bc: Comparison with Other Implementations
8 Comparison with Other Implementations
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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'.
Info Catalog
bc: Readline and Libedit Options
bc: Top
bc: Limits