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8. GNU bc and Other Implementations

This version of bc was implemented from the POSIX P1003.2/D11 draft and contains several differences and extensions relative to the draft and traditional implementations. It is not implemented in the traditional way using dc. This version is a single process which parses and runs a byte code translation of the program. There is an "undocumented" option (-c) that causes the program to output the byte code to the standard output instead of running it. It was mainly used for debugging the parser and preparing the math library.

A major source of differences is extensions, where a feature is extended to add more functionality and additions, where new features are added. The following is the list of differences and extensions.

LANG environment

This version does not conform to the POSIX standard in the processing of the LANG environment variable and all environment variables starting with LC_.

names

Traditional and POSIX bc have single letter names for functions, variables and arrays. They have been extended to be multi-character names that start with a letter and may contain letters, numbers and the underscore character.

Strings

Strings are not allowed to contain NUL characters. POSIX says all characters must be included in strings.

last

POSIX bc does not have a \fBlast variable. Some implementations of bc use the period (.) in a similar way.

comparisons

POSIX bc allows comparisons only in the if statement, the while statement, and the second expression of the for statement. Also, only one relational operation is allowed in each of those statements.

if statement, else clause

POSIX bc does not have an else clause.

for statement

POSIX bc requires all expressions to be present in the for statement.

&&, ||, !

POSIX bc does not have the logical operators.

read function

POSIX bc does not have a read function.

print statement

POSIX bc does not have a print statement.

continue statement

POSIX bc does not have a continue statement.

array parameters

POSIX bc does not (currently) support array parameters in full. The POSIX grammar allows for arrays in function definitions, but does not provide a method to specify an array as an actual parameter. (This is most likely an oversight in the grammar.) Traditional implementations of bc have only call by value array parameters.

function format

POSIX bc requires the opening brace on the same line as the define key word and the auto statement on the next line.

=+, =-, =*, =/, =%, =^

POSIX bc does not require these "old style" assignment operators to be defined. This version may allow these "old style" assignments. Use the limits statement to see if the installed version supports them. If it does support the "old style" assignment operators, the statement "a =- 1" will decrement a by 1 instead of setting a to the value -1.

spaces in numbers

Other implementations of bc allow spaces in numbers. For example, "x=1 3" would assign the value 13 to the variable x. The same statement would cause a syntax error in this version of bc.

errors and execution

This implementation varies from other implementations in terms of what code will be executed when syntax and other errors are found in the program. If a syntax error is found in a function definition, error recovery tries to find the beginning of a statement and continue to parse the function. Once a syntax error is found in the function, the function will not be callable and becomes undefined. Syntax errors in the interactive execution code will invalidate the current execution block. The execution block is terminated by an end of line that appears after a complete sequence of statements. For example,

 
a = 1
b = 2

has two execution blocks and

 
{ a = 1
  b = 2 }

has one execution block. Any runtime error will terminate the execution of the current execution block. A runtime warning will not terminate the current execution block.

Interrupts

During an interactive session, the SIGINT signal (usually generated by the control-C character from the terminal) will cause execution of the current execution block to be interrupted. It will display a "runtime" error indicating which function was interrupted. After all runtime structures have been cleaned up, a message will be printed to notify the user that bc is ready for more input. All previously defined functions remain defined and the value of all non-auto variables are the value at the point of interruption. All auto variables and function parameters are removed during the clean up process. During a non-interactive session, the SIGINT signal will terminate the entire run of bc.


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