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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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BC(1P) POSIX Programmer's Manual BC(1P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
bc — arbitrary-precision arithmetic language
bc [-l] [file...]
The bc utility shall implement an arbitrary precision calculator.
It shall take input from any files given, then read from the
standard input. If the standard input and standard output to bc
are attached to a terminal, the invocation of bc shall be
considered to be interactive, causing behavioral constraints
described in the following sections.
The bc utility shall conform to the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
-l (The letter ell.) Define the math functions and
initialize scale to 20, instead of the default zero; see
the EXTENDED DESCRIPTION section.
The following operand shall be supported:
file A pathname of a text file containing bc program
statements. After all files have been read, bc shall
read the standard input.
See the INPUT FILES section.
Input files shall be text files containing a sequence of comments,
statements, and function definitions that shall be executed as
they are read.
The following environment variables shall affect the execution of
bc:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base
Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables for the precedence of
internationalization variables used to determine the
values of locale categories.)
LC_ALL If set to a non-empty string value, override the values
of all the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences
of bytes of text data as characters (for example,
single-byte as opposed to multi-byte characters in
arguments and input files).
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
Default.
The output of the bc utility shall be controlled by the program
read, and consist of zero or more lines containing the value of
all executed expressions without assignments. The radix and
precision of the output shall be controlled by the values of the
obase and scale variables; see the EXTENDED DESCRIPTION section.
The standard error shall be used only for diagnostic messages.
None.
Grammar
The grammar in this section and the lexical conventions in the
following section shall together describe the syntax for bc
programs. The general conventions for this style of grammar are
described in Section 1.3, Grammar Conventions. A valid program
can be represented as the non-terminal symbol program in the
grammar. This formal syntax shall take precedence over the text
syntax description.
%token EOF NEWLINE STRING LETTER NUMBER
%token MUL_OP
/* '*', '/', '%' */
%token ASSIGN_OP
/* '=', '+=', '-=', '*=', '/=', '%=', '^=' */
%token REL_OP
/* '==', '<=', '>=', '!=', '<', '>' */
%token INCR_DECR
/* '++', '--' */
%token Define Break Quit Length
/* 'define', 'break', 'quit', 'length' */
%token Return For If While Sqrt
/* 'return', 'for', 'if', 'while', 'sqrt' */
%token Scale Ibase Obase Auto
/* 'scale', 'ibase', 'obase', 'auto' */
%start program
%%
program : EOF
| input_item program
;
input_item : semicolon_list NEWLINE
| function
;
semicolon_list : /* empty */
| statement
| semicolon_list ';' statement
| semicolon_list ';'
;
statement_list : /* empty */
| statement
| statement_list NEWLINE
| statement_list NEWLINE statement
| statement_list ';'
| statement_list ';' statement
;
statement : expression
| STRING
| Break
| Quit
| Return
| Return '(' return_expression ')'
| For '(' expression ';'
relational_expression ';'
expression ')' statement
| If '(' relational_expression ')' statement
| While '(' relational_expression ')' statement
| '{' statement_list '}'
;
function : Define LETTER '(' opt_parameter_list ')'
'{' NEWLINE opt_auto_define_list
statement_list '}'
;
opt_parameter_list : /* empty */
| parameter_list
;
parameter_list : LETTER
| define_list ',' LETTER
;
opt_auto_define_list : /* empty */
| Auto define_list NEWLINE
| Auto define_list ';'
;
define_list : LETTER
| LETTER '[' ']'
| define_list ',' LETTER
| define_list ',' LETTER '[' ']'
;
opt_argument_list : /* empty */
| argument_list
;
argument_list : expression
| LETTER '[' ']' ',' argument_list
;
relational_expression : expression
| expression REL_OP expression
;
return_expression : /* empty */
| expression
;
expression : named_expression
| NUMBER
| '(' expression ')'
| LETTER '(' opt_argument_list ')'
| '-' expression
| expression '+' expression
| expression '-' expression
| expression MUL_OP expression
| expression '^' expression
| INCR_DECR named_expression
| named_expression INCR_DECR
| named_expression ASSIGN_OP expression
| Length '(' expression ')'
| Sqrt '(' expression ')'
| Scale '(' expression ')'
;
named_expression : LETTER
| LETTER '[' expression ']'
| Scale
| Ibase
| Obase
;
Lexical Conventions in bc
The lexical conventions for bc programs, with respect to the
preceding grammar, shall be as follows:
1. Except as noted, bc shall recognize the longest possible token
or delimiter beginning at a given point.
