fwait - wait for asynchronous I/O operations to complete
#define _MINIX_SOURCE 1
int fwait(struct fwait *fwp)
Fwait allows a program to wait for the completion of asynchronous I/O
operations, and to get the results of completed operations. Asynchronous
I/O operations are started by marking a file descriptor asynchronous with
fcntl(2). After that read(2), write(2), and ioctl(2) system calls are
Fwait takes one argument, a pointer to an fwait structure, which is
defined in <fcntl.h> as follows:
typedef struct fwait
unsigned char (*fw_bits);
The first three fields (fw_flags, fw_bits, and fw_maxfd) have to be
filled-in by the user. Results are returned in the last four fields
(fw_result, fw_errno, fw_fd, and fw_operation) and in the fw_flags field.
The following flags are defined for the fw_flags field:
#define FWF_NONBLOCK 1
#define FWF_MORE 2
If the FWF_NONBLOCK flag is present, fwait does not block but returns
immediately. In this case, if no completed I/O operations are available,
fwait sets errno(2) to EAGAIN. If FWF_NONBLOCK is not set, fwait blocks
until a result can be reported, or until the process gets a signal.
The FWF_MORE flag is ignored by fwait, but instead, fwait sets this flag
when more results are available, i.e. the next call to fwait will not
block. The user program can use this flag to retrieve all available
results in a loop.
The second field, fw_bits contains a pointer to a 2-dimensional bit-
array. Bits set in the array specify about which file
descriptor/operation combinations fwait should report completion results.
The easiest way to create such an array is with the asio_fd_set_t
datatype and the macros ASIO_FD_SET, ASIO_FD_CLR, ASIO_FD_ISSET, and
ASIO_FD_ZERO. asio_fd_set_t is defined as follows:
#define ASIO_FD_SETSIZE 64
typedef unsigned char asio_fd_mask;
typedef struct asio_fd_set
asio_fd_mask afds_bits[howmany(ASIO_FD_SETSIZE, ASIO_NFDBITS)];
ASIO_FD_ZERO(p), with p a pointer to a asio_fd_set structure, clears all
bits. ASIO_FD_SET(n,o,p) and ASIO_FD_CLR(n,o,p) set respectively reset
bits in the asio_fd_set pointed to by p. N specifies the file descriptor
and o takes one of the following values depending on the operation:
#define ASIO_READ 0
#define ASIO_WRITE 1
#define ASIO_IOCTL 2
There is also ASIO_FD_ISSET(n,o,p) to test whether a certain bit is set.
Using an asio_fd_set, the fw_bits field is filled in as follows:
Finally, fw_maxfd is to be filled-in with the size of the bit-array.
Normally this is the value of the macro ASIO_FD_SETSIZE.
Fwait will examine the bits set in the bit-array to see whether
- The file descriptor refers to an open file. If not errno is set to
- The specified operation is in progress, or has completed without
reporting it's status. If not errno is set to EINVAL.
- For the first completed operation, fwait fills in the fields
fw_result, fw_errno, fw_fd, and fw_operation with resp. the return
value, the error code, the file descriptor number and the operation
- If a second completed operation is found, fwait sets the FWF_MORE
more flag, and updates the bit-array to reflect it's internal state.
The nonblocking behavior of the FWF_MORE is only guaranteed if the
bit-array is left unmodified.
Fwait returns 0 upon success and -1 upon failure.
Either the pointer to the fwait structure or the pointer to the bit-
array is invalid.
Fw_maxfd is invalid, or a bit is set for a file descriptor/operation
combination without a pending reult or a call in progress.
FWF_NONBLOCK was set but there was no result to report.
Fwait is interrupted by a signal.
asynchio(3), errno(2), fcntl(2), ioctl(2), read(2), write(2).
Philip Homburg (email@example.com)