STREAMS Programming Guide

Ordinary Messages

The message types found in sys/stream.h are described in this appendix, classified according to their message queueing priority. Ordinary messages are described first, with high-priority messages following. In certain cases, two message types may perform similar functions, differing only in priority. Message construction is described in Chapter 3, STREAMS Application-Level Mechanisms. The use of the word module generally implies module or driver.

Ordinary messages are also called normal or nonpriority messages. Ordinary messages are subject to flow control whereas high-priority messages are not.


Sent to a driver to request that BREAK be transmitted on whatever media the driver is controlling.

The message format is not defined by STREAMS and its use is developer dependent. This message may be considered a special case of an M_CTL message. An M_BREAK message cannot be generated by a user-level process and is always discarded if passed to the stream head.


Generated by modules that send information to a particular module or type of module. M_CTL messages are typically used for intermodule communication, as when adjacent STREAMS protocol modules negotiate the terms of their interface. An M_CTL message cannot be generated by a user-level process and is always discarded if passed to the stream head.


Contains ordinary data. Messages allocated by allocb(9F) are M_DATA type by default. M_DATA messages are generally sent bidirectionally on a stream and their contents can be passed between a process and the stream head. In the getmsg(2) and putmsg(2) system calls, the contents of M_DATA message blocks are referred to as the data part. Messages composed of multiple message blocks typically have M_DATA as the message type for all message blocks following the first.


Sent to a media driver to request a real-time delay on output. The data buffer associated with this message is expected to contain an integer to indicate the number of machine cycles of delay desired. M_DELAY messages are typically used to prevent transmitted data from exceeding the buffering capacity of slower terminals.

The message format is not defined by STREAMS and its use is developer dependent. Not all media drivers may understand this message. This message may be considered a special case of an M_CTL message. An M_DELAY message cannot be generated by a user-level process and is always discarded if passed to the stream head.


Generated by the stream head in response to I_STR, I_LINK, I_UNLINK, I_PLINK, and I_PUNLINK ioctls (see streamio(7I)). This message is also generated in response to ioctl calls that contain a command argument value not defined in streamio(7I). When one of these ioctl(2) is received from a user process, the stream head uses values supplied in the call and values from the process to create an M_IOCTL message containing them, then sends the message downstream. M_IOCTL messages perform the general ioctl(2) functions of character device drivers.

For an I_STR ioctl(2), the user values are supplied in a structure of the following form, provided as an argument to the ioctl(2) call (see I_STR in streamio(7I)):

struct strioctl
	int   ic_cmd;          /* downstream request */
	int   ic_timout;       /* ACK/NAK timeout */
	int   ic_len;          /* length of data arg */
	char  *ic_dp;          /* ptr to data arg */

where ic_cmd is the request (or command) defined by a downstream module or driver, ic_timout is the time the stream head waits for acknowledgement to the M_IOCTL message before timing out, and ic_dp points to an optional data buffer. On input, ic_len contains the length of the data in the buffer that was passed in. On return from the call, it contains the length of the data, if any, being returned to the user in the same buffer.

The M_IOCTL message format is one M_IOCTL message block followed by zero or more M_DATA message blocks. STREAMS constructs an M_IOCTL message block by placing an iocblk(9S) structure, defined in <sys/stream.h>, in its data buffer. The iocblk(9S) structure differs for 64–bit and 32–bit architectures.

