You need to use the raw I/O functions in a loop:
ssize_t todo = count;
for (ssize_t n; todo > 0; )
{
n = write(fd, str, todo);
if (n == -1 && errno != EINTR)
{
// error
break;
}
str += n;
todo -= n;
}
if (todo != 0) { /* error */ }
The special condition concerning EINTR
allows the write call to be interrupted by a signal without causing the entire operation to fail. Otherwise, we expect to be able to write all data eventually.
If you can't finish writing all data because your file descriptor is non-blocking and cannot accept any data at the moment, you have to store the remaining data and try again later when the file descriptor has signalled that it's ready for writing again.