manuals/dc/HN.1.md

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# Name

dc - arbitrary-precision decimal reverse-Polish notation calculator

# SYNOPSIS

**dc** [**-hiPRvVx**] [**-\-version**] [**-\-help**] [**-\-interactive**] [**-\-no-prompt**] [**-\-no-read-prompt**] [**-\-extended-register**] [**-e** *expr*] [**-\-expression**=*expr*...] [**-f** *file*...] [**-\-file**=*file*...] [*file*...]

# DESCRIPTION

dc(1) is an arbitrary-precision calculator. It uses a stack (reverse Polish
notation) to store numbers and results of computations. Arithmetic operations
pop arguments off of the stack and push the results.

If no files are given on the command-line, then dc(1) reads from **stdin** (see
the **STDIN** section). Otherwise, those files are processed, and dc(1) will
then exit.

If a user wants to set up a standard environment, they can use **DC_ENV_ARGS**
(see the **ENVIRONMENT VARIABLES** section). For example, if a user wants the
**scale** always set to **10**, they can set **DC_ENV_ARGS** to **-e 10k**, and
this dc(1) will always start with a **scale** of **10**.

# OPTIONS

The following are the options that dc(1) accepts.

**-h**, **-\-help**

:   Prints a usage message and quits.

**-v**, **-V**, **-\-version**

:   Print the version information (copyright header) and exit.

**-i**, **-\-interactive**

:   Forces interactive mode. (See the **INTERACTIVE MODE** section.)

    This is a **non-portable extension**.

**-P**, **-\-no-prompt**

:   Disables the prompt in TTY mode. (The prompt is only enabled in TTY mode.
    See the **TTY MODE** section.) This is mostly for those users that do not
    want a prompt or are not used to having them in dc(1). Most of those users
    would want to put this option in **DC_ENV_ARGS**.

    These options override the **DC_PROMPT** and **DC_TTY_MODE** environment
    variables (see the **ENVIRONMENT VARIABLES** section).

    This is a **non-portable extension**.

**-R**, **-\-no-read-prompt**

:   Disables the read prompt in TTY mode. (The read prompt is only enabled in
    TTY mode. See the **TTY MODE** section.) This is mostly for those users that
    do not want a read prompt or are not used to having them in dc(1). Most of
    those users would want to put this option in **BC_ENV_ARGS** (see the
    **ENVIRONMENT VARIABLES** section). This option is also useful in hash bang
    lines of dc(1) scripts that prompt for user input.

    This option does not disable the regular prompt because the read prompt is
    only used when the **?** command is used.

    These options *do* override the **DC_PROMPT** and **DC_TTY_MODE**
    environment variables (see the **ENVIRONMENT VARIABLES** section), but only
    for the read prompt.

    This is a **non-portable extension**.

**-x** **-\-extended-register**

:   Enables extended register mode. See the *Extended Register Mode* subsection
    of the **REGISTERS** section for more information.

    This is a **non-portable extension**.

**-e** *expr*, **-\-expression**=*expr*

:   Evaluates *expr*. If multiple expressions are given, they are evaluated in
    order. If files are given as well (see below), the expressions and files are
    evaluated in the order given. This means that if a file is given before an
    expression, the file is read in and evaluated first.

    If this option is given on the command-line (i.e., not in **DC_ENV_ARGS**,
    see the **ENVIRONMENT VARIABLES** section), then after processing all
    expressions and files, dc(1) will exit, unless **-** (**stdin**) was given
    as an argument at least once to **-f** or **-\-file**, whether on the
    command-line or in **DC_ENV_ARGS**. However, if any other **-e**,
    **-\-expression**, **-f**, or **-\-file** arguments are given after **-f-**
    or equivalent is given, dc(1) will give a fatal error and exit.

    This is a **non-portable extension**.

**-f** *file*, **-\-file**=*file*

:   Reads in *file* and evaluates it, line by line, as though it were read
    through **stdin**. If expressions are also given (see above), the
    expressions are evaluated in the order given.

    If this option is given on the command-line (i.e., not in **DC_ENV_ARGS**,
    see the **ENVIRONMENT VARIABLES** section), then after processing all
    expressions and files, dc(1) will exit, unless **-** (**stdin**) was given
    as an argument at least once to **-f** or **-\-file**. However, if any other
    **-e**, **-\-expression**, **-f**, or **-\-file** arguments are given after
    **-f-** or equivalent is given, dc(1) will give a fatal error and exit.

    This is a **non-portable extension**.

All long options are **non-portable extensions**.

# STDIN

If no files are given on the command-line and no files or expressions are given
by the **-f**, **-\-file**, **-e**, or **-\-expression** options, then dc(1)
read from **stdin**.

However, there is a caveat to this.

First, **stdin** is evaluated a line at a time. The only exception to this is if
a string has been finished, but not ended. This means that, except for escaped
brackets, all brackets must be balanced before dc(1) parses and executes.

# STDOUT

Any non-error output is written to **stdout**. In addition, if history (see the
**HISTORY** section) and the prompt (see the **TTY MODE** section) are enabled,
both are output to **stdout**.