2. A comment shall consist of any characters beginning with the
two adjacent characters "/*" and terminated by the next
occurrence of the two adjacent characters "*/". Comments
shall have no effect except to delimit lexical tokens.
3. The <newline> shall be recognized as the token NEWLINE.
4. The token STRING shall represent a string constant; it shall
consist of any characters beginning with the double-quote
character ('"') and terminated by another occurrence of the
double-quote character. The value of the string is the
sequence of all characters between, but not including, the two
double-quote characters. All characters shall be taken
literally from the input, and there is no way to specify a
string containing a double-quote character. The length of the
value of each string shall be limited to {BC_STRING_MAX}
bytes.
5. A <blank> shall have no effect except as an ordinary character
if it appears within a STRING token, or to delimit a lexical
token other than STRING.
6. The combination of a <backslash> character immediately
followed by a <newline> shall have no effect other than to
delimit lexical tokens with the following exceptions:
* It shall be interpreted as the character sequence
"\<newline>" in STRING tokens.
* It shall be ignored as part of a multi-line NUMBER token.
7. The token NUMBER shall represent a numeric constant. It shall
be recognized by the following grammar:
NUMBER : integer
| '.' integer
| integer '.'
| integer '.' integer
;
integer : digit
| integer digit
;
digit : 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7
| 8 | 9 | A | B | C | D | E | F
;
8. The value of a NUMBER token shall be interpreted as a numeral
in the base specified by the value of the internal register
ibase (described below). Each of the digit characters shall
have the value from 0 to 15 in the order listed here, and the
<period> character shall represent the radix point. The
behavior is undefined if digits greater than or equal to the
value of ibase appear in the token. However, note the
exception for single-digit values being assigned to ibase and
obase themselves, in Operations in bc.
9. The following keywords shall be recognized as tokens:
auto ibase length return while
break if obase scale
define for quit sqrt
10. Any of the following characters occurring anywhere except
within a keyword shall be recognized as the token LETTER:
a b c d e f g h i j k l m n o p q r s t u v w x y z
11. The following single-character and two-character sequences
shall be recognized as the token ASSIGN_OP:
= += -= *= /= %= ^=
12. If an '=' character, as the beginning of a token, is followed
by a '-' character with no intervening delimiter, the behavior
is undefined.
13. The following single-characters shall be recognized as the
token MUL_OP:
* / %
14. The following single-character and two-character sequences
shall be recognized as the token REL_OP:
== <= >= != < >
15. The following two-character sequences shall be recognized as
the token INCR_DECR:
++ --
16. The following single characters shall be recognized as tokens
whose names are the character:
<newline> ( ) , + - ; [ ] ^ { }
17. The token EOF is returned when the end of input is reached.
Operations in bc
There are three kinds of identifiers: ordinary identifiers, array
identifiers, and function identifiers. All three types consist of
single lowercase letters. Array identifiers shall be followed by
square brackets ("[]"). An array subscript is required except in
an argument or auto list. Arrays are singly dimensioned and can
contain up to {BC_DIM_MAX} elements. Indexing shall begin at zero
so an array is indexed from 0 to {BC_DIM_MAX}-1. Subscripts shall
be truncated to integers. The application shall ensure that
function identifiers are followed by parentheses, possibly
enclosing arguments. The three types of identifiers do not
conflict.
