Techniques for handling short reads/writes with scatter-gather?

Question

Scatter-gather - readv()/writev()/preadv()/pwritev() - reads/writes a variable number of iovec structs in a single system call. Basically it reads/write each buffer sequentiall

Answer 1

AFAICS the vectored read/write functions work the same wrt short reads/writes as the normal ones. That is, you get back the number of bytes read/written, but this might well point into the middle of a struct, just like with read()/write(). There is no guarantee that the possible "interruption points" (for lack of a better term) coincide with the vector boundaries. So unfortunately the vectored IO functions offer no more help for dealing with short reads/writes than the normal IO functions. In fact, it's more complicated since you need to map the byte count into an IO vector element and offset within the element.

Also note that the idea of using vectored IO for individual structs or data items might not work that well; the max allowed value for the iovcnt argument (IOV_MAX) is usually quite small, something like 1024 or so. So if you data is contiguous in memory, just pass it as a single element rather than artificially splitting it up.