path: root/man3p/getenv.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "GETENV" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" getenv 
.SH NAME
getenv \- get value of an environment variable
.SH SYNOPSIS
.LP
\fB#include <stdlib.h>
.br
.sp
char *getenv(const char *\fP\fIname\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIgetenv\fP() function shall search the environment of the calling
process (see the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Chapter 8, Environment Variables) for the
environment
variable \fIname\fP if it exists and return a pointer to the value
of the environment variable. If the specified environment
variable cannot be found, a null pointer shall be returned. The application
shall ensure that it does not modify the string pointed
to by the \fIgetenv\fP() function.
.LP
The string pointed to may be overwritten by a subsequent call to \fIgetenv\fP(),
\fIsetenv\fP(), or \fIunsetenv\fP(),  but shall not be overwritten
by a call to any other function in this volume
of IEEE\ Std\ 1003.1-2001.
.LP
If
the application modifies \fIenviron\fP or the pointers to which it
points, the behavior of \fIgetenv\fP() is undefined.
.LP
The \fIgetenv\fP() function need not be reentrant. A function that
is not required to be reentrant is not required to be
thread-safe. 
.SH RETURN VALUE
.LP
Upon successful completion, \fIgetenv\fP() shall return a pointer
to a string containing the \fIvalue\fP for the specified
\fIname\fP. If the specified \fIname\fP cannot be found in the environment
of the calling process, a null pointer shall be
returned.
.LP
The return value from \fIgetenv\fP() may point to static data which
may be overwritten by subsequent calls to \fIgetenv\fP(),
\fIsetenv\fP(), or \fIunsetenv\fP(). 
.LP
On XSI-conformant systems, the return value from \fIgetenv\fP() may
point to static data which may also be overwritten by
subsequent calls to \fIputenv\fP(). 
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Getting the Value of an Environment Variable
.LP
The following example gets the value of the \fIHOME\fP environment
variable.
.sp
.RS
.nf

\fB#include <stdlib.h>
\&...
const char *name = "HOME";
char *value;
.sp

value = getenv(name);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
The \fIclearenv\fP() function was considered but rejected. The \fIputenv\fP()
function
has now been included for alignment with the Single UNIX Specification.
.LP
The \fIgetenv\fP() function is inherently not reentrant because it
returns a value pointing to static data.
.LP
Conforming applications are required not to modify \fIenviron\fP directly,
but to use only the functions described here to
manipulate the process environment as an abstract object. Thus, the
implementation of the environment access functions has complete
control over the data structure used to represent the environment
(subject to the requirement that \fIenviron\fP be maintained as
a list of strings with embedded equal signs for applications that
wish to scan the environment). This constraint allows the
implementation to properly manage the memory it allocates, either
by using allocated storage for all variables (copying them on the
first invocation of \fIsetenv\fP() or \fIunsetenv\fP()), or keeping
track of which strings are currently in allocated space and which
are not, via a separate table or some other means. This enables the
implementation to free any allocated space used by strings (and
perhaps the pointers to them) stored in \fIenviron\fP when \fIunsetenv\fP()
is called. A
C runtime start-up procedure (that which invokes \fImain\fP() and
perhaps initializes \fIenviron\fP) can also initialize a flag
indicating that none of the environment has yet been copied to allocated
storage, or that the separate table has not yet been
initialized.
.LP
In fact, for higher performance of \fIgetenv\fP(), the implementation
could also maintain a separate copy of the environment in
a data structure that could be searched much more quickly (such as
an indexed hash table, or a binary tree), and update both it and
the linear list at \fIenviron\fP when \fIsetenv\fP() or \fIunsetenv\fP()
is invoked.
.LP
Performance of \fIgetenv\fP() can be important for applications which
have large numbers of environment variables. Typically,
applications like this use the environment as a resource database
of user-configurable parameters. The fact that these variables
are in the user's shell environment usually means that any other program
that uses environment variables (such as \fIls\fP, which attempts
to use \fICOLUMNS ),\fP or really almost any utility ( \fILANG ,\fP
\fILC_ALL ,\fP and so on) is similarly slowed down by the linear search
through the variables.
.LP
An implementation that maintains separate data structures, or even
one that manages the memory it consumes, is not currently
required as it was thought it would reduce consensus among implementors
who do not want to change their historical
implementations.
.LP
The POSIX Threads Extension states that multi-threaded applications
must not modify \fIenviron\fP directly, and that
IEEE\ Std\ 1003.1-2001 is providing functions which such applications
can use in the future to manipulate the environment
in a thread-safe manner. Thus, moving away from application use of
\fIenviron\fP is desirable from that standpoint as well.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIexec\fP() , \fIputenv\fP() , \fIsetenv\fP() , \fIunsetenv\fP()
, the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, Chapter 8, Environment Variables, \fI<stdlib.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .