iBlog - Ilia Alshanetsky

A friend of mine gave me a good idea of making a wrapper around the sendfile() syscall that can be used for very quick data transfers between socket and file descriptors. This syscall in some instances can replace 3 other syscalls (seek, read, write) when you need to write a portion of 1 file into another file. Another words a very neat functionality that I could see myself using in my own applications, such as the message file compactor for FUDforum who's entire job is to seek and write data from one file to another. Further tests have shown that this syscall can also be used to create a much faster copy() that compared to PHP's current copy mechanism is about 2x faster, not a bad improvement if I do say so myself.
However, when it came to testing the code in my primary development box it just would not work, neither would any sendfile example I could find on the net that dealt with transferring data across two files. After much searching on Google, IRC and reviewing the relevant kernel sources I've discovered the problem. It appears that as for Linux kernel 2.6.X sendfile() syscall can only be used to transfer data from a file descriptor to a socket descriptor. The so called "misfeature" of allowing data transfer between two files has been removed, while I could not find any reported problems about it in 2.4.X kernels where it can work with 2 file descriptors. The frustration is further escalated by the fact that sendfile() man page does not acknowledge this little tid-bit of information and only the very recent version even documents the EINVAL error code returned, ARGHH!!!
I am still quite curios as to why this functionality was removed, since even after much searching I was not able to find the reason for it, only the fact that it was removed sometime in the 2.5.X kernel series.