c - 流和文件有什么区别？

Question

C 标准谈​​论流。例如 fopen(3)手册页告诉 fopen 是一个流打开函数。

谁能解释一下流到底是什么，以及它们与文件的关系？

Answer 1

在 C 标准库的上下文中，流是执行某些 I/O 操作的通用接口(interface)。您可以从流中读取，写入流，有些流是可搜索的。将文件作为流打开只是将流作为应用程序的 I/O 接口(interface)的一种方法。

Let me quote:

11.1.1 Streams and File Descriptors

When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams. File descriptors are represented as objects of type int, while streams are represented as FILE * objects.

File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. [...]

... and further:

12.1 Streams

For historical reasons, the type of the C data structure that represents a stream is called FILE rather than “stream”. Since most of the library functions deal with objects of type FILE *, sometimes the term file pointer is also used to mean “stream”. This leads to unfortunate confusion over terminology in many books on C.

C 中 I/O 流的例子:

• 标准流: https://linux.die.net/man/3/stdin
• 文件流: https://linux.die.net/man/3/fopen
• 管道: https://linux.die.net/man/3/popen
• 流套接字: https://linux.die.net/man/2/socket

要进一步阅读，还可以查看这些链接:

• https://www.gnu.org/software/libc/manual/html_mono/libc.html#I_002fO-Overview
• https://www.gnu.org/software/libc/manual/html_mono/libc.html#I_002fO-on-Streams

基于流的 API 构建在低级文件描述符 API 之上，并提供额外的功能。然而，一些低级功能只能通过低级 API 获得，例如，内存映射 I/O、非阻塞 I/O 或“事件驱动”输入/输出:

• https://www.gnu.org/software/libc/manual/html_node/Memory_002dmapped-I_002fO.html
• https://linux.die.net/man/2/poll
• https://linux.die.net/man/4/epoll