Can someone please explain how stdio buffering works?

问题

I don't understand what the buffer is doing and how it's used. (Also, if you can explain what a buffer normally does) In particular, why do I need fflush in this example?

int main(int argc, char **argv)
{
    int pid, status;
    int newfd;  /* new file descriptor */

    if (argc != 2) {
        fprintf(stderr, "usage: %s output_file\n", argv[0]);
        exit(1);
    }
    if ((newfd = open(argv[1], O_CREAT|O_TRUNC|O_WRONLY, 0644)) < 0) {
        perror(argv[1]);    /* open failed */
        exit(1);
    }
    printf("This goes to the standard output.\n");
    printf("Now the standard output will go to \"%s\".\n", argv[1]);
    fflush(stdout);

    /* this new file will become the standard output */
    /* standard output is file descriptor 1, so we use dup2 to */
    /* to copy the new file descriptor onto file descriptor 1 */
    /* dup2 will close the current standard output */

    dup2(newfd, 1); 

    printf("This goes to the standard output too.\n");
    exit(0);
}

回答1:

In a UNIX system the stdout buffering happens to improve I/O performance. It would be very expensive to do I/O every time.

If you really don't want to buffer there's some options:

Disable buffering calling setvbuf http://www.cplusplus.com/reference/cstdio/setvbuf/
Call flush when you want to flush the buffer
Output to stderr (that's unbuffered by default)

Here you've more details: http://www.turnkeylinux.org/blog/unix-buffering

I/O is an expensive operation, so to reduce the number of I/O operations the system store the information in a temporary memory location, and delay the I/O operation to a moment when it has a good amount of data.

This way you've a much smaller number of I/O operations, what means, a faster application.

回答2:

danielfraca answers most of the question, but there's another part: what is the default buffering on a stream?

An output stream is line-buffered by default if and only if it refers to a terminal. Otherwise it is full-buffered. Also note that both kinds of buffering will auto-flush if more than BUFSIZ bytes are written (usually a power of two between 512 and 8192).

So this program:

#include <stdio.h>
#include <unistd.h>

int main()
{
    puts("Hello");
    fork();
    puts("World");
}

produces this output:

% ./fork 
Hello
World
World
% ./fork | cat
Hello
World
Hello
World

来源：https://stackoverflow.com/questions/29176636/can-someone-please-explain-how-stdio-buffering-works

标签

buffer

stdio