GETENV(3P)                                          POSIX Programmer's Manual                                         GETENV(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

NAME
       getenv - get value of an environment variable

SYNOPSIS
       #include <stdlib.h>

       char *getenv(const char *name);


DESCRIPTION
       The  getenv()  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 name  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 getenv() function.

       The string pointed to may be overwritten by a subsequent call to getenv(), setenv(), or unsetenv(),   but  shall  not  be
       overwritten by a call to any other function in this volume of IEEE Std 1003.1-2001.

       If the application modifies environ or the pointers to which it points, the behavior of getenv() is undefined.

       The  getenv()  function  need  not  be  reentrant.  A function that is not required to be reentrant is not required to be
       thread-safe.

RETURN VALUE
       Upon successful completion, getenv() shall return a pointer to a string containing the value for the specified  name.  If
       the specified name cannot be found in the environment of the calling process, a null pointer shall be returned.

       The  return  value  from  getenv()  may  point  to  static data which may be overwritten by subsequent calls to getenv(),
       setenv(), or unsetenv().

       On XSI-conformant systems, the return value from getenv() may point to static data which may also be overwritten by  sub-
       sequent calls to putenv().

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
   Getting the Value of an Environment Variable
       The following example gets the value of the HOME environment variable.


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


              value = getenv(name);

APPLICATION USAGE
       None.

RATIONALE
       The  clearenv()  function was considered but rejected. The putenv() function has now been included for alignment with the
       Single UNIX Specification.

       The getenv() function is inherently not reentrant because it returns a value pointing to static data.

       Conforming applications are required not to modify environ 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  environ
       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 setenv() or unsetenv()), 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 environ when unsetenv() is
       called. A C runtime start-up procedure (that which invokes main() and perhaps initializes environ) 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.

       In fact, for higher performance of getenv(), 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 environ when setenv() or unsetenv() is invoked.

       Performance of getenv() 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 ls, which attempts to use COLUMNS ), or really almost any utility ( LANG, LC_ALL,  and so on) is similarly slowed down
       by the linear search through the variables.

       An implementation that maintains separate data structures, or even one that manages the memory it consumes, is  not  cur-
       rently required as it was thought it would reduce consensus among implementors who do not want to change their historical
       implementations.

       The POSIX Threads Extension  states  that  multi-threaded  applications  must  not  modify  environ  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 environ is desirable from that standpoint as well.

FUTURE DIRECTIONS
       None.

SEE ALSO
       exec(), putenv(), setenv(), unsetenv(), the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Vari-
       ables, <stdlib.h>

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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                    GETENV(3P)