getenv(3p) — Linux manual page

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 (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 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.1‐2017 defers to the ISO C standard.

       The getenv() function shall search the environment of the calling
       process (see the Base Definitions volume of POSIX.1‐2017, 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 POSIX.1‐2017.

       The returned string pointer might also be invalidated if the
       calling thread is terminated.

       The getenv() function need not 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.

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 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.

FUTURE DIRECTIONS

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

SEE ALSO

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

       The Base Definitions volume of POSIX.1‐2017, Chapter 8,
       Environment Variables, stdlib.h(0p)

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                        GETENV(3P)

Pages that refer to this page: stdlib.h(0p), exec(3p), putenv(3p), setenv(3p), unsetenv(3p)

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