[php内核]----文件系统函数用于普通文件时的一些情况
今天和同事讨论到fflush函数的作用, 本想php的文件系统函数应该是构建在系统的标准I/O库之上的, 所以, 武断的认为fflush的作用就是刷出标准I/O库的缓冲, 相当于标准I/O库的fflush函数....
后来跟踪了一下代码, 发现结果大相径庭...
先说下结果吧:
1. php中的文件系统函数(fopen, fwrite, fread, fseek等)应用在普通文件上时, 内部使用的是open, write, read, seek等系统调用进行处理, 而没有经过标准I/O库.
2. fflush函数应用在普通文件上时, 不产生任何作用.
跟踪过程:
从ext/standard/file.c中的
PHP_NAMED_FUNCTION(php_if_fopen)
作为入口, 最终找到main/streams/plain_wrapper.c中的如下定义
PHPAPI php_stream_ops php_stream_stdio_ops = {
php_stdiop_write, php_stdiop_read,
php_stdiop_close, php_stdiop_flush,
"STDIO",
php_stdiop_seek,
php_stdiop_cast,
php_stdiop_stat,
php_stdiop_set_option
};
这就是应用在普通文件上的主要的文件系统函数的底层实现
以php_stdiop_flush(对应php用户态的fflush)为例:
static int php_stdiop_flush(php_stream *stream TSRMLS_DC)
{
php_stdio_stream_data *data = (php_stdio_stream_data*)stream->abstract;
assert(data != NULL);
/*
* stdio buffers data in user land. By calling fflush(3), this
* data is send to the kernel using write(2). fsync'ing is
* something completely different.
*/
if (data->file) {
return fflush(data->file);
}
return 0;
}
而普通文件在初始化过程中(跟踪open的过程中可以看到), 是没有设置data->file这个字段的, 而是使用了data->fd....
因此这里fflush不会被调用, 也就是说当fflush用在普通文件时, 无任何效果.
author: selfimpr mail: lgg860911@yahoo.com.cn

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