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

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 sequentially from the 0th iovec to the Nth. However according to the documentation it can also return less on the readv/writev calls than was requested. I was wondering if there is a standard/best practice/elegant way to handle that situation.
If we are just handling a bunch of character buffers or similar this isn't a big deal. But one of the niceties is using scatter-gather for structs and/or discrete variables as the individual iovec items. How do you handle the situation where the readv/writev only reads/writes a portion of a struct or half of a long or something like that.
Below is some contrived code of what I am getting at:
int fd;
struct iovec iov[3];
long aLong = 74775767;
int aInt = 949;
char aBuff[100]; //filled from where ever
ssize_t bytesWritten = 0;
ssize_t bytesToWrite = 0;
iov[0].iov_base = &aLong;
iov[0].iov_len = sizeof(aLong);
bytesToWrite += iov[0].iov_len;
iov[1].iov_base = &aInt;
iov[1].iov_len = sizeof(aInt);
bytesToWrite += iov[1].iov_len;
iov[2].iov_base = &aBuff;
iov[2].iov_len = sizeof(aBuff);
bytesToWrite += iov[2].iov_len;
bytesWritten = writev(fd, iov, 3);
if (bytesWritten == -1)
{
//handle error
}
if (bytesWritten < bytesToWrite)
//how to gracefully continue?.........

Answer 1

Use a loop like the following to advance the partially-processed iov:
for (;;) {
written = writev(fd, iov+cur, count-cur);
if (written < 0) goto error;
while (cur < count && written >= iov[cur].iov_len)
written -= iov[cur++].iov_len;
if (cur == count) break;
iov[cur].iov_base = (char *)iov[cur].iov_base + written;
iov[cur].iov_len -= written;
}
Note that if you don't check for cur < count you will read past the end of iov which might contain zero.

Answer 2

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.

Answer 3

Vectored write will write all the data you have provided with one call to "writev" function. So byteswritten will be always be equal to total number of bytes provided as input. this is what my understanding is.
Please correct me if I am wrong