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.