libc/stdlib/getenv.3

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.\"     @(#)getenv.3	8.2 (Berkeley) 12/11/93
.\" $FreeBSD: head/lib/libc/stdlib/getenv.3 251069 2013-05-28 20:57:40Z emaste $
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.Dd September 9, 2015
.Dt GETENV 3
.Os
.Sh NAME
.Nm getenv ,
.Nm putenv ,
.Nm setenv ,
.Nm unsetenv
.Nd environment variable functions
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft char *
.Fn getenv "const char *name"
.Ft int
.Fn setenv "const char *name" "const char *value" "int overwrite"
.Ft int
.Fn putenv "char *string"
.Ft int
.Fn unsetenv "const char *name"
.Sh DESCRIPTION
These functions set, unset and fetch environment variables from the
host
.Em environment list .
.Pp
The
.Fn getenv
function obtains the current value of the environment variable,
.Fa name .
The application should not modify the string pointed
to by the
.Fn getenv
function.
.Pp
The
.Fn setenv
function inserts or resets the environment variable
.Fa name
in the current environment list.
If the variable
.Fa name
does not exist in the list,
it is inserted with the given
.Fa value .
If the variable does exist, the argument
.Fa overwrite
is tested; if
.Fa overwrite
is zero, the
variable is not reset, otherwise it is reset
to the given
.Fa value .
.Pp
The
.Fn putenv
function takes an argument of the form ``name=value'' and
puts it directly into the current environment,
so altering the argument shall change the environment.
If the variable
.Fa name
does not exist in the list,
it is inserted with the given
.Fa value .
If the variable
.Fa name
does exist, it is reset to the given
.Fa value .
.Pp
The
.Fn unsetenv
function
deletes all instances of the variable name pointed to by
.Fa name
from the list.
.Pp
If corruption (e.g., a name without a value) is detected while making a copy of
environ for internal usage, then
.Fn setenv ,
.Fn unsetenv
and
.Fn putenv
will output a warning to stderr about the issue, drop the corrupt entry and
complete the task without error.
.Sh RETURN VALUES
The
.Fn getenv
function returns the value of the environment variable as a
.Dv NUL Ns
-terminated string.
If the variable
.Fa name
is not in the current environment,
.Dv NULL
is returned.
.Pp
.Rv -std setenv putenv unsetenv
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The function
.Fn getenv ,
.Fn setenv
or
.Fn unsetenv
failed because the
.Fa name
is a
.Dv NULL
pointer, points to an empty string, or points to a string containing an
.Dq Li \&=
character.
.Pp
The function
.Fn putenv
failed because
.Fa string
is a
.Dv NULL
pointer,
.Fa string
is without an
.Dq Li \&=
character or
.Dq Li \&=
is the first character in
.Fa string .
This does not follow the
.Tn POSIX
specification.
.It Bq Er ENOMEM
The function
.Fn setenv ,
.Fn unsetenv
or
.Fn putenv
failed because they were unable to allocate memory for the environment.
.El
.Sh SEE ALSO
.Xr csh 1 ,
.Xr sh 1 ,
.Xr execve 2 ,
.Xr environ 7
.Sh STANDARDS
The
.Fn getenv
function conforms to
.St -isoC .
The
.Fn setenv ,
.Fn putenv
and
.Fn unsetenv
functions conforms to
.St -p1003.1-2001 .
.Sh HISTORY
The functions
.Fn setenv
and
.Fn unsetenv
appeared in
.At v7 .
The
.Fn putenv
function appeared in
.Bx 4.3 Reno .
.Pp
Until
.Fx 7.0 ,
.Fn putenv
would make a copy of
.Fa string
and insert it into the environment using
.Fn setenv .
This was changed to use
.Fa string
as the memory location of the ``name=value'' pair to follow the
.Tn POSIX
specification.
.Sh BUGS
Successive calls to
.Fn setenv
that assign a larger-sized
.Fa value
than any previous value to the same
.Fa name
will result in a memory leak.
The
.Dx
semantics for this function
(namely, that the contents of
.Fa value
are copied and that old values remain accessible indefinitely) make this
bug unavoidable.
Future versions may eliminate one or both of these
semantic guarantees in order to fix the bug.