A Cup of tee() and a splice() of Cake

2 minute read

Apologies for the terrible pun in the title - I just couldn’t resist :-) I was hard at work on my current project the other day, a user-mode Linux server daemon, when I realized that I would need to both copy incoming data to disk and forward it to another daemon via a socket. This caused me a moment’s consternation, since I was using splice() to move incoming data from a socket to a file without needing an intermediate copy in a user-mode buffer, but then I remembered mention of tee(), a companion to splice(). Where splice() moves data directly from a socket (or file) to a pipe (or vice versa), tee() copies data from one pipe to another leaving the data intact in the source pipe. You can then use splice() again to move the data from tee()’s destination pipe to another file descriptor.

It was the work of a few minutes to code up a quick sample app to test this. Since it’s short, and there seems to be a dearth of tee()/splice() examples, here it is in its entirety:

#define _GNU_SOURCE // needed for splice
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static void test_splice(int in, int out, int out2, int number_of_bytes) {
    int rcvd = 0, sent = 0, teed = 0, remaining = number_of_bytes;
    int pipe1[2], pipe2[2];

    if (pipe(pipe1) < 0) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    if (pipe(pipe2) < 0) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    while (remaining > 0) {
        if ((rcvd = splice(in, NULL, pipe1[1], NULL, remaining,
                SPLICE_F_MORE | SPLICE_F_MOVE)) < 0) {
            perror("splice");
            exit(EXIT_FAILURE);
        }

        if (rcvd == 0) {
            printf("Reached end of input file\n");
            break;
        }

        printf("Wrote %d bytes to pipe1\n", rcvd);

        if ((teed = tee(pipe1[0], pipe2[1], rcvd, 0)) < 0) {
            perror("tee");
            exit(EXIT_FAILURE);
        }

        printf("Copied %d bytes from pipe1 to pipe2\n", teed);

        if ((sent = splice(pipe1[0], NULL, out, NULL, rcvd, SPLICE_F_MORE
                | SPLICE_F_MOVE)) < 0) {
            perror("splice");
            exit(EXIT_FAILURE);
        }

        printf("Read %d bytes from pipe1\n", sent);

        if ((sent = splice(pipe2[0], NULL, out2, NULL, teed, SPLICE_F_MORE
                | SPLICE_F_MOVE)) < 0) {
            perror("splice");
            exit(EXIT_FAILURE);
        }

        printf("Read %d bytes from pipe2\n", sent);

        remaining -= rcvd;
    }
}

int main(int argc, char *argv[]) {
    int infile, outfile1, outfile2, number_of_bytes = INT_MAX;

    if (argc < 4) {
        fprintf(stderr,
                "Usage: %s infile outfile1 outfile2 [number_of_bytes]\n",
                argv[0]);
        exit(EXIT_FAILURE);
    }

    if ((infile = open(argv[1], O_RDONLY)) < 0) {
        fprintf(stderr, "Can't open %s for reading\n", argv[1]);
        exit(EXIT_FAILURE);
    }

    if ((outfile1 = open(argv[2], O_WRONLY | O_CREAT | O_TRUNC, 0644)) < 0) {
        fprintf(stderr, "Can't create/open %s for writing\n", argv[2]);
        exit(EXIT_FAILURE);
    }

    if ((outfile2 = open(argv[3], O_WRONLY | O_CREAT | O_TRUNC, 0644)) < 0) {
        fprintf(stderr, "Can't create/open %s for writing\n", argv[3]);
        exit(EXIT_FAILURE);
    }

    if (argc > 4) {
        number_of_bytes = atoi(argv[4]);
    }

    test_splice(infile, outfile1, outfile2, number_of_bytes);

    return EXIT_SUCCESS;
}

The example doesn’t need much explanation; I added the ‘number_of_bytes’ parameter so that you can copy a limited amount of data from an infinite source such as /dev/zero or /dev/urandom. Note that a ‘real’ implementation needs a bit more code, since it’s not safe to assume that all the bytes get moved to the destination pipes in one hit, but that would obscure the example :-)