M commands may be abbreviated to a defined prefix. Most commands have arguments. However, some commands have either optional arguments or no arguments. When a command has no argument and is followed by more commands on the same line, at least two spaces (<SP>) must follow the command without arguments. Commands that accept arguments generally accept multiple arguments on the same command. M treats multiple arguments the same as multiple occurrences of the same command, each with its own argument.
M provides postconditionals as a tool for placing a condition on the execution of a single command and, in some cases, a single command argument. A postconditional consists of a colon (:) delimiter followed by a truth-valued expression. When the expression evaluates to true, M executes the command occurrence. When the expression evaluates to false, M does not execute the command occurrence.
Command postconditionals appear immediately following a command and apply to all arguments for the command when it has multiple arguments. All commands except commands that themselves have a conditional aspect accept a command postconditional. Among the M standard commands, ELSE, FOR, and IF do not accept command postconditionals. All the GT.M command extensions accept command postconditionals. When a postconditional evaluates to a literal FALSE (0), GT.M discards the command and its arguments at compile time, which means it does not perform any validity checking on the arguments.
Commands that affect the flow of control may accept postconditionals on individual command arguments. Because multiple arguments act as multiple commands, this is a straight-forward application of the same principal as command postconditional. The only M standard commands that accept argument postconditionals are DO, GOTO, and XECUTE. The GT.M command extensions that accept argument postconditionals are BREAK, ZGOTO, and ZSYSTEM.
M provides timeouts as a tool to retain program control over commands of indefinite duration. A timeout consists of a colon (:) delimiter on an argument, followed by a numeric expression specifying the number of seconds to millisecond (three decimal place) precision for M to attempt to execute the command. When the timeout is zero (0), M makes a single attempt to complete the command.
GT.M caps the maximum timeout to 2,147,483.647 seconds (about 24.8 days), and converts values greater than the maximum timeout to that cap. When a command has a timeout, M maintains the $TEST intrinsic special variable as the command completes. If the command completes successfully, M sets $TEST to TRUE (1). If the command times out before successful completion, M sets $TEST to FALSE (0). When a command argument does not specify a timeout, M does not maintain $TEST.
The following commands accept timeouts:
LOCK
JOB
OPEN
READ
ZALLOCATE
When a READ times out, M returns any characters that have arrived between the start of the command and the timeout. M does not produce any partial results for any of the other timed commands.
GT.M process execution is interruptible with the following events:
Typing <CTRL-C> or getting SIGINT if CTRAP=$CHAR(3) for the terminal device or on a $PRINCIPAL terminal if its mode is CENABLE.
Typing <CTRL-n> if CTRAP=$CHAR(n) on a terminal device performing a READ
Getting a MUPIP INTRPT (SIGUSR1)
Exceeding $ZMAXTPTIME in a transaction
$ZTIMEOUT expires
A terminal disconnect ("hangup")
A terminal output error during an asynchronous flush
GT.CM network error
A MALLOCLIM error
+$ZTEXit evaluates to a truth value at the outermost TCOMMIT or TROLLBACK
When GT.M detects any of these events, it transfers control to a vector that depends on the event. For most events, GT.M uses the $ETRAP or $ZTRAP vectors described in more detail in the Error Processing chapter. For INTRPT and $ZTEXit, it XECUTEs the interrupt handler code placed in $ZINTERRUPT. If $ZINTERRUPT is an empty string, the process ignores any MUPIP INTRPT directed at it. The default value of $ZINTERRUPT is "IF $ZJOBEXAM()" which redirects a dump of ZSHOW "*" to a file and reports each such occasion to the operator log. For $ZTIMEOUT, the value may specify a vector that takes precedence over the current error handling vector. For <CTRL-C> without CENABLE, it transfers control as if there was an error; with CENABLE, GT.M enters Direct Mode to give the programmer control. Without CENABLE or CTRAP, GT.M ignores <CTRL-C> on a $PRINCIPAL terminal. The GT.M terminal handler only recognizes other <CTRL> characters if CTRAP enabled when the OS terminal handling delivers and they appear in the terminal input stream.
GT.M recognizes most of these events when they occur but transfers control to the interrupt vector at the start of each M line, at each iteration of a FOR loop, at certain points during the execution of commands which may take a "long" time. For example, ZWRITE, HANG, LOCK, MERGE, ZSHOW "V", OPEN of terminals, disk files, PIPEs, FIFOs, and SOCKETs (unless zero timeout,) WRITE /WAIT for SOCKETs, and READ for terminals, SOCKETs, FIFOs, PIPEs, and sequential files in FOLLOW mode.
The HANG command and timed restartable I/O commands such as timed READ for terminals, SOCKETs, FIFO, PIPE, and sequential files in FOLLOW mode as well as SOCKET OPEN CONNECT and WRITE /WAIT account for time spent in handling the interrupt. However, the LOCK command pauses the timeout countdown until the interrupt handling is complete.
If +$ZTEXIT evaluates to a truth value at the outermost TCOMMIT or TROLLBACK, GT.M XECUTEs $ZINTERRUPT after completing the commit or rollback. Except for <CTRL-C> GT.M recognizes CTRAP characters when READ. CTRAP characters other than <CTRL-C> tend to be limited by terminal configuration.