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freebsd-dev/share/man/man9/getenv.9
Mitchell Horne cba446e2c2 Add getenv(9) boolean parsing functions 
This adds the getenv_bool() function, to parse a boolean value from a
kernel environment variable or tunable. This works for traditional
boolean values like "0" and "1", and also "true" and "false"
(case-insensitive). These semantics do not yet apply to sysctls declared
using SYSCTL_BOOL with CTLFLAG_TUN (they still only parse 1 and 0).

Also added are two wrapper functions, getenv_is_true() and
getenv_is_false(). These are slightly simpler for callers wishing to
perform a single check of a configuration variable.

Reviewed by:	jhb (slightly earlier version)
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D26270
2020-09-21 15:24:44 +00:00

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.\" -*- nroff -*-
.\"
.\" Copyright (c) 2013 Hudson River Trading LLC
.\" Written by: John H. Baldwin <jhb@FreeBSD.org>
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd September 21, 2020
.Dt GETENV 9
.Os
.Sh NAME
.Nm freeenv ,
.Nm kern_getenv ,
.Nm getenv_int ,
.Nm getenv_long ,
.Nm getenv_string ,
.Nm getenv_quad ,
.Nm getenv_uint ,
.Nm getenv_ulong ,
.Nm getenv_bool ,
.Nm getenv_is_true ,
.Nm getenv_is_false ,
.Nm kern_setenv ,
.Nm testenv ,
.Nm kern_unsetenv
.Nd kernel environment variable functions
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.Ft void
.Fn freeenv "char *env"
.Ft char *
.Fn kern_getenv "const char *name"
.Ft int
.Fn getenv_int "const char *name" "int *data"
.Ft int
.Fn getenv_long "const char *name" "long *data"
.Ft int
.Fn getenv_string "const char *name" "char *data" "int size"
.Ft int
.Fn getenv_quad "const char *name" "quad_t *data"
.Ft int
.Fn getenv_uint "const char *name" "unsigned int *data"
.Ft int
.Fn getenv_ulong "const char *name" "unsigned long *data"
.Ft int
.Fn getenv_bool "const char *name" "bool *data"
.Ft bool
.Fn getenv_is_true "const char *name"
.Ft bool
.Fn getenv_is_false "const char *name"
.Ft int
.Fn kern_setenv "const char *name" "const char *value"
.Ft int
.Fn testenv "const char *name"
.Ft int
.Fn kern_unsetenv "const char *name"
.Sh DESCRIPTION
These functions set, unset, fetch, and parse variables from the kernel's
environment.
.Pp
The
.Fn kern_getenv
function obtains the current value of the kernel environment variable
.Fa name
and returns a pointer to the string value.
The caller should not modify the string pointed to by the return value.
The
.Fn kern_getenv
function may allocate temporary storage,
so the
.Fn freeenv
function must be called to release any allocated resources when the value
returned by
.Fn kern_getenv
is no longer needed.
.Pp
The
.Fn freeenv
function is used to release the resources allocated by a previous call to
.Fn kern_getenv .
The
.Fa env
argument passed to
.Fn freeenv
is the pointer returned by the earlier call to
.Fn kern_getenv .
Like
.Xr free 3 ,
the
.Fa env
argument can be
.Va NULL ,
in which case no action occurs.
.Pp
The
.Fn kern_setenv
function inserts or resets the kernel environment variable
.Fa name
to
.Fa value .
If the variable
.Fa name
already exists,
its value is replaced.
This function can fail if an internal limit on the number of environment
variables is exceeded.
.Pp
The
.Fn kern_unsetenv
function deletes the kernel environment variable
.Fa name .
.Pp
The
.Fn testenv
function is used to determine if a kernel environment variable exists.
It returns a non-zero value if the variable
.Fa name
exists and zero if it does not.
.Pp
The
.Fn getenv_int ,
.Fn getenv_long ,
.Fn getenv_quad ,
.Fn getenv_uint ,
and
.Fn getenv_ulong
functions look for a kernel environment variable
.Fa name
and parse it as a signed integer,
long integer,
signed 64-bit integer,
unsigned integer,
or an unsigned long integer,
respectively.
These functions fail and return zero if
.Fa name
does not exist or if any invalid characters are present in its value.
On success,
these function store the parsed value in the integer variable pointed to
by
.Fa data .
If the parsed value overflows the integer type,
a truncated value is stored in
.Fa data
and zero is returned.
If the value begins with a prefix of
.Dq 0x
it is interpreted as hexadecimal.
If it begins with a prefix of
.Dq 0
it is interpreted as octal.
Otherwise,
the value is interpreted as decimal.
The value may contain a single character suffix specifying a unit for
the value.
The interpreted value is multiplied by the unit's magnitude before being
returned.
The following unit suffixes are supported:
.Bl -column -offset indent ".Sy Unit" ".Sy Magnitude"
.It Sy Unit Ta Sy Magnitude
.It k Ta 2^10
.It m Ta 2^20
.It g Ta 2^30
.It t Ta 2^40
.El
.Pp
The
.Fn getenv_string
function stores a copy of the kernel environment variable
.Fa name
in the buffer described by
.Fa data
and
.Fa size .
If the variable does not exist,
zero is returned.
If the variable exists,
up to
.Fa size - 1
characters of its value are copied to the buffer pointed to by
.Fa data
followed by a null character and a non-zero value is returned.
.Pp
The
.Fn getenv_bool
function interprets the value of the kernel environment variable
.Fa name
as a boolean value by performing a case-insensitive comparison against the
strings "1",
"0",
"true",
and "false".
If the environment variable exists and has a valid boolean value, then that
value will be copied to the variable pointed to by
.Fa data .
If the environment variable exists but is not a boolean value, then a warning
will be printed to the kernel message buffer.
The
.Fn getenv_is_true
and
.Fn getenv_is_false
functions are wrappers around
.Fn getenv_bool
that simplify testing for a desired boolean value.
.Sh RETURN VALUES
The
.Fn kern_getenv
function returns a pointer to an environment variable's value on success or
.Dv NULL
if the variable does not exist.
.Pp
The
.Fn kern_setenv
and
.Fn kern_unsetenv
functions return zero on success and -1 on failure.
.Pp
The
.Fn testenv
function returns zero if the specified environment variable does not exist and
a non-zero value if it does exist.
.Pp
The
.Fn getenv_int ,
.Fn getenv_long ,
.Fn getenv_string ,
.Fn getenv_quad ,
.Fn getenv_uint ,
.Fn getenv_ulong ,
and
.Fn getenv_bool
functions return a non-zero value on success and zero on failure.
.Pp
The
.Fn getenv_is_true
and
.Fn getenv_is_false
functions return
.Dv true
if the specified environment variable exists and its value matches the desired
boolean condition, and
.Dv false
otherwise.
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