fallocate和ftruncate有什么区别 [英] what's the difference between fallocate and ftruncate

问题描述

他们都可以根据我的测试更改文件大小. 他们为什么都可以将文件更改为更大和更短? fallocate和ftruncate有什么区别?

They can all change file size according to my test. why can they all change file to larger and to shorter? what's the difference between fallocate and ftruncate?

推荐答案

ftruncate是一个简单的单用途函数. 根据POSIX文档，它只是将文件设置为所需的长度:

ftruncate is a simple, single-purpose function. Per the POSIX documentation, it simply sets the file to the requested length:

如果fildes引用常规文件，则ftruncate()函数将导致文件大小被截断为length. ...

If fildes refers to a regular file, the ftruncate() function shall cause the size of the file to be truncated to length. ...

ftruncate()也是标准的POSIX功能，并且是可移植的.请注意，POSIX未指定操作系统设置文件长度的方式，例如，设置为任何长度的文件是否为

ftruncate() is also a standard POSIX function and is portable. Note that POSIX does not specify how an OS sets the file length, such as whether or not a file set to any length is a sparse file.

fallocate()是特定于Linux的功能并以非常特定的方式做更多的事情:

fallocate() is a Linux-specific function that does a lot more, and in very specific ways:

分配磁盘空间

fallocate()的默认操作(即，模式为零)分配 在offset和len指定的范围内的磁盘空间.这 如果offset+len为 大于文件大小.指定范围内的任何子区域 通过offset和len在呼叫之前不包含数据的方式将是 初始化为零.此默认行为与 posix_fallocate(3)库函数的行为，并且旨在 作为最佳实现该功能的一种方法.

The default operation (i.e., mode is zero) of fallocate() allocates the disk space within the range specified by offset and len. The file size (as reported by stat(2)) will be changed if offset+len is greater than the file size. Any subregion within the range specified by offset and len that did not contain data before the call will be initialized to zero. This default behavior closely resembles the behavior of the posix_fallocate(3) library function, and is intended as a method of optimally implementing that function.

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取消分配文件空间

指定FALLOC_FL_PUNCH_HOLE标志(从Linux开始可用 2.6.38)在模式下释放字节空间(即创建一个空位) 范围从offset开始并持续到len个字节.内 指定范围，部分文件系统块被清零，整个 文件系统块将从文件中删除.成功之后 调用，此范围内的后续读取将返回零.

Specifying the FALLOC_FL_PUNCH_HOLE flag (available since Linux 2.6.38) in mode deallocates space (i.e., creates a hole) in the byte range starting at offset and continuing for len bytes. Within the specified range, partial filesystem blocks are zeroed, and whole filesystem blocks are removed from the file. After a successful call, subsequent reads from this range will return zeroes.

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折叠文件空间

指定FALLOC_FL_COLLAPSE_RANGE标志(从Linux开始可用 3.15)在模式下从文件中删除字节范围，而不会留下 孔.要折叠的字节范围从offset开始并继续 len个字节.操作完成后， 从位置offset+len开始的文件将被附加到 位置偏移，文件将缩小len个字节.

Specifying the FALLOC_FL_COLLAPSE_RANGE flag (available since Linux 3.15) in mode removes a byte range from a file, without leaving a hole. The byte range to be collapsed starts at offset and continues for len bytes. At the completion of the operation, the contents of the file starting at the location offset+len will be appended at the location offset, and the file will be len bytes smaller.

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归零文件空间

指定FALLOC_FL_ZERO_RANGE标志(从Linux 3.15开始可用) 在模式下，从offset开始的字节范围中的空间为零，并且 连续len个字节.在指定范围内，块为 为文件中跨越孔的区域预先分配.后 成功调用后，此范围内的后续读取将返回 零.

Specifying the FALLOC_FL_ZERO_RANGE flag (available since Linux 3.15) in mode zeroes space in the byte range starting at offset and continuing for len bytes. Within the specified range, blocks are preallocated for the regions that span the holes in the file. After a successful call, subsequent reads from this range will return zeroes.

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增加文件空间

指定FALLOC_FL_INSERT_RANGE标志(从Linux开始可用 4.1)在模式下，通过在 文件大小而不会覆盖任何现有数据.孔会开始 在offset处并继续len个字节.将孔插入内部时 文件，从offset开始的文件内容将被移动 len个字节向上(即，到更高的文件偏移量).插入一个 文件内的孔会使文件大小增加len个字节.

Specifying the FALLOC_FL_INSERT_RANGE flag (available since Linux 4.1) in mode increases the file space by inserting a hole within the file size without overwriting any existing data. The hole will start at offset and continue for len bytes. When inserting the hole inside file, the contents of the file starting at offset will be shifted upward (i.e., to a higher file offset) by len bytes. Inserting a hole inside a file increases the file size by len bytes.

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