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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 dif-
       fer (consult 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 overwrit-
       ten by subsequent 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 allo-
       cates, 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 sepa-
       rate 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 initial-
       ized.

       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 currently 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, Envi-
       ronment Variables, <stdlib.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions 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 Stan-
       dard, 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 .



IEEE/The Open Group                  2003                           GETENV(3P)