How can I use Linux's splice() function to copy a file to another file?

Question

Hello--here's another question about splice(). I'm hoping to use it to copy files, and am trying to use two splice calls joined by a pipe like the example on splice's Wikipedia page. I wrote a simple test case which only tries to read the first 32K bytes from one file and write them to another:

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

int main(int argc, char **argv) {
    int pipefd[2];
    int result;
    FILE *in_file;
    FILE *out_file;

    result = pipe(pipefd);

    in_file = fopen(argv[1], "rb");
    out_file = fopen(argv[2], "wb");

    result = splice(fileno(in_file), 0, pipefd[1], NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
    printf("%d\n", result);

    result = splice(pipefd[0], NULL, fileno(out_file), 0, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
    printf("%d\n", result);

    if (result == -1)
        printf("%d - %s\n", errno, strerror(errno));

    close(pipefd[0]);
    close(pipefd[1]);
    fclose(in_file);
    fclose(out_file);

    return 0;
}

When I run this, the input file seems to be read properly, but the second splice call fails with EINVAL. Anybody know what I'm doing wrong here?

Thanks!

Answer 1

From the splice manpage:

   EINVAL Target  file  system  doesn't  support  splicing; target file is
          opened in append mode; neither of the descriptors  refers  to  a
          pipe; or offset given for non-seekable device.

We know one of the descriptors is a pipe, and the file's not open in append mode. We also know no offset is given (0 is equivalent to NULL - did you mean to pass in a pointer to a zero offset?), so that's not the problem. Therefore, the filesystem you're using doesn't support splicing to files.

Answer 2

What kind of file system(s) are you copying to/from?

Your example runs on my system when both files are on ext3 but fails when I use an external drive (I forget offhand if it is DOS or NTFS). My guess is that one or both of your files are on a file system that splice does not support.