The following table summarizes the rules for precedence and
associativity of all operators. Operators on the same line shall
have the same precedence; rows are in order of decreasing
precedence.
Table: Operators in bc
┌───────────────────────────┬───────────────┐
│ Operator │ Associativity │
├───────────────────────────┼───────────────┤
│ ++, -- │ N/A │
│ unary - │ N/A │
│ ^ │ Right to left │
│ *, /, % │ Left to right │
│ +, binary - │ Left to right │
│ =, +=, -=, *=, /=, %=, ^= │ Right to left │
│ ==, <=, >=, !=, <, > │ None │
└───────────────────────────┴───────────────┘
Each expression or named expression has a scale, which is the
number of decimal digits that shall be maintained as the
fractional portion of the expression.
Named expressions are places where values are stored. Named
expressions shall be valid on the left side of an assignment. The
value of a named expression shall be the value stored in the place
named. Simple identifiers and array elements are named
expressions; they have an initial value of zero and an initial
scale of zero.
The internal registers scale, ibase, and obase are all named
expressions. The scale of an expression consisting of the name of
one of these registers shall be zero; values assigned to any of
these registers are truncated to integers. The scale register
shall contain a global value used in computing the scale of
expressions (as described below). The value of the register scale
is limited to 0 ≤ scale ≤ {BC_SCALE_MAX} and shall have a default
value of zero. The ibase and obase registers are the input and
output number radix, respectively. The value of ibase shall be
limited to:
2 ≤ ibase ≤ 16
The value of obase shall be limited to:
2 ≤ obase ≤ {BC_BASE_MAX}
When either ibase or obase is assigned a single digit value from
the list in Lexical Conventions in bc, the value shall be assumed
in hexadecimal. (For example, ibase=A sets to base ten, regardless
of the current ibase value.) Otherwise, the behavior is undefined
when digits greater than or equal to the value of ibase appear in
the input. Both ibase and obase shall have initial values of 10.
Internal computations shall be conducted as if in decimal,
regardless of the input and output bases, to the specified number
of decimal digits. When an exact result is not achieved (for
example, scale=0; 3.2/1), the result shall be truncated.
For all values of obase specified by this volume of POSIX.1‐2017,
bc shall output numeric values by performing each of the following
steps in order:
1. If the value is less than zero, a <hyphen-minus> ('-')
character shall be output.
2. One of the following is output, depending on the numerical
value:
* If the absolute value of the numerical value is greater
than or equal to one, the integer portion of the value
shall be output as a series of digits appropriate to obase
(as described below), most significant digit first. The
most significant non-zero digit shall be output next,
followed by each successively less significant digit.
* If the absolute value of the numerical value is less than
one but greater than zero and the scale of the numerical
value is greater than zero, it is unspecified whether the
character 0 is output.
* If the numerical value is zero, the character 0 shall be
output.
3. If the scale of the value is greater than zero and the numeric
value is not zero, a <period> character shall be output,
followed by a series of digits appropriate to obase (as
described below) representing the most significant portion of
the fractional part of the value. If s represents the scale of
the value being output, the number of digits output shall be s
if obase is 10, less than or equal to s if obase is greater
than 10, or greater than or equal to s if obase is less than
10. For obase values other than 10, this should be the number
of digits needed to represent a precision of 10s.
For obase values from 2 to 16, valid digits are the first obase of
the single characters:
0 1 2 3 4 5 6 7 8 9 A B C D E F
which represent the values zero to 15, inclusive, respectively.
For bases greater than 16, each digit shall be written as a
separate multi-digit decimal number. Each digit except the most
significant fractional digit shall be preceded by a single
<space>. For bases from 17 to 100, bc shall write two-digit
decimal numbers; for bases from 101 to 1000, three-digit decimal
strings, and so on. For example, the decimal number 1024 in base
25 would be written as:
01 15 24
and in base 125, as:
008 024
Very large numbers shall be split across lines with 70 characters
per line in the POSIX locale; other locales may split at different
character boundaries. Lines that are continued shall end with a
<backslash>.
