Zero copy system call

从 splice(2) 中的描述来看，它是基于reference count完成的

stackoverflow Is Linux kernel splice() zero copy?

splice(2)

NOTE:

"splice" 的中文意思是 "移接"

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <fcntl.h>

ssize_t splice(int fd_in, off64_t *off_in, int fd_out,
               off64_t *off_out, size_t len, unsigned int flags);

splice() moves data between two file descriptors without copying between kernel address space and user address space. It transfers up to len bytes of data from the file descriptor fd_in to the file descriptor fd_out, where one of the file descriptors must refer to a pipe.

NOTE:

在两个fd之间进行transfer，这些transfer是直接由kernel完成的，因此 "without copying between kernel address space and user address space"

NOTE

The three system calls splice(), vmsplice(2), and tee(2), provide user-space programs with full control over an arbitrary kernel buffer, implemented within the kernel using the same type of buffer that is used for a pipe. In overview, these system calls perform the following tasks:

1、splice() moves data from the buffer to an arbitrary file descriptor, or vice versa, or from one buffer to another.

2、tee(2) "copies" the data from one buffer to another.

3、vmsplice(2) "copies" data from user space into the buffer.

Though we talk of copying, actual copies are generally avoided. The kernel does this by implementing a pipe buffer as a set of reference-counted pointers to pages of kernel memory. The kernel creates "copies" of pages in a buffer by creating new pointers (for the output buffer) referring to the pages, and increasing the reference counts for the pages: only pointers are copied, not the pages of the buffer.

NOTE: 显然也是基于reference count的

vmsplice(2)

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <fcntl.h>
#include <sys/uio.h>

ssize_t vmsplice(int fd, const struct iovec *iov, size_t nr_segs, unsigned int flags);

If fd is opened for writing, the vmsplice() system call maps nr_segs ranges of user memory described by iov into a pipe.

If fd is opened for reading, the vmsplice() system call fills nr_segs ranges of user memory described by iov from a pipe.

The file descriptor fd must refer to a pipe.

tee(2)

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <fcntl.h>

ssize_t tee(int fd_in, int fd_out, size_t len, unsigned int flags);

tee() duplicates up to len bytes of data from the pipe referred to by the file descriptor fd_in to the pipe referred to by the file descriptor fd_out. It does not consume the data that is duplicated from fd_in; therefore, that data can be copied by a subsequent splice(2).

Conceptually, tee() copies the data between the two pipes. In reality no real data copying takes place though: under the covers, tee() assigns data to the output by merely grabbing a reference to the input.

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>

int
    main(int argc, char *argv[])
{
    int fd;
    int len, slen;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <file>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd == -1) {
        perror("open");
        exit(EXIT_FAILURE);
    }

    do {
        /*
                * tee stdin to stdout.
                */
        len = tee(STDIN_FILENO, STDOUT_FILENO,
                  INT_MAX, SPLICE_F_NONBLOCK);

        if (len < 0) {
            if (errno == EAGAIN)
                continue;
            perror("tee");
            exit(EXIT_FAILURE);
        } else
            if (len == 0)
                break;

        /*
                * Consume stdin by splicing it to a file.
                */
        while (len > 0) {
            slen = splice(STDIN_FILENO, NULL, fd, NULL,
                          len, SPLICE_F_MOVE);
            if (slen < 0) {
                perror("splice");
                break;
            }
            len -= slen;
        }
    } while (1);

    close(fd);
    exit(EXIT_SUCCESS);
}