a low-level seek */
    if (stream->ops->seek && (stream->flags & PHP_STREAM_FLAG_NO_SEEK) == 0 && stream->readpos != stream->writepos) {
        stream->readpos = stream->writepos = 0;

        stream->ops->seek(stream, stream->position, SEEK_SET, &stream->position TSRMLS_CC);
    }


    while (count > 0) {
        towrite = count;
　　　　//chunk_size是8K
        if (towrite > stream->chunk_size)
            towrite = stream->chunk_size;
　　　　//分多次调用
        justwrote = stream->ops->write(stream, buf, towrite TSRMLS_CC);

        /* convert justwrote to an integer, since normally it is unsigned */
        if ((int)justwrote > 0) {
            buf += justwrote;
            count -= justwrote;
            didwrite += justwrote;

            /* Only screw with the buffer if we can seek, otherwise we lose data
             * buffered from fifos and sockets */
            if (stream->ops->seek && (stream->flags & PHP_STREAM_FLAG_NO_SEEK) == 0) {
                stream->position += justwrote;
            }
        } else {
            break;
        }
    }
    return didwrite;

}

上图中红色注释是我加的。可以看出，fwrite会判断写入的内容长度，如果大于8K，会分多次write。每次write是直接调用了系统的write，这个是原子的。因为有多次write，就会有被中断的可能，就可能会出现错乱。

结论：写入小于8K，是原子性操作，不用加锁。反之需要。

error_log和file_put_contents函数

看这两个函数的源代码，发现最终也会调用_php_stream_write_buffer，所以结果是一样的。

程序验证：

先构造一个脚本，每次写10000字节（超过8K）

$a=str_repeat($argv[1],10000);
$i=0;
while($i++<10000)
file_put_contents('temp',$a."
",FILE_APPEND);

用shell模拟并发

for((j=0;j<10;j++));do
        php test.php a&
        php test.php b&
done

结果可以明显看出来，出现了大量的错乱。

wc -l temp 
200000 temp
grep -P '(ab|ba)' temp|wc -l
54579

如果把每次10000字节，改成8000如何呢，下面结果很明显了？

wc -l temp 
200000 temp
grep -P '(ab|ba)' temp|wc -l       
0

TIP:怎么快速找到源代码呢？参考我的一篇博客中的方法：http://www.cnblogs.com/hxdoit/p/5193598.html