#if defined(_LP64)
struct iocblk {
	int     ioc_cmd;       /* ioctl command type */
	cred_t  *ioc_cr;       /* full credentials */
	uint    ioc_id;        /* ioctl id */
	uint    ioc_flag;      /* see below */
	size_t  ioc_count;     /* count of bytes in data field */
	int     ioc_rval;      /* return value  */
	int     ioc_error;     /* error code */
struct iocblk {
	int     ioc_cmd;       /* ioctl command type */
	cred_t  *ioc_cr;       /* full credentials */
	uint    ioc_id;        /* ioctl id */
	size_t  ioc_count;     /* count of bytes in data field */
	int     ioc_error;     /* error code */
	int     ioc_rval;      /* return value  */
	intt    ioc_fill1;
	uint    ioc_flag;      /* see below */
	int     ioc_filler[2]; /* reserved for future use */
#endif  /* _LP64 */

For an I_STR ioctl(2), ioc_cmd corresponds to ic_cmd of the strioctl structure. ioc_cr points to a credentials structure defining the user process's permissions (see cred.hfile).. Its contents can be tested to determine whether the user issuing the ioctl(2) call is authorized to do so. For an I_STR ioctl(2), ioc_count is the number of data bytes, if any, contained in the message and corresponds to ic_len.

ioc_id is an identifier generated internally, and is used by the stream head to match each M_IOCTL message sent downstream with response messages sent upstream to the stream head. The response message that completes the stream-head processing for the ioctl(2) is an M_IOCACK (positive acknowledgement) or an M_IOCNAK (negative acknowledgement) message.

For an I_STR ioctl(2), if a user supplies data to be sent downstream, the stream head copies the data (pointed to by ic_dp in the strioctl structure) into M_DATA message blocks and links the blocks to the initial M_IOCTL message block. ioc_count is copied from ic_len. If there are no data, ioc_count is zero.

If the stream head does not recognize the command argument of an ioctl(2), the head creates a transparent M_IOCTL message. The format of a transparent M_IOCTL message is one M_IOCTL message block followed by one M_DATA block. The form of the iocblk structure is the same as above. However, ioc_cmd is set to the value of the command argument in the ioctl(2) and ioc_count is set to TRANSPARENT, defined in <sys/stream.h>. TRANSPARENT distinguishes the case where an I_STR ioctl(2) specifies a value of ioc_cmd equivalent to the command argument of a transparent ioctl(2). The M_DATA block of the message contains the value of the arg parameter in the ioctl(2).

The first module or driver that understands the ioc_cmd request contained in the M_IOCTL acts on it. For an I_STR ioctl(2), this action generally includes an immediate upstream transmission of an M_IOCACK message. For transparent M_IOCTLs, this action generally includes the upstream transmission of an M_COPYIN or M_COPYOUT message.

Intermediate modules that do not recognize a particular request must pass the message on. If a driver does not recognize the request, or the receiving module can not acknowledge it, an M_IOCNAK message must be returned.

M_IOCACK and M_IOCNAK message types have the same format as an M_IOCTL message and contain an iocblk structure in the first block. An M_IOCACK block may be linked to following M_DATA blocks. If one of these messages reaches the stream head with an identifier that does not match that of the currently outstanding M_IOCTL message, the response message is discarded. A common means of ensuring that the correct identifier is returned is for the replying module to convert the M_IOCTL message into the appropriate response type and set ioc_count to 0 if no data is returned. Then, qreply(9F) is used to send the response to the stream head.

In an M_IOCACK or M_IOCNAK message, ioc_error holds any return error condition set by a downstream module. If this value is non-zero, it is returned to the user in errno. Note that both an M_IOCNAK and an M_IOCACK can return an error. However, only an M_IOCACK can have a return value. For an M_IOCACK, ioc_rval holds any return value set by a responding module. For an M_IOCNAK, ioc_rval is ignored by the stream head.

If a module processing an I_STR ioctl(2) is sending data to a user process, it must use the M_IOCACK message that it constructs such that the M_IOCACK block is linked to one or more following M_DATA blocks containing the user data. The module must set ioc_count to the number of data bytes sent. The stream head places the data in the address pointed to by ic_dp in the user I_STR strioctl structure.

A module processing a transparent ioctl(2) that is sending data to a user process can use only an M_COPYOUT message. For a transparent ioctl(2), no data can be sent to the user process in an M_IOCACK message. All data must be sent in a preceding M_COPYOUT message. The stream head ignores any data contained in an M_IOCACK message (in M_DATA blocks) and frees the blocks.

