getenv(3p) — Linux manual page


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

PROLOG         top

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

NAME         top

       getenv — get value of an environment variable

SYNOPSIS         top

       #include <stdlib.h>

       char *getenv(const char *name);

DESCRIPTION         top

       The functionality described on this reference page is aligned with
       the ISO C standard. Any conflict between the requirements described
       here and the ISO C standard is unintentional. This volume of
       POSIX.1‐2008 defers to the ISO C standard.

       The getenv() function shall search the environment of the calling
       process (see the Base Definitions volume of POSIX.1‐2008, 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 returned string pointer might be invalidated or the string
       content might be overwritten by a subsequent call to getenv(),
       setenv(), unsetenv(), or (if supported) putenv() but they shall not
       be affected by a call to any other function in this volume of

       The getenv() function need not be thread-safe.

RETURN VALUE         top

       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.

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

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

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

           value = getenv(name);



RATIONALE         top

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

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

       Conforming applications are required not to directly modify the
       pointers to which environ points, but to use only the setenv(),
       unsetenv(), and putenv() functions, or assignment to environ itself,
       to manipulate the process environment. This constraint allows the
       implementation to properly manage the memory it allocates. This
       enables the implementation to free any space it has allocated to
       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. If the application switches to a complete new
       environment by assigning a new value to environ, this can be detected
       by getenv(), setenv(), unsetenv(), or putenv() and the implementation
       can at that point reinitialize based on the new environment. (This
       may include copying the environment strings into a new array and
       assigning environ to point to it.)

       In fact, for higher performance of getenv(), implementations that do
       not provide putenv() 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. On implementations that do provide putenv(), such a copy
       might still be worthwhile but would need to allow for the fact that
       applications can directly modify the content of environment strings
       added with putenv().  For example, if an environment string found by
       searching the copy is one that was added using putenv(), the
       implementation would need to check that the string in environ still
       has the same name (and value, if the copy includes values), and
       whenever searching the copy produces no match the implementation
       would then need to search each environment string in environ that was
       added using putenv() in case any of them have changed their names and
       now match. Thus, each use of putenv() to add to the environment would
       reduce the speed advantage of having the copy.

       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.


       A future version may add one or more functions to access and modify
       the environment in a thread-safe manner.

SEE ALSO         top

       exec(1p), putenv(3p), setenv(3p), unsetenv(3p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
       Variables, stdlib.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see .

IEEE/The Open Group                 2013                          GETENV(3P)

Pages that refer to this page: stdlib.h(0p)exec(3p)execl(3p)execle(3p)execlp(3p)execv(3p)execve(3p)execvp(3p)putenv(3p)setenv(3p)unsetenv(3p)