──SIGNAL───┬─ labelname ──────────────────────────┬─────;─
├─┬───────┬─ expression ───────────────┤
│ └ VALUE ┘ │
├── OFF ────┬─ ERROR ─────┬────────────┤
│ ├─ FAILURE ───┤ │
│ ├─ HALT ──────┤ │
│ ├─ NOVALUE ───┤ │
│ ├─ SYNTAX ────┤ │
│ └─ NOTREADY ──┘ │
│ │
└─ ON ─┬─ ERROR ───┬┬─────────────────┬┘
├─ FAILURE ─┤└ NAME ─ trapname ┘
├─ HALT ────┤
├─ NOVALUE ─┤
├─ SYNTAX ──┤
└─ NOTREADY ┘
SIGNAL is used to cause an abnormal change in the flow of control, or, if ON or OFF is specified, controls the trapping of certain conditions.
To control trapping, you specify OFF or ON and the condition you want to trap. OFF turns off the specified condition trap. ON turns on the specified condition trap.
┴╓: For information on condition traps, see the OS/2 Procedures Language 2/REXX Reference.
To change the flow of control, a label name is derived from labelname or taken from the result of evaluating the expression after VALUE. The labelname you specify must be a symbol, treated literally, or a literal string that is taken as a constant. The subkeyword VALUE can be omitted if expression does not begin with a symbol or literal string (for example, if it starts with a special character, such as an operator or parenthesis). All active pending DO, IF, SELECT, and INTERPRET instructions in the current routine are then terminated; that is, they cannot be reactivated. Control then passes to the first label in the program that matches the required string, as though the search had started from the top of the program.
Example:
Signal fred; /* Jump to label "FRED" below */ .... .... Fred: say 'Hi!'
Because the search effectively starts at the top of the program, if duplicates are present, control always passes to the first occurrence of the label in the program
When control reaches the specified label, the line number of the SIGNAL instruction is assigned to the special variable SIGL. This can aid debugging because SIGNAL can determine the source of a jump to a label.
Using SIGNAL with the INTERPRET Instruction
If, as the result of an INTERPRET instruction, a SIGNAL instruction is issued or a trapped event occurs, the remainder of the strings being interpreted are not searched for the given label. In effect, labels within interpreted strings are ignored.