A function call shall consist of a function name followed by
parentheses containing a <comma>-separated list of expressions,
which are the function arguments. A whole array passed as an
argument shall be specified by the array name followed by empty
square brackets. All function arguments shall be passed by value.
As a result, changes made to the formal parameters shall have no
effect on the actual arguments. If the function terminates by
executing a return statement, the value of the function shall be
the value of the expression in the parentheses of the return
statement or shall be zero if no expression is provided or if
there is no return statement.
The result of sqrt(expression) shall be the square root of the
expression. The result shall be truncated in the least significant
decimal place. The scale of the result shall be the scale of the
expression or the value of scale, whichever is larger.
The result of length(expression) shall be the total number of
significant decimal digits in the expression. The scale of the
result shall be zero.
The result of scale(expression) shall be the scale of the
expression. The scale of the result shall be zero.
A numeric constant shall be an expression. The scale shall be the
number of digits that follow the radix point in the input
representing the constant, or zero if no radix point appears.
The sequence ( expression ) shall be an expression with the same
value and scale as expression. The parentheses can be used to
alter the normal precedence.
The semantics of the unary and binary operators are as follows:
-expression
The result shall be the negative of the expression. The
scale of the result shall be the scale of expression.
The unary increment and decrement operators shall not modify the
scale of the named expression upon which they operate. The scale
of the result shall be the scale of that named expression.
++named-expression
The named expression shall be incremented by one. The result
shall be the value of the named expression after
incrementing.
--named-expression
The named expression shall be decremented by one. The result
shall be the value of the named expression after
decrementing.
named-expression++
The named expression shall be incremented by one. The result
shall be the value of the named expression before
incrementing.
named-expression--
The named expression shall be decremented by one. The result
shall be the value of the named expression before
decrementing.
The exponentiation operator, <circumflex> ('^'), shall bind right
to left.
expression^expression
The result shall be the first expression raised to the power
of the second expression. If the second expression is not
an integer, the behavior is undefined. If a is the scale of
the left expression and b is the absolute value of the right
expression, the scale of the result shall be:
if b >= 0 min(a * b, max(scale, a)) if b < 0 scale
The multiplicative operators ('*', '/', '%') shall bind left to
right.
expression*expression
The result shall be the product of the two expressions. If a
and b are the scales of the two expressions, then the scale
of the result shall be:
min(a+b,max(scale,a,b))
expression/expression
The result shall be the quotient of the two expressions. The
scale of the result shall be the value of scale.
expression%expression
For expressions a and b, a%b shall be evaluated equivalent
to the steps:
1. Compute a/b to current scale.
2. Use the result to compute:
a - (a / b) * b
to scale:
max(scale + scale(b), scale(a))
The scale of the result shall be:
max(scale + scale(b), scale(a))
When scale is zero, the '%' operator is the mathematical
remainder operator.
The additive operators ('+', '-') shall bind left to right.
expression+expression
The result shall be the sum of the two expressions. The
scale of the result shall be the maximum of the scales of
the expressions.
expression-expression
The result shall be the difference of the two expressions.
The scale of the result shall be the maximum of the scales
of the expressions.
The assignment operators ('=', "+=", "-=", "*=", "/=", "%=", "^=")
shall bind right to left.
named-expression=expression
This expression shall result in assigning the value of the
expression on the right to the named expression on the left.
The scale of both the named expression and the result shall
be the scale of expression.
The compound assignment forms:
named-expression <operator>= expression
shall be equivalent to:
named-expression=named-expression <operator> expression
except that the named-expression shall be evaluated only once.