No data can be sent with an M_IOCNAK message for any type of M_IOCTL. The stream head ignores and frees any M_DATA blocks.

The stream head blocks the user process until an M_IOCACK or M_IOCNAK response to the M_IOCTL (same ioc_id) is received. For an M_IOCTL generated from an I_STR ioctl(2), the stream head times out if no response is received in ic_timout interval (the user can specify an explicit interval or specify use of the default interval). For M_IOCTL messages generated from all other ioctl(2)s, the default (infinite) is used.


The M_PASSFP message passes a file pointer from the stream head at one end of a stream pipe to the stream head at the other end of the same stream pipe.

The message is generated as a result of an I_SENDFD ioctl(2) (see streamio(7I)) issued by a process to the sending stream head. STREAMS places the M_PASSFP message directly on the destination stream head's read queue to be retrieved by an _RECVFD ioctl(2) (see streamio(7I)). The message is placed without passing it through the stream (that is, it is not seen by any modules or drivers in the stream). This message should never be present on any queue except the read queue of a stream head. Consequently, modules and drivers do not need to recognize this message, and it can be ignored by module and driver developers.


The M_PROTO message contains control information and associated data. The message format is one or more M_PROTO message blocks followed by zero or more M_DATA message blocks. The semantics of the M_DATA and M_PROTO message blocks are determined by the STREAMS module that receives the message.

Note –

On the write side, the user can only generate M_PROTO messages containing one M_PROTO message block.

The M_PROTO message block typically contains implementation dependent control information. M_PROTO messages are generally sent bidirectionally on a stream, and their contents can be passed between a process and the stream head. The contents of the first message block of an M_PROTO message is generally referred to as the control part, and the contents of any following M_DATA message blocks are referred to as the data part. In the getmsg(2) and putmsg(2), the control and data parts are passed separately.

The format of M_PROTO and M_PCPROTO (generically PROTO) messages sent upstream to the stream head allows multiple PROTO blocks at the beginning of the message although its use is not recommended. getmsg(2)) compacts the blocks into a single control part when passing them to the user process.


This message is reserved for internal use. Modules that do not recognize this message must pass it on. Drivers that do not recognize it must free it.


This message is used to alter some characteristics of the stream head. It is generated by any downstream module, and is interpreted by the stream head. The data buffer of the message has the following structure, as defined in stream.h:

struct stroptions {
	uint     so_flags;        /* options to set */
	shor     so_readopt;      /* read option */
	ushort   so_wroff;        /* write offset */
	ssize_t  so_minpsz;       /* minimum read packet size */
	ssize_t  so_maxpsz;       /* maximum read packet size */
	size_t   so_hiwat;        /* read queue high–water mark */
	size_t   so_lowat;        /* read queue low–water mark */
	unsigned char so_band;    /* band for water marks */
	ushort   so_erropt;       /* error option */
	ssize_t  so_maxblk;       /* maximum message block size */
	ushort   so_copyopt;      /* copy options (see stropts.h) */

where so_flags specifies which options are to be altered, and can be any combination of the following:

The offset must be less than the maximum message buffer size (system dependent). Under certain circumstances, a write offset may not be inserted. A module or driver must test that b_rptr in the msgb(9S)structure is greater than db_base in the datab(9S) structure to determine that an offset has been inserted in the first message block.

Note –

For conformance with the POSIX standard, new applications should use the O_NONBLOCK flag whose behavior is the same as that of O_NDELAY unless otherwise noted.


The M_SIG message is sent upstream by modules or drivers to post a signal to a process. When the message reaches the front of the stream-head read queue, it evaluates the first data byte of the message as a signal number, defined in <sys/signal.h>. (The signal is not generated until it reaches the front of the stream-head read queue.) The associated signal will be sent to processes under the following conditions: