As far as the actual I/O goes, the code I've written a million times in various guises for copying data from one stream to another goes something like this. It returns 0 on success, or -1 with errno set on error (in which case any number of bytes might have been copied).
Note that for copying regular files, you can skip the EAGAIN stuff, since regular files are always blocking I/O. But inevitably if you write this code, someone will use it on other types of file descriptors, so consider it a freebie.
There's a file-specific optimisation that GNU cp does, which I haven't bothered with here, that for long blocks of 0 bytes instead of writing you just extend the output file by seeking off the end.
void block(int fd, int event) {
pollfd topoll;
topoll.fd = fd;
topoll.events = event;
poll(&topoll, 1, -1);
// no need to check errors - if the stream is bust then the
// next read/write will tell us
}
int copy_data_buffer(int fdin, int fdout, void *buf, size_t bufsize) {
for(;;) {
void *pos;
// read data to buffer
ssize_t bytestowrite = read(fdin, buf, bufsize);
if (bytestowrite == 0) break; // end of input
if (bytestowrite == -1) {
if (errno == EINTR) continue; // signal handled
if (errno == EAGAIN) {
block(fdin, POLLIN);
continue;
}
return -1; // error
}
// write data from buffer
pos = buf;
while (bytestowrite > 0) {
ssize_t bytes_written = write(fdout, pos, bytestowrite);
if (bytes_written == -1) {
if (errno == EINTR) continue; // signal handled
if (errno == EAGAIN) {
block(fdout, POLLOUT);
continue;
}
return -1; // error
}
bytestowrite -= bytes_written;
pos += bytes_written;
}
}
return 0; // success
}
// Default value. I think it will get close to maximum speed on most
// systems, short of using mmap etc. But porters / integrators
// might want to set it smaller, if the system is very memory
// constrained and they don't want this routine to starve
// concurrent ops of memory. And they might want to set it larger
// if I'm completely wrong and larger buffers improve performance.
// It's worth trying several MB at least once, although with huge
// allocations you have to watch for the linux
// "crash on access instead of returning 0" behaviour for failed malloc.
#ifndef FILECOPY_BUFFER_SIZE
#define FILECOPY_BUFFER_SIZE (64*1024)
#endif
int copy_data(int fdin, int fdout) {
// optional exercise for reader: take the file size as a parameter,
// and don't use a buffer any bigger than that. This prevents
// memory-hogging if FILECOPY_BUFFER_SIZE is very large and the file
// is small.
for (size_t bufsize = FILECOPY_BUFFER_SIZE; bufsize >= 256; bufsize /= 2) {
void *buffer = malloc(bufsize);
if (buffer != NULL) {
int result = copy_data_buffer(fdin, fdout, buffer, bufsize);
free(buffer);
return result;
}
}
// could use a stack buffer here instead of failing, if desired.
// 128 bytes ought to fit on any stack worth having, but again
// this could be made configurable.
return -1; // errno is ENOMEM
}
To open the input file:
int fdin = open(infile, O_RDONLY|O_BINARY, 0);
if (fdin == -1) return -1;
Opening the output file is tricksy. As a basis, you want:
int fdout = open(outfile, O_WRONLY|O_BINARY|O_CREAT|O_TRUNC, 0x1ff);
if (fdout == -1) {
close(fdin);
return -1;
}
But there are confounding factors:
- you need to special-case when the files are the same, and I can't remember how to do that portably.
- if the output filename is a directory, you might want to copy the file into the directory.
- if the output file already exists (open with O_EXCL to determine this and check for EEXIST on error), you might want to do something different, as
cp -i does.
- you might want the permissions of the output file to reflect those of the input file.
- you might want other platform-specific meta-data to be copied.
- you may or may not wish to unlink the output file on error.
Obviously the answers to all these questions could be "do the same as cp". In which case the answer to the original question is "ignore everything I or anyone else has said, and use the source of cp".
Btw, getting the filesystem's cluster size is next to useless. You'll almost always see speed increasing with buffer size long after you've passed the size of a disk block.