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.\"     from: @(#)getenv.3	8.2 (Berkeley) 12/11/93
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.Dd October 25, 2010
.Dt GETENV 3
.Os
.Sh NAME
.Nm getenv ,
.Nm getenv_r ,
.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 getenv_r "const char *name" "char *buf" "size_t len"
.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 .
For compatibility with differing environment conventions,
the
.Fn getenv
or
.Fn getenv_r
given argument
.Ar name
may be appended with an equal sign
.Dq Li \&= .
.Pp
The
.Fn getenv
function obtains the current value of the environment variable
.Ar name .
If the variable
.Ar name
is not in the current environment, a
.Dv NULL
pointer is returned.
.Pp
The
.Fn getenv_r
function obtains the current value of the environment variable
.Fa name
and copies it to
.Fa buf .
If
.Fa name
is not in the current environment, or the string length of the value of
.Fa name
is longer than
.Fa len
characters, then \-1 is returned and
.Va errno
is set to indicate the error.
.Pp
The
.Fn setenv
function inserts or resets the environment variable
.Ar name
in the current environment list.
If the variable
.Ar name
does not exist in the list,
it is inserted with the given
.Ar value .
If the variable does exist, the argument
.Ar overwrite
is tested; if
.Ar overwrite is
zero, the
variable is not reset, otherwise it is reset
to the given
.Ar value .
.Pp
The
.Fn putenv
function takes an argument of the form
.Dq name=value
and it will set the environment variable
.Dq name
equal to
.Dq value
by altering an existing entry, or creating a new one if an existing
one does not exist.
The actual string argument passed to
.Fn putenv
will become part of the environment.
If one changes the string, the environment will also change.
.Pp
The
.Fn unsetenv
function
deletes all instances of the variable name pointed to by
.Fa name
from the list.
.Sh RETURN VALUES
The functions
.Fn getenv_r ,
.Fn setenv ,
.Fn putenv ,
and
.Fn unsetenv
return zero if successful; otherwise the global variable
.Va errno
is set to indicate the error and a
\-1 is returned.
.Pp
If
.Fn getenv
is successful, the string returned should be considered read-only.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa name
argument to
.Fn setenv
or
.Fn unsetenv
is a null pointer, points to an empty string, or points to a string
containing an
.Dq Li \&=
character.
The
.Fa value
argument to
.Fn setenv
is a null pointer.
The
.Fa string
argument to
.Fn putenv
is a null pointer, or points to a string that either starts with a
.Dq Li \&=
character or does not contain one at all.
.It Bq Er ENOMEM
The function
.Fn setenv
or
.Fn putenv
failed because they were unable to allocate memory for the environment.
.El
.Pp
The function
.Fn getenv_r
can return the following errors:
.Bl -tag -width Er
.It Bq Er ENOENT
The variable
.Fa name
was not found in the environment.
.It Bq Er ERANGE
The value of the named variable is too long to fit in the supplied buffer.
.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 -ansiC .
The
.Fn putenv
function conforms to
.St -xpg4 .
The
.Fn unsetenv
function 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 .