**Note**: Unlike other dc(1) implementations, this dc(1) will issue a fatal
error (see the **EXIT STATUS** section) if it cannot write to **stdout**, so if
**stdout** is closed, as in **dc <file> >&-**, it will quit with an error. This
is done so that dc(1) can report problems when **stdout** is redirected to a
file.

If there are scripts that depend on the behavior of other dc(1) implementations,
it is recommended that those scripts be changed to redirect **stdout** to
**/dev/null**.

# STDERR

Any error output is written to **stderr**.

**Note**: Unlike other dc(1) implementations, this dc(1) will issue a fatal
error (see the **EXIT STATUS** section) if it cannot write to **stderr**, so if
**stderr** is closed, as in **dc <file> 2>&-**, it will quit with an error. This
is done so that dc(1) can exit with an error code when **stderr** is redirected
to a file.

If there are scripts that depend on the behavior of other dc(1) implementations,
it is recommended that those scripts be changed to redirect **stderr** to
**/dev/null**.

# SYNTAX

Each item in the input source code, either a number (see the **NUMBERS**
section) or a command (see the **COMMANDS** section), is processed and executed,
in order. Input is processed immediately when entered.

**ibase** is a register (see the **REGISTERS** section) that determines how to
interpret constant numbers. It is the "input" base, or the number base used for
interpreting input numbers. **ibase** is initially **10**. The max allowable
value for **ibase** is **16**. The min allowable value for **ibase** is **2**.
The max allowable value for **ibase** can be queried in dc(1) programs with the
**T** command.

**obase** is a register (see the **REGISTERS** section) that determines how to
output results. It is the "output" base, or the number base used for outputting
numbers. **obase** is initially **10**. The max allowable value for **obase** is
**DC_BASE_MAX** and can be queried with the **U** command. The min allowable
value for **obase** is **0**. If **obase** is **0**, values are output in
scientific notation, and if **obase** is **1**, values are output in engineering
notation. Otherwise, values are output in the specified base.

Outputting in scientific and engineering notations are **non-portable
extensions**.

The *scale* of an expression is the number of digits in the result of the
expression right of the decimal point, and **scale** is a register (see the
**REGISTERS** section) that sets the precision of any operations (with
exceptions). **scale** is initially **0**. **scale** cannot be negative. The max
allowable value for **scale** can be queried in dc(1) programs with the **V**
command.

**seed** is a register containing the current seed for the pseudo-random number
generator. If the current value of **seed** is queried and stored, then if it is
assigned to **seed** later, the pseudo-random number generator is guaranteed to
produce the same sequence of pseudo-random numbers that were generated after the
value of **seed** was first queried.

Multiple values assigned to **seed** can produce the same sequence of
pseudo-random numbers. Likewise, when a value is assigned to **seed**, it is not
guaranteed that querying **seed** immediately after will return the same value.
In addition, the value of **seed** will change after any call to the **'**
command or the **"** command that does not get receive a value of **0** or
**1**. The maximum integer returned by the **'** command can be queried with the
**W** command.

**Note**: The values returned by the pseudo-random number generator with the
**'** and **"** commands are guaranteed to **NOT** be cryptographically secure.
This is a consequence of using a seeded pseudo-random number generator. However,
they *are* guaranteed to be reproducible with identical **seed** values. This
means that the pseudo-random values from dc(1) should only be used where a
reproducible stream of pseudo-random numbers is *ESSENTIAL*. In any other case,
use a non-seeded pseudo-random number generator.

The pseudo-random number generator, **seed**, and all associated operations are
**non-portable extensions**.

## Comments

Comments go from **#** until, and not including, the next newline. This is a
**non-portable extension**.

# NUMBERS

Numbers are strings made up of digits, uppercase letters up to **F**, and at
most **1** period for a radix. Numbers can have up to **DC_NUM_MAX** digits.
Uppercase letters are equal to **9** + their position in the alphabet (i.e.,
**A** equals **10**, or **9+1**). If a digit or letter makes no sense with the
current value of **ibase**, they are set to the value of the highest valid digit
in **ibase**.

Single-character numbers (i.e., **A** alone) take the value that they would have
if they were valid digits, regardless of the value of **ibase**. This means that
**A** alone always equals decimal **10** and **F** alone always equals decimal
**15**.

In addition, dc(1) accepts numbers in scientific notation. These have the form
**\<number\>e\<integer\>**. The exponent (the portion after the **e**) must be
an integer. An example is **1.89237e9**, which is equal to **1892370000**.
Negative exponents are also allowed, so **4.2890e_3** is equal to **0.0042890**.

**WARNING**: Both the number and the exponent in scientific notation are
interpreted according to the current **ibase**, but the number is still
multiplied by **10\^exponent** regardless of the current **ibase**. For example,
if **ibase** is **16** and dc(1) is given the number string **FFeA**, the
resulting decimal number will be **2550000000000**, and if dc(1) is given the
number string **10e_4**, the resulting decimal number will be **0.0016**.

Accepting input as scientific notation is a **non-portable extension**.

