Home Man
Today's Posts

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

Linux 2.6 - man page for getenv (linux section 3posix)

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

       getenv - get value of an environment variable

       #include <stdlib.h>

       char *getenv(const char *name);

       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 environ-
       ment 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 reen-
       trant is not required to be thread-safe.

       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 subse-
       quent 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 subsequent calls to putenv().

       No errors are defined.

       The following sections are informative.

   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);


       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 sep-
       arate 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 envi-
       ron 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.


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

       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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					GETENV(P)

All times are GMT -4. The time now is 08:12 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
Show Password