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AIX Version 4.3 Communications Technical Reference, Volume 1

dlcwrite Entry Point of the GDLC Device Manager


Writes transmit data to generic data link control (GDLC).


#include <sys/uio.h>
#include <sys/device.h>
#include <sys/gdlextcb.h>
int dlcwrite (devno, uiop, chan, ext)
dev_t devno;
struct uio *uiop;
int chan, ext;
Note: The dlc prefix is replaced with the three-digit prefix for the specific GDLC device manager being written.


The dlcwrite entry point is called when a user application program invokes a writex subroutine or when a kernel user calls the fp_write kernel service. An extended write is used in order to specify the type of data being sent, as well as the service access point (SAP) and link station (LS) identifiers.

The following fields in the uio and iov structures are used to control the write data transfer operation:

uio_iov Points to an iovec structure.
uio_iovcnt Indicates the number of elements in the iovec structure. This must be set to a value of 1 for the kernel user, indicating that there is a single communications memory buffer (mbuf) chain associated with the write subroutine.
uio_offset Specifies the file offset established by a previous fp_lseek kernel service. This field is ignored by GDLC.
uio_segflag Indicates whether the data area is in application or kernel space. This field is set to the UIO_USERSPACE value by the file I/O subsystem if the data area is in application space. The field must be set to the UIO_SYSSPACE value by the kernel user to indicate kernel space.
uio_fmode Contains the value of the file mode set during an application open subroutine to GDLC or can be set directly during a fp_open kernel service to GDLC.
uio_resid Contains the total byte count of the transmit data area for application users. For kernel users, GDLC ignores this field since the communications memory buffer (mbuf) also carries this information.
iovec structure Contains the starting address and length of the transmit. (See the iov_base and iov_len fields.)
iov_base Specifies a variable in the iovec structure where GDLC gets the address of the application user's transmit data area or the address of the kernel user's transmit mbuf.
iov_len Specifies a variable in the iovec structure that contains the byte length of the application user's transmit data area. This variable is ignored by GDLC for kernel users, since the transmit mbuf contains a length field.


devno Indicates major and minor device numbers. This is a dev_t device number that specifies both the major and minor device numbers of the GDLC device manager. One dev_t device number exists for each type of GDLC, such as Ethernet, Token-Ring, or SDLC.
uiop Points to the uio structure containing the write parameters.
chan Specifies the channel ID assigned by GDLC in the dlcmpx routine at open time.
ext Specifies the extended subroutine parameter. This is a pointer to the extended I/O structure. This data must be in the application space if the uio_fmode field indicates an application subroutine or in the kernel space if the uio_fmode field indicates a kernel subroutine. See the "write Subroutine Extended Parameters for DLC" for more information on this parameter.

Return Values

The following return values are defined in the /usr/include/sys/errno.h file:

0 Indicates a successful operation.
EAGAIN Indicates that transmit is temporarily blocked and a sleep cannot be issued.
EBADF Indicates a bad file number (application).
EINTR Indicates that a signal interrupted the routine before it could complete successfully.
EINVAL Indicates an invalid value, such as too much data for a single packet.
ENOMEM Indicates insufficient resources to satisfy the write subroutine, such as a lack of communications memory buffers (mbufs).
ENXIO Indicates an invalid file pointer (kernel).

Implementation Specifics

Each GDLC supports the dlcwrite entry point as its switch table entry for the writex subroutine. The file system calls this entry point from the process environment only.

Related Information

The open subroutine, writex subroutine.

The dlcmpx entry point of the GDLC device manager, dlcread entry point of the GDLC device manager, ddwrite device entry point.

The fp_lseek kernel service, fp_open kernel service, fp_write kernel service.

write Subroutine Extended Parameters for DLC.

Generic Data Link Control (GDLC) Environment Overview in AIX Version 4.3 Communications Programming Concepts.

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