# COMMANDS

The valid commands are listed below.

## Printing

These commands are used for printing.

Note that both scientific notation and engineering notation are available for
printing numbers. Scientific notation is activated by assigning **0** to
**obase** using **0o**, and engineering notation is activated by assigning **1**
to **obase** using **1o**. To deactivate them, just assign a different value to
**obase**.

Printing numbers in scientific notation and/or engineering notation is a
**non-portable extension**.

**p**

:   Prints the value on top of the stack, whether number or string, and prints a
    newline after.

    This does not alter the stack.

**n**

:   Prints the value on top of the stack, whether number or string, and pops it
    off of the stack.

**P**

:   Pops a value off the stack.

    If the value is a number, it is truncated and the absolute value of the
    result is printed as though **obase** is **256** and each digit is
    interpreted as an 8-bit ASCII character, making it a byte stream.

    If the value is a string, it is printed without a trailing newline.

    This is a **non-portable extension**.

**f**

:   Prints the entire contents of the stack, in order from newest to oldest,
    without altering anything.

    Users should use this command when they get lost.

## Arithmetic

These are the commands used for arithmetic.

**+**

:   The top two values are popped off the stack, added, and the result is pushed
    onto the stack. The *scale* of the result is equal to the max *scale* of
    both operands.

**-**

:   The top two values are popped off the stack, subtracted, and the result is
    pushed onto the stack. The *scale* of the result is equal to the max
    *scale* of both operands.

**\***

:   The top two values are popped off the stack, multiplied, and the result is
    pushed onto the stack. If **a** is the *scale* of the first expression and
    **b** is the *scale* of the second expression, the *scale* of the result
    is equal to **min(a+b,max(scale,a,b))** where **min()** and **max()** return
    the obvious values.

**/**

:   The top two values are popped off the stack, divided, and the result is
    pushed onto the stack. The *scale* of the result is equal to **scale**.

    The first value popped off of the stack must be non-zero.

**%**

:   The top two values are popped off the stack, remaindered, and the result is
    pushed onto the stack.

    Remaindering is equivalent to 1) Computing **a/b** to current **scale**, and
    2) Using the result of step 1 to calculate **a-(a/b)\*b** to *scale*
    **max(scale+scale(b),scale(a))**.

    The first value popped off of the stack must be non-zero.

**~**

:   The top two values are popped off the stack, divided and remaindered, and
    the results (divided first, remainder second) are pushed onto the stack.
    This is equivalent to **x y / x y %** except that **x** and **y** are only
    evaluated once.

    The first value popped off of the stack must be non-zero.

    This is a **non-portable extension**.

**\^**

:   The top two values are popped off the stack, the second is raised to the
    power of the first, and the result is pushed onto the stack. The *scale* of
    the result is equal to **scale**.

    The first value popped off of the stack must be an integer, and if that
    value is negative, the second value popped off of the stack must be
    non-zero.

**v**

:   The top value is popped off the stack, its square root is computed, and the
    result is pushed onto the stack. The *scale* of the result is equal to
    **scale**.

    The value popped off of the stack must be non-negative.

**\_**

:   If this command *immediately* precedes a number (i.e., no spaces or other
    commands), then that number is input as a negative number.

    Otherwise, the top value on the stack is popped and copied, and the copy is
    negated and pushed onto the stack. This behavior without a number is a
    **non-portable extension**.

**b**

:   The top value is popped off the stack, and if it is zero, it is pushed back
    onto the stack. Otherwise, its absolute value is pushed onto the stack.

    This is a **non-portable extension**.

**|**

:   The top three values are popped off the stack, a modular exponentiation is
    computed, and the result is pushed onto the stack.

    The first value popped is used as the reduction modulus and must be an
    integer and non-zero. The second value popped is used as the exponent and
    must be an integer and non-negative. The third value popped is the base and
    must be an integer.

    This is a **non-portable extension**.

**\$**

:   The top value is popped off the stack and copied, and the copy is truncated
    and pushed onto the stack.

    This is a **non-portable extension**.

**\@**

:   The top two values are popped off the stack, and the precision of the second
    is set to the value of the first, whether by truncation or extension.

    The first value popped off of the stack must be an integer and non-negative.

    This is a **non-portable extension**.

**H**

:   The top two values are popped off the stack, and the second is shifted left
    (radix shifted right) to the value of the first.

    The first value popped off of the stack must be an integer and non-negative.

    This is a **non-portable extension**.

**h**

:   The top two values are popped off the stack, and the second is shifted right
    (radix shifted left) to the value of the first.

    The first value popped off of the stack must be an integer and non-negative.

    This is a **non-portable extension**.

**G**

:   The top two values are popped off of the stack, they are compared, and a
    **1** is pushed if they are equal, or **0** otherwise.

    This is a **non-portable extension**.

**N**

:   The top value is popped off of the stack, and if it a **0**, a **1** is
    pushed; otherwise, a **0** is pushed.

    This is a **non-portable extension**.

**(**

:   The top two values are popped off of the stack, they are compared, and a
    **1** is pushed if the first is less than the second, or **0** otherwise.