Unlike all other operators, the relational operators ('<', '>',
"<=", ">=", "==", "!=") shall be only valid as the object of an
if, while, or inside a for statement.
expression1<expression2
The relation shall be true if the value of expression1 is
strictly less than the value of expression2.
expression1>expression2
The relation shall be true if the value of expression1 is
strictly greater than the value of expression2.
expression1<=expression2
The relation shall be true if the value of expression1 is
less than or equal to the value of expression2.
expression1>=expression2
The relation shall be true if the value of expression1 is
greater than or equal to the value of expression2.
expression1==expression2
The relation shall be true if the values of expression1 and
expression2 are equal.
expression1!=expression2
The relation shall be true if the values of expression1 and
expression2 are unequal.
There are only two storage classes in bc: global and automatic
(local). Only identifiers that are local to a function need be
declared with the auto command. The arguments to a function shall
be local to the function. All other identifiers are assumed to be
global and available to all functions. All identifiers, global and
local, have initial values of zero. Identifiers declared as auto
shall be allocated on entry to the function and released on
returning from the function. They therefore do not retain values
between function calls. Auto arrays shall be specified by the
array name followed by empty square brackets. On entry to a
function, the old values of the names that appear as parameters
and as automatic variables shall be pushed onto a stack. Until the
function returns, reference to these names shall refer only to the
new values.
References to any of these names from other functions that are
called from this function also refer to the new value until one of
those functions uses the same name for a local variable.
When a statement is an expression, unless the main operator is an
assignment, execution of the statement shall write the value of
the expression followed by a <newline>.
When a statement is a string, execution of the statement shall
write the value of the string.
Statements separated by <semicolon> or <newline> characters shall
be executed sequentially. In an interactive invocation of bc, each
time a <newline> is read that satisfies the grammatical
production:
input_item : semicolon_list NEWLINE
the sequential list of statements making up the semicolon_list
shall be executed immediately and any output produced by that
execution shall be written without any delay due to buffering.
In an if statement (if(relation) statement), the statement shall
be executed if the relation is true.
The while statement (while(relation) statement) implements a loop
in which the relation is tested; each time the relation is true,
the statement shall be executed and the relation retested. When
the relation is false, execution shall resume after statement.
A for statement(for(expression; relation; expression) statement)
shall be the same as:
first-expression
while (relation) {
statement
last-expression
}
The application shall ensure that all three expressions are
present.
The break statement shall cause termination of a for or while
statement.
The auto statement (auto identifier [,identifier] ...) shall cause
the values of the identifiers to be pushed down. The identifiers
can be ordinary identifiers or array identifiers. Array
identifiers shall be specified by following the array name by
empty square brackets. The application shall ensure that the auto
statement is the first statement in a function definition.
A define statement:
define LETTER ( opt_parameter_list ) {
opt_auto_define_list
statement_list
}
defines a function named LETTER. If a function named LETTER was
previously defined, the define statement shall replace the
previous definition. The expression:
LETTER ( opt_argument_list )
shall invoke the function named LETTER. The behavior is undefined
if the number of arguments in the invocation does not match the
number of parameters in the definition. Functions shall be defined
before they are invoked. A function shall be considered to be
defined within its own body, so recursive calls are valid. The
values of numeric constants within a function shall be interpreted
in the base specified by the value of the ibase register when the
function is invoked.
The return statements (return and return(expression)) shall cause
termination of a function, popping of its auto variables, and
specification of the result of the function. The first form shall
be equivalent to return(0). The value and scale of the result
returned by the function shall be the value and scale of the
expression returned.
The quit statement (quit) shall stop execution of a bc program at
the point where the statement occurs in the input, even if it
occurs in a function definition, or in an if, for, or while
statement.
The following functions shall be defined when the -l option is
specified:
s( expression )
Sine of argument in radians.
c( expression )
Cosine of argument in radians.
a( expression )
Arctangent of argument.
l( expression )
Natural logarithm of argument.
e( expression )
Exponential function of argument.
j( expression1, expression2 )
Bessel function of expression2 of the first kind of integer
order expression1.
