STREAMS Programming Guide

Autopush Facility

The autopush facility (see autopush(1M)) lets administrators specify a list of modules to be automatically pushed onto the Stream whenever a STREAMS device is opened. A prespecified list (/etc/iu.ap) of modules can be pushed onto the Stream if the STREAMS device is not already open.

The STREAMS Administrative Driver (SAD) (see sad(7D)) provides an interface to the autopush mechanism. System administrators can open the SAD and set or get autopush information on other drivers. The SAD caches the list of modules to push for each driver. When the driver is opened, if not already open, the Stream head checks the SAD's cache to determine if the device is configured to have modules automatically pushed. If an entry is found, the modules are pushed. If the device is open, another open does not cause the list of the prespecified modules to be pushed again.

Three options configure the module list:

When the configuration list is cleared, a range of minor devices has to be cleared as a range and not in parts.

Application Interface

The SAD is accessed through /dev/sad/admin or /dev/sad/user. After the device is initialized, a program can be run to perform any needed autopush configuration. The program should open the SAD, read a configuration file to find out what modules must be configured for which devices, format the information into strapush structures, and perform the necessary SAD_SAP ioctl(2)s. See sad(7D) for more information.

All autopush operations are performed through an ioctl(2) command to set or get autopush information. Only root can set autopush information, but any user can get the autopush information for a device.

In the ioctl(2) call, the parameters are the file descriptor of the SAD, either SAD_SAP (set autopush information) or SAD_GAP (get autopush information), and a pointer to a strapush structure.

strapush is defined as:

 * maximum number of modules that can be pushed on a
 * Stream using the autopush feature should be no greater
 * than nstrpush
#define MAXAPUSH 8

/* autopush information common to user and kernel */

struct apcommon {
 	uint apc_cmd;          /* command - see below */
 	major_t apc_major;     /* major device number */
 	minor_t apc_minor;     /* minor device number */
 	minor_t apc_lastminor; /* last minor dev # for range */
 	uint apc_npush;        /* number of modules to push */

/* ap_cmd - various options of autopush */
#define SAP_CLEAR    0     /* remove configuration list */
#define SAP_ONE      1     /* configure one minor device*/
#define SAP_RANGE    2     /* config range of minor devices */
#define SAP_ALL      3     /* configure all minor devices */

/* format of autopush ioctls */
struct strapush {
 	struct apcommon sap_common;
 	char sap_list[MAXAPUSH] [FMNAMESZ + 1]; /* module list */

#define sap_cmd              sap_common.apc_cmd
#define sap_major            sap_common.apc_major
#define sap_minor            sap_common.apc_minor
#define sap_lastminor        sap_common.apc_lastminor
#define sap_npush            sap_common.apc_npush

A device is identified by its major device number, sap_major. A SAD_CMD ioctl(2) is one of the following commands:


Configures a single minor device, sap_minor, of a driver


Configures a range of minor devices from sap_minor to sap_lastminor, inclusive


Configures all minor devices of a device


Clears the previous settings by removing the entry with the matching sap_major and sap_minor fields

The list of modules is specified as a list of module names in sap_list. The maximum number of modules to push automatically is defined by MAXAPUSH.

A user can query the current configuration status of a given major or minor device by issuing the SAD_GAP ioctl(2) with sap_major and sap_minor values of the device set. On successful return from this system call, the strapush structure will be filled in with the corresponding information for that device. The maximum number of entries the SAD driver can cache is determined by the tunable parameter NAUTOPUSH found in the SAD driver's master file.

The following is an example of an autopush configuration file in /etc/iu.ap:

# /dev/console and /dev/contty autopush setup
#	major    minor   lastminor   modules
		wc       0       0           ldterm ttcompat
		zs       0       1           ldterm ttcompat
		ptsl     0       15          ldterm ttcompat

The first line is the configuration of a single minor device whose major name is wc and minor numbers start at 0 and end at 0, creating only one minor number. The modules automatically pushed are ldterm and ttcompat. The second line represents the configuration of the zs driver. The minor device numbers are 0 and 1. The last line allows minor device numbers from 0 to 15 for the ptsl driver.