    This is a **non-portable extension**.

**{**

:   The top two values are popped off of the stack, they are compared, and a
    **1** is pushed if the first is less than or equal to the second, or **0**
    otherwise.

    This is a **non-portable extension**.

**)**

:   The top two values are popped off of the stack, they are compared, and a
    **1** is pushed if the first is greater than the second, or **0** otherwise.

    This is a **non-portable extension**.

**}**

:   The top two values are popped off of the stack, they are compared, and a
    **1** is pushed if the first is greater than or equal to the second, or
    **0** otherwise.

    This is a **non-portable extension**.

**M**

:   The top two values are popped off of the stack. If they are both non-zero, a
    **1** is pushed onto the stack. If either of them is zero, or both of them
    are, then a **0** is pushed onto the stack.

    This is like the **&&** operator in bc(1), and it is *not* a short-circuit
    operator.

    This is a **non-portable extension**.

**m**

:   The top two values are popped off of the stack. If at least one of them is
    non-zero, a **1** is pushed onto the stack. If both of them are zero, then a
    **0** is pushed onto the stack.

    This is like the **||** operator in bc(1), and it is *not* a short-circuit
    operator.

    This is a **non-portable extension**.

## Pseudo-Random Number Generator

dc(1) has a built-in pseudo-random number generator. These commands query the
pseudo-random number generator. (See Parameters for more information about the
**seed** value that controls the pseudo-random number generator.)

The pseudo-random number generator is guaranteed to **NOT** be
cryptographically secure.

**'**

:   Generates an integer between 0 and **DC_RAND_MAX**, inclusive (see the
    **LIMITS** section).

    The generated integer is made as unbiased as possible, subject to the
    limitations of the pseudo-random number generator.

    This is a **non-portable extension**.

**"**

:   Pops a value off of the stack, which is used as an **exclusive** upper bound
    on the integer that will be generated. If the bound is negative or is a
    non-integer, an error is raised, and dc(1) resets (see the **RESET**
    section) while **seed** remains unchanged. If the bound is larger than
    **DC_RAND_MAX**, the higher bound is honored by generating several
    pseudo-random integers, multiplying them by appropriate powers of
    **DC_RAND_MAX+1**, and adding them together. Thus, the size of integer that
    can be generated with this command is unbounded. Using this command will
    change the value of **seed**, unless the operand is **0** or **1**. In that
    case, **0** is pushed onto the stack, and **seed** is *not* changed.

    The generated integer is made as unbiased as possible, subject to the
    limitations of the pseudo-random number generator.

    This is a **non-portable extension**.

## Stack Control

These commands control the stack.

**c**

:   Removes all items from ("clears") the stack.

**d**

:   Copies the item on top of the stack ("duplicates") and pushes the copy onto
    the stack.

**r**

:   Swaps ("reverses") the two top items on the stack.

**R**

:   Pops ("removes") the top value from the stack.

## Register Control

These commands control registers (see the **REGISTERS** section).

**s**_r_

:   Pops the value off the top of the stack and stores it into register *r*.

**l**_r_

:   Copies the value in register *r* and pushes it onto the stack. This does not
    alter the contents of *r*.

**S**_r_

:   Pops the value off the top of the (main) stack and pushes it onto the stack
    of register *r*. The previous value of the register becomes inaccessible.

**L**_r_

:   Pops the value off the top of the stack for register *r* and push it onto
    the main stack. The previous value in the stack for register *r*, if any, is
    now accessible via the **l**_r_ command.

## Parameters

These commands control the values of **ibase**, **obase**, **scale**, and
**seed**. Also see the **SYNTAX** section.

**i**

:   Pops the value off of the top of the stack and uses it to set **ibase**,
    which must be between **2** and **16**, inclusive.

    If the value on top of the stack has any *scale*, the *scale* is ignored.

**o**

:   Pops the value off of the top of the stack and uses it to set **obase**,
    which must be between **0** and **DC_BASE_MAX**, inclusive (see the
    **LIMITS** section and the **NUMBERS** section).

    If the value on top of the stack has any *scale*, the *scale* is ignored.

**k**

:   Pops the value off of the top of the stack and uses it to set **scale**,
    which must be non-negative.

    If the value on top of the stack has any *scale*, the *scale* is ignored.

**j**

:   Pops the value off of the top of the stack and uses it to set **seed**. The
    meaning of **seed** is dependent on the current pseudo-random number
    generator but is guaranteed to not change except for new major versions.

    The *scale* and sign of the value may be significant.

    If a previously used **seed** value is used again, the pseudo-random number
    generator is guaranteed to produce the same sequence of pseudo-random
    numbers as it did when the **seed** value was previously used.

    The exact value assigned to **seed** is not guaranteed to be returned if the
    **J** command is used. However, if **seed** *does* return a different value,
    both values, when assigned to **seed**, are guaranteed to produce the same
    sequence of pseudo-random numbers. This means that certain values assigned
    to **seed** will not produce unique sequences of pseudo-random numbers.

