Expression Expression

1.3.6.1.2.1.90.1.2.1.1.3

The expression to be evaluated. This object is the same as a DisplayString (RFC 1903) except for its maximum length. Except for the variable names the expression is in ANSI C syntax. Only the subset of ANSI C operators and functions listed here is allowed. Variables are expressed as a dollar sign ('$') and an integer that corresponds to an expObjectIndex. An example of a valid expression is: ($1-$5)*100 Expressions must not be recursive, that is although an expression may use the results of another expression, it must not contain any variable that is directly or indirectly a result of its own evaluation. The managed system must check for recursive expressions. The only allowed operators are: ( ) - (unary) + - * / % & | ^ << >> ~ ! && || == != > >= < <= Note the parentheses are included for parenthesizing the expression, not for casting data types. The only constant types defined are: int (32-bit signed) long (64-bit signed) unsigned int unsigned long hexadecimal character string oid The default type for a positive integer is int unless it is too large in which case it is long. All but oid are as defined for ANSI C. Note that a hexadecimal constant may end up as a scalar or an array of 8-bit integers. A string constant is enclosed in double quotes and may contain back-slashed individual characters as in ANSI C. An oid constant comprises 32-bit, unsigned integers and at least one period, for example: 0. .0 1.3.6.1 No additional leading or trailing subidentifiers are automatically added to an OID constant. The constant is taken as expressed. Integer-typed objects are treated as 32- or 64-bit, signed or unsigned integers, as appropriate. The results of mixing them are as for ANSI C, including the type of the result. Note that a 32-bit value is thus promoted to 64 bits only in an operation with a 64-bit value. There is no provision for larger values to handle overflow. Relative to SNMP data types, a resulting value becomes unsigned when calculating it uses any unsigned value, including a counter. To force the final value to be of data type counter the expression must explicitly use the counter32() or counter64() function (defined below). OCTET STRINGS and OBJECT IDENTIFIERs are treated as one-dimensioned arrays of unsigned 8-bit integers and unsigned 32-bit integers, respectively. IpAddresses are treated as 32-bit, unsigned integers in network byte order, that is, the hex version of 255.0.0.0 is 0xff000000. Conditional expressions result in a 32-bit, unsigned integer of value 0 for false or 1 for true. When an arbitrary value is used as a boolean 0 is false and non-zero is true. Rules for the resulting data type from an operation, based on the operator: For << and >> the result is the same as the left hand operand. For &&, ||, ==, !=, <, <=, >, and >= the result is always Unsigned32. For unary - the result is always Integer32. For +, -, *, /, %, &, |, and ^ the result is promoted according to the following rules, in order from most to least preferred: If left hand and right hand operands are the same type, use that. If either side is Counter64, use that. If either side is IpAddress, use that. If either side is TimeTicks, use that. If either side is Counter32, use that. Otherwise use Unsigned32. The following rules say what operators apply with what data types. Any combination not explicitly defined does not work. For all operators any of the following can be the left hand or right hand operand: Integer32, Counter32, Unsigned32, Counter64. The operators +, -, *, /, %, <, <=, >, and >= work with TimeTicks. The operators &, |, and ^ work with IpAddress. The operators << and >> work with IpAddress but only as the left hand operand. The + operator performs a concatenation of two OCTET STRINGs or two OBJECT IDENTIFIERs. The operators &, | perform bitwise operations on OCTET STRINGs. If the OCTET STRING happens to be a DisplayString the results may be meaningless, but the agent system does not check this as some such systems do not have this information. The operators << and >> perform bitwise operations on OCTET STRINGs appearing as the left hand operand. The only functions defined are: counter32 counter64 arraySection stringBegins stringEnds stringContains oidBegins oidEnds oidContains average maximum minimum sum exists The following function definitions indicate their parameters by naming the data type of the parameter in the parameter's position in the parameter list. The parameter must be of the type indicated and generally may be a constant, a MIB object, a function, or an expression. counter32(integer) - wrapped around an integer value counter32 forces Counter32 as a data type. counter64(integer) - similar to counter32 except that the resulting data type is 'counter64'. arraySection(array, integer, integer) - selects a piece of an array (i.e. part of an OCTET STRING or OBJECT IDENTIFIER). The integer arguments are in the range 0 to 4,294,967,295. The first is an initial array index (one-dimensioned) and the second is an ending array index. A value of 0 indicates first or last element, respectively. If the first element is larger than the array length the result is 0 length. If the second integer is less than or equal to the first, the result is 0 length. If the second is larger than the array length it indicates last element. stringBegins/Ends/Contains(octetString, octetString) - looks for the second string (which can be a string constant) in the first and returns the one-dimensioned arrayindex where the match began. A return value of 0 indicates no match (i.e. boolean false). oidBegins/Ends/Contains(oid, oid) - looks for the second OID (which can be an OID constant) in the first and returns the the one-dimensioned index where the match began. A return value of 0 indicates no match (i.e. boolean false). average/maximum/minimum(integer) - calculates the average, minimum, or maximum value of the integer valued object over multiple sample times. If the object disappears for any sample period, the accumulation and the resulting value object cease to exist until the object reappears at which point the calculation starts over. sum(integerObject*) - sums all available values of the wildcarded integer object, resulting in an integer scalar. Must be used with caution as it wraps on overflow with no notification. exists(anyTypeObject) - verifies the object instance exists. A return value of 0 indicates NoSuchInstance (i.e. boolean false).