fscanf和fprintf是否在C中缓冲？

Question

I wish to write an efficient C program, that makes a copy of a file. There doesn't seem to be a function (such as rename ) that performs this. I plan to use fscanf an fprintf in stdio.h, but their descriptions do not say how or if they are buffered. Are they buffered between different cache levels? (I assume disk to memory buffer is handled by the OS)

Answer 1

When you open a file with fopen , it will be fully buffered.

You can change the buffering before doing any other operations on the file, using setvbuf ( reference ).

Using any normal I/O functions on a FILE object will take advantage of the buffering.

If you are just copying the data, you will be doing sequential reads and writes, and will not necessarily need buffering. But doing that efficiently does require choosing an appropriate block size for I/O operations. Traditionally, this is related to the size of a disk sector (4096 bytes) but that value isn't future-proof. The default used by fopen is BUFSIZ .

As with any optimisation, construct actual tests to verify your performance gains (or losses).

In the end, for the fastest I/O you might have to use OS-specific APIs. The CI/O functions just map to the general case of those APIs, but there may be special performance settings for an OS that you cannot control through the C library. I certainly ran into this when writing a fast AVI writer for Windows. Using platform-specific I/OI was able to achieve the maximum read/write speed of the disk: twice the speed of buffered I/O ( <stdio.h> ) or the native AVI API, and about 20% faster than traditional C unbuffered I/O.

Answer 2

The printf and scanf family of functions are all part of the same buffered "interface". man 3 stdio :

The standard I/O library provides a simple and efficient buffered stream I/O interface. Input and output is mapped into logical data streams and the physical I/O characteristics are concealed. The functions and macros are listed below; more information is available from the individual man pages.

If you want to avoid buffering, you will have to use a different C library.