io_trywrite(3) write to a descriptor without blocking


#include <io.h>

int io_trywrite(int64 fd,const char* buf,int64 len);


io_trywrite tries to write len bytes of data from buf[0], buf[1], ..., buf[len-1] to descriptor fd. (The effects are undefined if len is 0 or smaller.) There are several possible results:

  • o_trywrite returns an integer between 1 and len: This number of bytes was immediately written from the beginning of buf. Note that this number can be, and often is, smaller than len; you must not assume that io_trywrite always succeeds in writing exactly len bytes.
  • io_trywrite returns -1, setting errno to EAGAIN: No bytes were written, because the descriptor is not ready. For example, the descriptor is writing to a full pipe that could still be read.
  • io_trywrite returns -3, setting errno to something other than EAGAIN: No bytes were written, because the write attempt encountered a persistent error, such as a serious disk failure (EIO), an unreachable network (ENETUNREACH), or an invalid descriptor number (EBADF).

io_trywrite does not pause waiting for a descriptor that is not ready. If you want to pause, use io_waitread or io_wait.

You can make io_trywrite faster and more efficient by making the socket non-blocking with io_nonblock().

Once upon a time, many UNIX programs neglected to check the success of their writes. They would often encounter EPIPE, and would blithely continue writing, rather than exiting with an appropriate exit code. The UNIX kernel developers decided to send a SIGPIPE signal, which terminates the process by default, along with returning EPIPE. This papers over the problem without fixing it: the same programs ignore other errors such as EIO. One hopes that the programs have been fixed by now; kernels nevertheless continue to generate the SIGPIPE signal. The first time io_trywrite or io_waitwrite is called, it arranges for SIGPIPE to be ignored. (Technically, for SIGPIPE to be caught by an empty signal handler, so this doesn't affect child processes.) Do not use SIGPIPE elsewhere in the program.