    There is no limit to the length (number of significant decimal digits) or
    *scale* of the value that can be assigned to **seed**.

    This is a **non-portable extension**.

**I**

:   Pushes the current value of **ibase** onto the main stack.

**O**

:   Pushes the current value of **obase** onto the main stack.

**K**

:   Pushes the current value of **scale** onto the main stack.

**J**

:   Pushes the current value of **seed** onto the main stack.

    This is a **non-portable extension**.

**T**

:   Pushes the maximum allowable value of **ibase** onto the main stack.

    This is a **non-portable extension**.

**U**

:   Pushes the maximum allowable value of **obase** onto the main stack.

    This is a **non-portable extension**.

**V**

:   Pushes the maximum allowable value of **scale** onto the main stack.

    This is a **non-portable extension**.

**W**

:   Pushes the maximum (inclusive) integer that can be generated with the **'**
    pseudo-random number generator command.

    This is a **non-portable extension**.

## Strings

The following commands control strings.

dc(1) can work with both numbers and strings, and registers (see the
**REGISTERS** section) can hold both strings and numbers. dc(1) always knows
whether the contents of a register are a string or a number.

While arithmetic operations have to have numbers, and will print an error if
given a string, other commands accept strings.

Strings can also be executed as macros. For example, if the string **[1pR]** is
executed as a macro, then the code **1pR** is executed, meaning that the **1**
will be printed with a newline after and then popped from the stack.

**\[**_characters_**\]**

:   Makes a string containing *characters* and pushes it onto the stack.

    If there are brackets (**\[** and **\]**) in the string, then they must be
    balanced. Unbalanced brackets can be escaped using a backslash (**\\**)
    character.

    If there is a backslash character in the string, the character after it
    (even another backslash) is put into the string verbatim, but the (first)
    backslash is not.

**a**

:   The value on top of the stack is popped.

    If it is a number, it is truncated and its absolute value is taken. The
    result mod **256** is calculated. If that result is **0**, push an empty
    string; otherwise, push a one-character string where the character is the
    result of the mod interpreted as an ASCII character.

    If it is a string, then a new string is made. If the original string is
    empty, the new string is empty. If it is not, then the first character of
    the original string is used to create the new string as a one-character
    string. The new string is then pushed onto the stack.

    This is a **non-portable extension**.

**x**

:   Pops a value off of the top of the stack.

    If it is a number, it is pushed back onto the stack.

    If it is a string, it is executed as a macro.

    This behavior is the norm whenever a macro is executed, whether by this
    command or by the conditional execution commands below.

**\>**_r_

:   Pops two values off of the stack that must be numbers and compares them. If
    the first value is greater than the second, then the contents of register
    *r* are executed.

    For example, **0 1>a** will execute the contents of register **a**, and
    **1 0>a** will not.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

**>**_r_**e**_s_

:   Like the above, but will execute register *s* if the comparison fails.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

    This is a **non-portable extension**.

**!\>**_r_

:   Pops two values off of the stack that must be numbers and compares them. If
    the first value is not greater than the second (less than or equal to), then
    the contents of register *r* are executed.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

**!\>**_r_**e**_s_

:   Like the above, but will execute register *s* if the comparison fails.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

    This is a **non-portable extension**.

**\<**_r_

:   Pops two values off of the stack that must be numbers and compares them. If
    the first value is less than the second, then the contents of register *r*
    are executed.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

**\<**_r_**e**_s_

:   Like the above, but will execute register *s* if the comparison fails.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

    This is a **non-portable extension**.

**!\<**_r_

:   Pops two values off of the stack that must be numbers and compares them. If
    the first value is not less than the second (greater than or equal to), then
    the contents of register *r* are executed.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

**!\<**_r_**e**_s_

:   Like the above, but will execute register *s* if the comparison fails.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

    This is a **non-portable extension**.

**=**_r_

:   Pops two values off of the stack that must be numbers and compares them. If
    the first value is equal to the second, then the contents of register *r*
    are executed.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

**=**_r_**e**_s_

:   Like the above, but will execute register *s* if the comparison fails.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

    This is a **non-portable extension**.

**!=**_r_

:   Pops two values off of the stack that must be numbers and compares them. If
    the first value is not equal to the second, then the contents of register
    *r* are executed.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

**!=**_r_**e**_s_

:   Like the above, but will execute register *s* if the comparison fails.

    If either or both of the values are not numbers, dc(1) will raise an error
    and reset (see the **RESET** section).

    This is a **non-portable extension**.

**?**

:   Reads a line from the **stdin** and executes it. This is to allow macros to
    request input from users.

**q**

:   During execution of a macro, this exits the execution of that macro and the
    execution of the macro that executed it. If there are no macros, or only one
    macro executing, dc(1) exits.

**Q**

:   Pops a value from the stack which must be non-negative and is used the
    number of macro executions to pop off of the execution stack. If the number
    of levels to pop is greater than the number of executing macros, dc(1)
    exits.

**,**

:   Pushes the depth of the execution stack onto the stack. The execution stack
    is the stack of string executions. The number that is pushed onto the stack
    is exactly as many as is needed to make dc(1) exit with the **Q** command,
    so the sequence **,Q** will make dc(1) exit.

