bc.1

Unix操作系统minix 2.0源码

.PP
The following is the definition of the exponential function used in the
math library.  This function is written in POSIX \fBbc\fR.
.nf
.RS
\fB
scale = 20

/* Uses the fact that e^x = (e^(x/2))^2
   When x is small enough, we use the series:
     e^x = 1 + x + x^2/2! + x^3/3! + ...
*/

define e(x) {
  auto  a, d, e, f, i, m, v, z

  /* Check the sign of x. */
  if (x<0) {
    m = 1
    x = -x
  } 

  /* Precondition x. */
  z = scale;
  scale = 4 + z + .44*x;
  while (x > 1) {
    f += 1;
    x /= 2;
  }

  /* Initialize the variables. */
  v = 1+x
  a = x
  d = 1

  for (i=2; 1; i++) {
    e = (a *= x) / (d *= i)
    if (e == 0) {
      if (f>0) while (f--)  v = v*v;
      scale = z
      if (m) return (1/v);
      return (v/1);
    }
    v += e
  }
}
\fR
.RE
.fi
.PP
The following is code that uses the extended features of \fBbc\fR to
implement a simple program for calculating checkbook balances.  This
program is best kept in a file so that it can be used many times 
without having to retype it at every use.
.nf
.RS
\fB
scale=2
print "\enCheck book program!\en"
print "  Remember, deposits are negative transactions.\en"
print "  Exit by a 0 transaction.\en\en"

print "Initial balance? "; bal = read()
bal /= 1
print "\en"
while (1) {
  "current balance = "; bal
  "transaction? "; trans = read()
  if (trans == 0) break;
  bal -= trans
  bal /= 1
}
quit
\fR
.RE
.fi
.PP
The following is the definition of the recursive factorial function.
.nf
.RS
\fB
define f (x) {
  if (x <= 1) return (1);
  return (f(x-1) * x);
}
\fR
.RE
.fi
.SS DIFFERENCES
This version of 
.B 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
.I dc(1).
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.
.PP
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.
.IP LANG 11n
This version does not conform to the POSIX standard in the processing
of the LANG environment variable and all environment variables starting
with LC_.
.IP names
Traditional and POSIX
.B 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.
.IP Strings
Strings are not allowed to contain NUL characters.  POSIX says all characters
must be included in strings.
.IP last
POSIX \fBbc\fR does not have a \fBlast\fR variable.  Some implementations
of \fBbc\fR use the period (.) in a similar way.  
.IP comparisons
POSIX \fBbc\fR 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.
.IP "if statement, else clause"
POSIX \fBbc\fR does not have an else clause.
.IP "for statement"
POSIX \fBbc\fR requires all expressions to be present in the for statement.
.IP "&&, ||, !"
POSIX \fBbc\fR does not have the logical operators.
.IP "read function"
POSIX \fBbc\fR does not have a read function.
.IP "print statement"
POSIX \fBbc\fR does not have a print statement .
.IP "continue statement"
POSIX \fBbc\fR does not have a continue statement.
.IP "array parameters"
POSIX \fBbc\fR does not have array parameters.  Other implementations
of \fBbc\fR may have call by value array parameters.
.IP "=+, =-, =*, =/, =%, =^"
POSIX \fBbc\fR 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 \fBa\fR by 1 instead of setting \fBa\fR to the
value -1.
.IP "spaces in numbers"
Other implementations of \fBbc\fR 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 \fBbc\fR.
.IP "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, 
.nf
.RS
a = 1
b = 2
.RE
.fi
has two execution blocks and
.nf
.RS
{ a = 1
  b = 2 }
.RE
.fi
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.
.IP "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 \fBbc\fR 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 \fBbc\fR.
.SS LIMITS
The following are the limits currently in place for this 
.B bc
processor.  Some of them may have been changed by an installation.
Use the limits statement to see the actual values.
.IP BC_BASE_MAX
The maximum output base is currently set at 999.  The maximum input base
is 16.
.IP BC_DIM_MAX
This is currently an arbitrary limit of 65535 as distributed.  Your
installation may be different.
.IP BC_SCALE_MAX
The number of digits after the decimal point is limited to INT_MAX digits.
Also, the number of digits before the decimal point is limited to INT_MAX
digits.
.IP BC_STRING_MAX
The limit on the number of characters in a string is INT_MAX characters.
.IP exponent
The value of the exponent in the raise operation (^) is limited to LONG_MAX.
.IP multiply
The multiply routine may yield incorrect results if a number
has more than LONG_MAX / 90 total digits.  For 32 bit longs, this number is
23,860,929 digits.
.IP "code size"
Each function and the "main" program are limited to 10240 bytes of
compiled byte code each.  This limit (BC_MAX_SEGS) can be easily changed
to have more than 10 segments of 1024 bytes.
.IP "variable names"
The current limit on the number of unique names is 32767 for each of
simple variables, arrays and functions.
.SH FILES
In most installations, \fBbc\fR is completely self-contained.
Where executable size is of importance or the C compiler does
not deal with very long strings, \fBbc\fR will read
the standard math library from the file /usr/local/lib/libmath.b.
(The actual location may vary.  It may be /lib/libmath.b.)
.SH DIAGNOSTICS
If any file on the command line can not be opened, \fBbc\fR will report
that the file is unavailable and terminate.  Also, there are compile
and run time diagnostics that should be self-explanatory.
.SH BUGS
Error recovery is not very good yet.
.SH AUTHOR
.nf
Philip A. Nelson
[e-mail]
.fi
.SH ACKNOWLEDGEMENTS
The author would like to thank Steve Sommars ([e-mail]) for
his extensive help in testing the implementation.  Many great suggestions
were given.  This is a much better product due to his involvement.