The scale of the result returned by these functions shall be the
value of the scale register at the time the function is invoked.
The value of the scale register after these functions have
completed their execution shall be the same value it had upon
invocation. The behavior is undefined if any of these functions is
invoked with an argument outside the domain of the mathematical
function.
The following exit values shall be returned:
0 All input files were processed successfully.
unspecified
An error occurred.
If any file operand is specified and the named file cannot be
accessed, bc shall write a diagnostic message to standard error
and terminate without any further action.
In an interactive invocation of bc, the utility should print an
error message and recover following any error in the input. In a
non-interactive invocation of bc, invalid input causes undefined
behavior.
The following sections are informative.
Automatic variables in bc do not work in exactly the same way as
in either C or PL/1.
For historical reasons, the exit status from bc cannot be relied
upon to indicate that an error has occurred. Returning zero after
an error is possible. Therefore, bc should be used primarily by
interactive users (who can react to error messages) or by
application programs that can somehow validate the answers
returned as not including error messages.
The bc utility always uses the <period> ('.') character to
represent a radix point, regardless of any decimal-point character
specified as part of the current locale. In languages like C or
awk, the <period> character is used in program source, so it can
be portable and unambiguous, while the locale-specific character
is used in input and output. Because there is no distinction
between source and input in bc, this arrangement would not be
possible. Using the locale-specific character in bc's input would
introduce ambiguities into the language; consider the following
example in a locale with a <comma> as the decimal-point character:
define f(a,b) {
...
}
...
f(1,2,3)
Because of such ambiguities, the <period> character is used in
input. Having input follow different conventions from output would
be confusing in either pipeline usage or interactive usage, so the
<period> is also used in output.
In the shell, the following assigns an approximation of the first
ten digits of 'π' to the variable x:
x=$(printf "%s\n" 'scale = 10; 104348/33215' | bc)
The following bc program prints the same approximation of 'π',
with a label, to standard output:
scale = 10
"pi equals "
104348 / 33215
The following defines a function to compute an approximate value
of the exponential function (note that such a function is
predefined if the -l option is specified):
scale = 20
define e(x){
auto a, b, c, i, s
a = 1
b = 1
s = 1
for (i = 1; 1 == 1; i++){
a = a*x
b = b*i
c = a/b
if (c == 0) {
return(s)
}
s = s+c
}
}
The following prints approximate values of the exponential
function of the first ten integers:
for (i = 1; i <= 10; ++i) {
e(i)
}
The bc utility is implemented historically as a front-end
processor for dc; dc was not selected to be part of this volume of
POSIX.1‐2017 because bc was thought to have a more intuitive
programmatic interface. Current implementations that implement bc
using dc are expected to be compliant.
The exit status for error conditions has been left unspecified for
several reasons:
* The bc utility is used in both interactive and non-interactive
situations. Different exit codes may be appropriate for the
two uses.
* It is unclear when a non-zero exit should be given; divide-by-
zero, undefined functions, and syntax errors are all
possibilities.
* It is not clear what utility the exit status has.
* In the 4.3 BSD, System V, and Ninth Edition implementations,
bc works in conjunction with dc. The dc utility is the
parent, bc is the child. This was done to cleanly terminate bc
if dc aborted.
The decision to have bc exit upon encountering an inaccessible
input file is based on the belief that bc file1 file2 is used most
often when at least file1 contains data/function
declarations/initializations. Having bc continue with prerequisite
files missing is probably not useful. There is no implication in
the CONSEQUENCES OF ERRORS section that bc must check all its
files for accessibility before opening any of them.