## Status

These commands query status of the stack or its top value.

**Z**

:   Pops a value off of the stack.

    If it is a number, calculates the number of significant decimal digits it
    has and pushes the result. It will push **1** if the argument is **0** with
    no decimal places.

    If it is a string, pushes the number of characters the string has.

**X**

:   Pops a value off of the stack.

    If it is a number, pushes the *scale* of the value onto the stack.

    If it is a string, pushes **0**.

**z**

:   Pushes the current depth of the stack (before execution of this command)
    onto the stack.

**y**_r_

:   Pushes the current stack depth of the register *r* onto the main stack.

    Because each register has a depth of **1** (with the value **0** in the top
    item) when dc(1) starts, dc(1) requires that each register's stack must
    always have at least one item; dc(1) will give an error and reset otherwise
    (see the **RESET** section). This means that this command will never push
    **0**.

    This is a **non-portable extension**.

## Arrays

These commands manipulate arrays.

**:**_r_

:   Pops the top two values off of the stack. The second value will be stored in
    the array *r* (see the **REGISTERS** section), indexed by the first value.

**;**_r_

:   Pops the value on top of the stack and uses it as an index into the array
    *r*. The selected value is then pushed onto the stack.

**Y**_r_

:   Pushes the length of the array *r* onto the stack.

    This is a **non-portable extension**.

# REGISTERS

Registers are names that can store strings, numbers, and arrays. (Number/string
registers do not interfere with array registers.)

Each register is also its own stack, so the current register value is the top of
the stack for the register. All registers, when first referenced, have one value
(**0**) in their stack, and it is a runtime error to attempt to pop that item
off of the register stack.

In non-extended register mode, a register name is just the single character that
follows any command that needs a register name. The only exceptions are: a
newline (**'\\n'**) and a left bracket (**'['**); it is a parse error for a
newline or a left bracket to be used as a register name.

## Extended Register Mode

Unlike most other dc(1) implentations, this dc(1) provides nearly unlimited
amounts of registers, if extended register mode is enabled.

If extended register mode is enabled (**-x** or **-\-extended-register**
command-line arguments are given), then normal single character registers are
used *unless* the character immediately following a command that needs a
register name is a space (according to **isspace()**) and not a newline
(**'\\n'**).

In that case, the register name is found according to the regex
**\[a-z\]\[a-z0-9\_\]\*** (like bc(1) identifiers), and it is a parse error if
the next non-space characters do not match that regex.

# RESET

When dc(1) encounters an error or a signal that it has a non-default handler
for, it resets. This means that several things happen.

First, any macros that are executing are stopped and popped off the stack.
The behavior is not unlike that of exceptions in programming languages. Then
the execution point is set so that any code waiting to execute (after all
macros returned) is skipped.

Thus, when dc(1) resets, it skips any remaining code waiting to be executed.
Then, if it is interactive mode, and the error was not a fatal error (see the
**EXIT STATUS** section), it asks for more input; otherwise, it exits with the
appropriate return code.

# PERFORMANCE

Most dc(1) implementations use **char** types to calculate the value of **1**
decimal digit at a time, but that can be slow. This dc(1) does something
different.

It uses large integers to calculate more than **1** decimal digit at a time. If
built in a environment where **DC_LONG_BIT** (see the **LIMITS** section) is
**64**, then each integer has **9** decimal digits. If built in an environment
where **DC_LONG_BIT** is **32** then each integer has **4** decimal digits. This
value (the number of decimal digits per large integer) is called
**DC_BASE_DIGS**.

In addition, this dc(1) uses an even larger integer for overflow checking. This
integer type depends on the value of **DC_LONG_BIT**, but is always at least
twice as large as the integer type used to store digits.

# LIMITS

The following are the limits on dc(1):

**DC_LONG_BIT**

:   The number of bits in the **long** type in the environment where dc(1) was
    built. This determines how many decimal digits can be stored in a single
    large integer (see the **PERFORMANCE** section).

**DC_BASE_DIGS**

:   The number of decimal digits per large integer (see the **PERFORMANCE**
    section). Depends on **DC_LONG_BIT**.

**DC_BASE_POW**

:   The max decimal number that each large integer can store (see
    **DC_BASE_DIGS**) plus **1**. Depends on **DC_BASE_DIGS**.

**DC_OVERFLOW_MAX**

:   The max number that the overflow type (see the **PERFORMANCE** section) can
    hold. Depends on **DC_LONG_BIT**.

**DC_BASE_MAX**

:   The maximum output base. Set at **DC_BASE_POW**.

**DC_DIM_MAX**

:   The maximum size of arrays. Set at **SIZE_MAX-1**.

**DC_SCALE_MAX**

:   The maximum **scale**. Set at **DC_OVERFLOW_MAX-1**.

**DC_STRING_MAX**

:   The maximum length of strings. Set at **DC_OVERFLOW_MAX-1**.

**DC_NAME_MAX**

:   The maximum length of identifiers. Set at **DC_OVERFLOW_MAX-1**.

