Thread: Using splice() to copy 1 file to another

The example code would appear to bail out when the pipe is full.

This is the wrong use of splice(). As you've realized, splice() only works with a pipe. You don't want to create a pipe just to copy a file.

To copy bytes from one open fd to another, use the sendfile() function.

Originally Posted by Angus

Then output file descriptor to sendfile() must be a socket.

I do see what you are talking about on the man page. But that isn't by design. The kernel is apparently buggy, or somebody is too lazy to make it work.

Originally Posted by Angus

http://ilia.ws/archives/13-sendfile-...el-sucks!.html gives some slightly different reasons for that. What about memory-mapping the input and output files then memcpy()ing between them?

In previous testing, it was found that simply reading the file with fread (or read) and writing with fwrite (or write) is faster than mmap'ing the file.

--
Mats

Originally Posted by Angus

What I find strange is that the code example on that page has a while loop writing the entire file to the pipe, and then having another loop read the entire file from the pipe.

In any case, wouldn't it be better that each splice() call that writes to the pipe be followed by a call that reads from the pipe, rather than packaging each type of splice call to its own while loop?

Have you actually tried this? Chances are each loop only iterates once anyway.

Originally Posted by Angus

Another question I have on the side is, why is the writing splice() call not handling a return value of -1 followed by an errno value of ENOMEM ("Out of memory")--something that seems a very likely event?

It is:

Code:

if (ret < 0)
                goto pipe;
pipe:
                close (filedes [0]);
                close (filedes [1]);
out:
                if (ret < 0)
                        return -errno;
                return 0;