5.4 I/O Status Block

The I/O status block (IOSB) formats for the read, write, set mode, set characteristics, sense mode, sense characteristics, and LAT port driver I/O functions are shown in Figures Figure 5-14, Figure 5-16, Figure 5-17, and Figure 5-18. Figure 5-15 shows the IOSB format for the itemlist read function. Appendix A lists the status returns for these functions. (The OpenVMS system messages documentation provides explanations and suggested user actions for these returns.)

In Figure 5-14, the offset to terminator at IOSB+2 is the count of characters before the terminator character (see “Read Function Terminators”). The terminator character is in the buffer at the offset specified in IOSB+2. When the buffer is full, the offset at IOSB+2 is equal to the requested buffer size. At the same time, IOSB+4 is equal to 0. In the case of multiple character escape sequences that act as terminators, the terminator at IOSB+4 is the first character (ESC) of the escape sequence. IOSB+6 contains the size of the terminator string, usually 1. However, in an escape sequence, IOSB+6 contains the size of the validated escape sequence (see “Escape and Control Sequences”). The sum of IOSB+2 and IOSB+6 is the number of characters in the buffer.

Figure 5-14 IOSB Contents—Read Function

In Figure 5-15 the terminator position word contains a number, the character of which is determined by the mode of operation. For itemlist read operations that do not specify TRM$K_EM_RDVERIFY, this word contains the number of characters from the end of the buffer to the cursor location at the time the terminator character was received. If TRM$K_EM_RDVERIFY is specified, the terminator position word contains the offset into the buffer from the nonverified character.

Figure 5-15 IOSB Contents—Itemlist Read Function

The byte at IOSB+5 passes the status information, listed in Table 5-30, on TRM$K_EM_RDVERIFY operations in which TRM$M_TM_ARROWS or TRM$M_TM_TOGGLE is set in TRM$_MODIFIERS.

In Figure 5-16, the remote terminal driver does not return the number of lines output or the cursor position.

Figure 5-16 IOSB Contents—Write Function

In Figure 5-17, the TT driver attempts to return the correct data in IOSB after a SETMODE or SETCHAR. To be sure the returned data is correct, the user should follow the SETMODE or SETCHAR with a SENSEMODE or SENSECHAR.

Figure 5-17 IOSB Contents—Set Mode, Set Characteristics, Sense Mode, and Sense Characteristics Functions

When an application program makes an I/O request for a connection to a remote device on a terminal server, the LAT port driver places status information about the request into the first word of the I/O status block, as shown in Figure 5-18. Table 5-29 lists the possible status returns.

If the server rejects the request, the LAT port driver returns a numeric LAT rejection code in the second word of the I/O status block. Table 5-31 lists the LAT rejection codes.

Figure 5-18 IOSB Contents—LAT Port Driver Function