**DC_NUM_MAX**

:   The maximum length of a number (in decimal digits), which includes digits
    after the decimal point. Set at **DC_OVERFLOW_MAX-1**.

**DC_RAND_MAX**

:   The maximum integer (inclusive) returned by the **'** command, if dc(1). Set
    at **2\^DC_LONG_BIT-1**.

Exponent

:   The maximum allowable exponent (positive or negative). Set at
    **DC_OVERFLOW_MAX**.

Number of vars

:   The maximum number of vars/arrays. Set at **SIZE_MAX-1**.

These limits are meant to be effectively non-existent; the limits are so large
(at least on 64-bit machines) that there should not be any point at which they
become a problem. In fact, memory should be exhausted before these limits should
be hit.

# ENVIRONMENT VARIABLES

dc(1) recognizes the following environment variables:

**DC_ENV_ARGS**

:   This is another way to give command-line arguments to dc(1). They should be
    in the same format as all other command-line arguments. These are always
    processed first, so any files given in **DC_ENV_ARGS** will be processed
    before arguments and files given on the command-line. This gives the user
    the ability to set up "standard" options and files to be used at every
    invocation. The most useful thing for such files to contain would be useful
    functions that the user might want every time dc(1) runs. Another use would
    be to use the **-e** option to set **scale** to a value other than **0**.

    The code that parses **DC_ENV_ARGS** will correctly handle quoted arguments,
    but it does not understand escape sequences. For example, the string
    **"/home/gavin/some dc file.dc"** will be correctly parsed, but the string
    **"/home/gavin/some \"dc\" file.dc"** will include the backslashes.

    The quote parsing will handle either kind of quotes, **'** or **"**. Thus,
    if you have a file with any number of single quotes in the name, you can use
    double quotes as the outside quotes, as in **"some 'dc' file.dc"**, and vice
    versa if you have a file with double quotes. However, handling a file with
    both kinds of quotes in **DC_ENV_ARGS** is not supported due to the
    complexity of the parsing, though such files are still supported on the
    command-line where the parsing is done by the shell.

**DC_LINE_LENGTH**

:   If this environment variable exists and contains an integer that is greater
    than **1** and is less than **UINT16_MAX** (**2\^16-1**), dc(1) will output
    lines to that length, including the backslash newline combo. The default
    line length is **70**.

**DC_SIGINT_RESET**

:   If dc(1) is not in interactive mode (see the **INTERACTIVE MODE** section),
    then this environment variable has no effect because dc(1) exits on
    **SIGINT** when not in interactive mode.

    However, when dc(1) is in interactive mode, then if this environment
    variable exists and contains an integer, a non-zero value makes dc(1) reset
    on **SIGINT**, rather than exit, and zero makes dc(1) exit. If this
    environment variable exists and is *not* an integer, then dc(1) will exit on
    **SIGINT**.

    This environment variable overrides the default, which can be queried with
    the **-h** or **-\-help** options.

**DC_TTY_MODE**

:   If TTY mode is *not* available (see the **TTY MODE** section), then this
    environment variable has no effect.

    However, when TTY mode is available, then if this environment variable
    exists and contains an integer, then a non-zero value makes dc(1) use TTY
    mode, and zero makes dc(1) not use TTY mode.

    This environment variable overrides the default, which can be queried with
    the **-h** or **-\-help** options.

**DC_PROMPT**

:   If TTY mode is *not* available (see the **TTY MODE** section), then this
    environment variable has no effect.

    However, when TTY mode is available, then if this environment variable
    exists and contains an integer, a non-zero value makes dc(1) use a prompt,
    and zero or a non-integer makes dc(1) not use a prompt. If this environment
    variable does not exist and **DC_TTY_MODE** does, then the value of the
    **DC_TTY_MODE** environment variable is used.

    This environment variable and the **DC_TTY_MODE** environment variable
    override the default, which can be queried with the **-h** or **-\-help**
    options.

# EXIT STATUS

dc(1) returns the following exit statuses:

**0**

:   No error.

**1**

:   A math error occurred. This follows standard practice of using **1** for
    expected errors, since math errors will happen in the process of normal
    execution.

    Math errors include divide by **0**, taking the square root of a negative
    number, using a negative number as a bound for the pseudo-random number
    generator, attempting to convert a negative number to a hardware integer,
    overflow when converting a number to a hardware integer, overflow when
    calculating the size of a number, and attempting to use a non-integer where
    an integer is required.

    Converting to a hardware integer happens for the second operand of the power
    (**\^**), places (**\@**), left shift (**H**), and right shift (**h**)
    operators.

**2**

:   A parse error occurred.

    Parse errors include unexpected **EOF**, using an invalid character, failing
    to find the end of a string or comment, and using a token where it is
    invalid.

**3**

:   A runtime error occurred.

    Runtime errors include assigning an invalid number to any global (**ibase**,
    **obase**, or **scale**), giving a bad expression to a **read()** call,
    calling **read()** inside of a **read()** call, type errors (including
    attempting to execute a number), and attempting an operation when the stack
    has too few elements.

