Language Syntax
This chapter describes the MDB language syntax, operators, and rules for command and symbol name resolution.
Syntax
The debugger processes commands from standard input. If standard input is a terminal, MDB provides terminal editing capabilities. MDB can also process commands from macro files and from dcmd pipelines, described below. The language syntax is designed around the concept of computing the value of an expression (typically a memory address in the target), and applying a dcmd to that address. The current address location is referred to as dot, and "." is used to reference its value.
A metacharacter is one of the following characters:
[ ] | ! / \ ? = > $ : ; NEWLINE SPACE TAB
A blank is a TAB or a SPACE. A word is a sequence of characters separated by one or more non-quoted metacharacters. Some of the metacharacters function only as delimiters in certain contexts, as described below. An identifier is a sequence of letters, digits, underscores, periods, or back quotes beginning with a letter, underscore, or period. Identifiers are used as the names of symbols, variables, dcmds, and walkers. Commands are delimited by a NEWLINE or semicolon ( ; ).
A dcmd is denoted by one of the following words or metacharacters:
/ \ ? = > $character :character ::identifier
dcmds named by metacharacters or prefixed by a single $ or : are provided as built-in operators, and implement complete compatibility with the command set of the legacy adb(1) utility. After a dcmd has been parsed, the /, \, ?, =, >, $, and : characters are no longer recognized as metacharacters until the termination of the argument list.
A simple-command is a dcmd followed by a sequence of zero or more blank-separated words. The words are passed as arguments to the invoked dcmd, except as specified under "Arithmetic Expansion" and "Quoting". Each dcmd returns an exit status that indicates it was either successful, failed, or was invoked with invalid arguments.
A pipeline is a sequence of one or more simple commands separated by |. Unlike the shell, dcmds in MDB pipelines are not executed as separate processes. After the pipeline has been parsed, each dcmd is invoked in order from left to right. Each dcmd's output is processed and stored as described in "dcmd Pipelines". After the left-hand dcmd is complete, its processed output is used as input for the next dcmd in the pipeline. If any dcmd does not return a successful exit status, the pipeline is aborted.
An expression is a sequence of words that is evaluated to compute a 64-bit unsigned integer value. The words are evaluated using the rules described in "Arithmetic Expansion".
Commands
A command is one of the following:
- pipeline [ ! word ... ] [ ; ]
A simple-command or pipeline can be optionally suffixed with the ! character, indicating that the debugger should open a pipe(2) and send the standard output of the last dcmd in the MDB pipeline to an external process created by executing $SHELL -c followed by the string formed by concatenating the words after the ! character. For more details, refer to "Shell Escapes".
- expression pipeline [ ! word ... ] [ ; ]
A simple-command or pipeline can be prefixed with an expression. Before execution of the pipeline, the value of dot (the variable denoted by ".") is set to the value of the expression.
- expression , expression pipeline [ ! word ... ] [ ; ]
A simple-command or pipeline can be prefixed with two expressions. The first is evaluated to determine the new value of dot, and the second is evaluated to determine a repeat count for the first dcmd in the pipeline. This dcmd will be executed count times before the next dcmd in the pipeline is executed. The repeat count applies only to the first dcmd in the pipeline.
- , expression pipeline [ ! word ... ] [ ; ]
If the initial expression is omitted, dot is not modified; however, the first dcmd in the pipeline will be repeated according to the value of the expression.
- expression [ ! word ... ] [ ; ]
A command can consist only of an arithmetic expression. The expression is evaluated and the dot variable is set to its value, then the previous dcmd and arguments are executed using the new value of dot.
- expression , expression [ ! word ... ] [ ; ]
A command can consist only of a dot expression and repeat count expression. After dot is set to the value of the first expression, the previous dcmd and arguments are repeatedly executed the number of times specified by the value of the second expression.
- , expression [ ! word ... ] [ ; ]
If the initial expression is omitted, dot is not modified but the previous dcmd and arguments are repeatedly executed the number of times specified by the value of the count expression.
- ! word ... [ ; ]
If the command begins with the ! character, no dcmds are executed and the debugger executes $SHELL -c followed by the string formed by concatenating the words after the ! character.
Comments
A word beginning with // causes that word and all the subsequent characters up to a NEWLINE to be ignored.
Arithmetic Expansion
Arithmetic expansion is performed when an MDB command is preceded by an optional expression representing a start address, or a start address and a repeat count. Arithmetic expansion can also be performed to compute a numerical argument for a dcmd. An arithmetic expression can appear in an argument list enclosed in square brackets preceded by a dollar sign ($[ expression ]), and will be replaced by the value of the expression.
Expressions can contain any of the following special words:
| integer | The specified integer value. Integer values can be prefixed with 0i or 0I to indicate binary values, 0o or 0O to indicate octal values, 0t or 0T to indicate decimal values, and 0x or 0X to indicate hexadecimal values (the default). |
| 0[tT][0-9]+.[0-9]+ | The specified decimal floating point value, converted to its IEEE double-precision floating point representation |
| 'cccccccc' | The integer value computed by converting each character to a byte equal to its ASCII value. Up to eight characters can be specified in a character constant. Characters are packed into the integer in reverse order (right-to-left), beginning at the least significant byte. |
| <identifier | The value of the variable named by identifier |
| identifier | The value of the symbol named by identifier |
| (expression) | The value of expression |
| . | The value of dot |
| & | The most recent value of dot used to execute a dcmd |
| + | The value of dot incremented by the current increment |
| ^ | The value of dot decremented by the current increment |
The increment is a global variable that stores the total bytes read by the last formatting dcmd. For more information on the increment, refer to the discussion of "Formatting dcmds".
Unary Operators
Unary operators are right associative and have higher precedence than binary operators. The unary operators are:
| #expression | Logical negation |
| ~expression | Bitwise complement |
| -expression | Integer negation |
| %expression | Value of a pointer-sized quantity at the object file location corresponding to virtual address expression in the target's virtual address space |
| %/[csil]/expression | Value of a char-, short-, int-, or long-sized quantity at the object file location corresponding to virtual address expression in the target's virtual address space |
| %/[1248]/expression | Value of a one-, two-, four-, or eight-byte quantity at the object file location corresponding to virtual address expression in the target's virtual address space |
| *expression | Value of a pointer-sized quantity at virtual address expression in the target's virtual address space |
| */[csil]/expression | Value of a char-, short-, int-, or long-sized quantity at virtual address expression in the target's virtual address space |
| */[1248]/expression | Value of a one-, two-, four-, or eight-byte quantity at virtual address expression in the target's virtual address space |
Binary Operators
Binary operators are left associative and have lower precedence than unary operators. The binary operators, in order of precedence from highest to lowest, are:
| * | Integer multiplication |
| % | Integer division |
| # | Left-hand side rounded up to next multiple of right-hand side |
| + | Integer addition |
| - | Integer subtraction |
| << | Bitwise shift left |
| >> | Bitwise shift right |
| == | Logical equality |
| != | Logical inequality |
| & | Bitwise AND |
| ^ | Bitwise exclusive OR |
| | | Bitwise inclusive OR |