There was considerable debate on the appropriateness of the
language accepted by bc. Several reviewers preferred to see
either a pure subset of the C language or some changes to make the
language more compatible with C. While the bc language has some
obvious similarities to C, it has never claimed to be compatible
with any version of C. An interpreter for a subset of C might be a
very worthwhile utility, and it could potentially make bc
obsolete. However, no such utility is known in historical
practice, and it was not within the scope of this volume of
POSIX.1‐2017 to define such a language and utility. If and when
they are defined, it may be appropriate to include them in a
future version of this standard. This left the following
alternatives:
1. Exclude any calculator language from this volume of
POSIX.1‐2017.
The consensus of the standard developers was that a simple
programmatic calculator language is very useful for both
applications and interactive users. The only arguments for
excluding any calculator were that it would become obsolete if
and when a C-compatible one emerged, or that the absence would
encourage the development of such a C-compatible one. These
arguments did not sufficiently address the needs of current
application developers.
2. Standardize the historical dc, possibly with minor
modifications.
The consensus of the standard developers was that dc is a
fundamentally less usable language and that that would be far
too severe a penalty for avoiding the issue of being similar
to but incompatible with C.
3. Standardize the historical bc, possibly with minor
modifications.
This was the approach taken. Most of the proponents of
changing the language would not have been satisfied until most
or all of the incompatibilities with C were resolved. Since
most of the changes considered most desirable would break
historical applications and require significant modification
to historical implementations, almost no modifications were
made. The one significant modification that was made was the
replacement of the historical bc assignment operators "=+",
and so on, with the more modern "+=", and so on. The older
versions are considered to be fundamentally flawed because of
the lexical ambiguity in uses like a=-1.
In order to permit implementations to deal with backwards-
compatibility as they see fit, the behavior of this one
ambiguous construct was made undefined. (At least three
implementations have been known to support this change
already, so the degree of change involved should not be
great.)
The '%' operator is the mathematical remainder operator when scale
is zero. The behavior of this operator for other values of scale
is from historical implementations of bc, and has been maintained
for the sake of historical applications despite its non-intuitive
nature.
Historical implementations permit setting ibase and obase to a
broader range of values. This includes values less than 2, which
were not seen as sufficiently useful to standardize. These
implementations do not interpret input properly for values of
ibase that are greater than 16. This is because numeric constants
are recognized syntactically, rather than lexically, as described
in this volume of POSIX.1‐2017. They are built from lexical tokens
of single hexadecimal digits and <period> characters. Since
<blank> characters between tokens are not visible at the syntactic
level, it is not possible to recognize the multi-digit ``digits''
used in the higher bases properly. The ability to recognize input
in these bases was not considered useful enough to require
modifying these implementations. Note that the recognition of
numeric constants at the syntactic level is not a problem with
conformance to this volume of POSIX.1‐2017, as it does not impact
the behavior of conforming applications (and correct bc programs).
Historical implementations also accept input with all of the
digits '0'-'9' and 'A'-'F' regardless of the value of ibase; since
digits with value greater than or equal to ibase are not really
appropriate, the behavior when they appear is undefined, except
for the common case of:
ibase=8;
/* Process in octal base. */
...
ibase=A
/* Restore decimal base. */
In some historical implementations, if the expression to be
written is an uninitialized array element, a leading <space>
and/or up to four leading 0 characters may be output before the
character zero. This behavior is considered a bug; it is unlikely
that any currently conforming application relies on:
echo 'b[3]' | bc
returning 00000 rather than 0.
Exact calculation of the number of fractional digits to output for
a given value in a base other than 10 can be computationally
expensive. Historical implementations use a faster approximation,
and this is permitted. Note that the requirements apply only to
values of obase that this volume of POSIX.1‐2017 requires
implementations to support (in particular, not to 1, 0, or
negative bases, if an implementation supports them as an
extension).
Historical implementations of bc did not allow array parameters to
be passed as the last parameter to a function. New implementations
are encouraged to remove this restriction even though it is not
required by the grammar.
None.
Section 1.3, Grammar Conventions, awk(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 BC(1P)
Pages that refer to this page: printf(1p)
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