**4**

:   A fatal error occurred.

    Fatal errors include memory allocation errors, I/O errors, failing to open
    files, attempting to use files that do not have only ASCII characters (dc(1)
    only accepts ASCII characters), attempting to open a directory as a file,
    and giving invalid command-line options.

The exit status **4** is special; when a fatal error occurs, dc(1) always exits
and returns **4**, no matter what mode dc(1) is in.

The other statuses will only be returned when dc(1) is not in interactive mode
(see the **INTERACTIVE MODE** section), since dc(1) resets its state (see the
**RESET** section) and accepts more input when one of those errors occurs in
interactive mode. This is also the case when interactive mode is forced by the
**-i** flag or **-\-interactive** option.

These exit statuses allow dc(1) to be used in shell scripting with error
checking, and its normal behavior can be forced by using the **-i** flag or
**-\-interactive** option.

# INTERACTIVE MODE

Like bc(1), dc(1) has an interactive mode and a non-interactive mode.
Interactive mode is turned on automatically when both **stdin** and **stdout**
are hooked to a terminal, but the **-i** flag and **-\-interactive** option can
turn it on in other situations.

In interactive mode, dc(1) attempts to recover from errors (see the **RESET**
section), and in normal execution, flushes **stdout** as soon as execution is
done for the current input. dc(1) may also reset on **SIGINT** instead of exit,
depending on the contents of, or default for, the **DC_SIGINT_RESET**
environment variable (see the **ENVIRONMENT VARIABLES** section).

# TTY MODE

If **stdin**, **stdout**, and **stderr** are all connected to a TTY, then "TTY
mode" is considered to be available, and thus, dc(1) can turn on TTY mode,
subject to some settings.

If there is the environment variable **DC_TTY_MODE** in the environment (see the
**ENVIRONMENT VARIABLES** section), then if that environment variable contains a
non-zero integer, dc(1) will turn on TTY mode when **stdin**, **stdout**, and
**stderr** are all connected to a TTY. If the **DC_TTY_MODE** environment
variable exists but is *not* a non-zero integer, then dc(1) will not turn TTY
mode on.

If the environment variable **DC_TTY_MODE** does *not* exist, the default
setting is used. The default setting can be queried with the **-h** or
**-\-help** options.

TTY mode is different from interactive mode because interactive mode is required
in the [bc(1) specification][1], and interactive mode requires only **stdin**
and **stdout** to be connected to a terminal.

## Prompt

If TTY mode is available, then a prompt can be enabled. Like TTY mode itself, it
can be turned on or off with an environment variable: **DC_PROMPT** (see the
**ENVIRONMENT VARIABLES** section).

If the environment variable **DC_PROMPT** exists and is a non-zero integer, then
the prompt is turned on when **stdin**, **stdout**, and **stderr** are connected
to a TTY and the **-P** and **-\-no-prompt** options were not used. The read
prompt will be turned on under the same conditions, except that the **-R** and
**-\-no-read-prompt** options must also not be used.

However, if **DC_PROMPT** does not exist, the prompt can be enabled or disabled
with the **DC_TTY_MODE** environment variable, the **-P** and **-\-no-prompt**
options, and the **-R** and **-\-no-read-prompt** options. See the **ENVIRONMENT
VARIABLES** and **OPTIONS** sections for more details.

# SIGNAL HANDLING

Sending a **SIGINT** will cause dc(1) to do one of two things.

If dc(1) is not in interactive mode (see the **INTERACTIVE MODE** section), or
the **DC_SIGINT_RESET** environment variable (see the **ENVIRONMENT VARIABLES**
section), or its default, is either not an integer or it is zero, dc(1) will
exit.

However, if dc(1) is in interactive mode, and the **DC_SIGINT_RESET** or its
default is an integer and non-zero, then dc(1) will stop executing the current
input and reset (see the **RESET** section) upon receiving a **SIGINT**.

Note that "current input" can mean one of two things. If dc(1) is processing
input from **stdin** in interactive mode, it will ask for more input. If dc(1)
is processing input from a file in interactive mode, it will stop processing the
file and start processing the next file, if one exists, or ask for input from
**stdin** if no other file exists.

This means that if a **SIGINT** is sent to dc(1) as it is executing a file, it
can seem as though dc(1) did not respond to the signal since it will immediately
start executing the next file. This is by design; most files that users execute
when interacting with dc(1) have function definitions, which are quick to parse.
If a file takes a long time to execute, there may be a bug in that file. The
rest of the files could still be executed without problem, allowing the user to
continue.

**SIGTERM** and **SIGQUIT** cause dc(1) to clean up and exit, and it uses the
default handler for all other signals.

# SEE ALSO

bc(1)

# STANDARDS

The dc(1) utility operators are compliant with the operators in the bc(1)
[IEEE Std 1003.1-2017 (“POSIX.1-2017”)][1] specification.

# BUGS

None are known. Report bugs at https://git.yzena.com/gavin/bc.

# AUTHOR

Gavin D. Howard <gavin@yzena.com> and contributors.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/bc.html