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| <a name="Environment-Access"></a> |
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| <h4 class="subsection">25.4.1 Environment Access</h4> |
| |
| <p><a name="index-environment-access-3100"></a><a name="index-environment-representation-3101"></a> |
| The value of an environment variable can be accessed with the |
| <code>getenv</code> function. This is declared in the header file |
| <samp><span class="file">stdlib.h</span></samp>. Modifications of enviroment variables are not |
| allowed in Multi-threaded programs. The <code>getenv</code> function |
| can be safely used in multi-threaded programs |
| <a name="index-stdlib_002eh-3102"></a> |
| <!-- stdlib.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Function: char * <b>getenv</b> (<var>const char *name</var>)<var><a name="index-getenv-3103"></a></var><br> |
| <blockquote><p>This function returns a string that is the value of the environment |
| variable <var>name</var>. You must not modify this string. In some non-Unix |
| systems not using the GNU library, it might be overwritten by subsequent |
| calls to <code>getenv</code> (but not by any other library function). If the |
| environment variable <var>name</var> is not defined, the value is a null |
| pointer. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- SVID --> |
| <div class="defun"> |
| — Function: int <b>putenv</b> (<var>char *string</var>)<var><a name="index-putenv-3104"></a></var><br> |
| <blockquote><p>The <code>putenv</code> function adds or removes definitions from the environment. |
| If the <var>string</var> is of the form ‘<samp><var>name</var><span class="samp">=</span><var>value</var></samp>’, the |
| definition is added to the environment. Otherwise, the <var>string</var> is |
| interpreted as the name of an environment variable, and any definition |
| for this variable in the environment is removed. |
| |
| <p>The difference to the <code>setenv</code> function is that the exact string |
| given as the parameter <var>string</var> is put into the environment. If the |
| user should change the string after the <code>putenv</code> call this will |
| reflect in automatically in the environment. This also requires that |
| <var>string</var> is no automatic variable which scope is left before the |
| variable is removed from the environment. The same applies of course to |
| dynamically allocated variables which are freed later. |
| |
| <p>This function is part of the extended Unix interface. Since it was also |
| available in old SVID libraries you should define either |
| <var>_XOPEN_SOURCE</var> or <var>_SVID_SOURCE</var> before including any header. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>setenv</b> (<var>const char *name, const char *value, int replace</var>)<var><a name="index-setenv-3105"></a></var><br> |
| <blockquote><p>The <code>setenv</code> function can be used to add a new definition to the |
| environment. The entry with the name <var>name</var> is replaced by the |
| value ‘<samp><var>name</var><span class="samp">=</span><var>value</var></samp>’. Please note that this is also true |
| if <var>value</var> is the empty string. To do this a new string is created |
| and the strings <var>name</var> and <var>value</var> are copied. A null pointer |
| for the <var>value</var> parameter is illegal. If the environment already |
| contains an entry with key <var>name</var> the <var>replace</var> parameter |
| controls the action. If replace is zero, nothing happens. Otherwise |
| the old entry is replaced by the new one. |
| |
| <p>Please note that you cannot remove an entry completely using this function. |
| |
| <p>This function was originally part of the BSD library but is now part of |
| the Unix standard. |
| </p></blockquote></div> |
| |
| <!-- stdlib.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>unsetenv</b> (<var>const char *name</var>)<var><a name="index-unsetenv-3106"></a></var><br> |
| <blockquote><p>Using this function one can remove an entry completely from the |
| environment. If the environment contains an entry with the key |
| <var>name</var> this whole entry is removed. A call to this function is |
| equivalent to a call to <code>putenv</code> when the <var>value</var> part of the |
| string is empty. |
| |
| <p>The function return <code>-1</code> if <var>name</var> is a null pointer, points to |
| an empty string, or points to a string containing a <code>=</code> character. |
| It returns <code>0</code> if the call succeeded. |
| |
| <p>This function was originally part of the BSD library but is now part of |
| the Unix standard. The BSD version had no return value, though. |
| </p></blockquote></div> |
| |
| <p>There is one more function to modify the whole environment. This |
| function is said to be used in the POSIX.9 (POSIX bindings for Fortran |
| 77) and so one should expect it did made it into POSIX.1. But this |
| never happened. But we still provide this function as a GNU extension |
| to enable writing standard compliant Fortran environments. |
| |
| <!-- stdlib.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>clearenv</b> (<var>void</var>)<var><a name="index-clearenv-3107"></a></var><br> |
| <blockquote><p>The <code>clearenv</code> function removes all entries from the environment. |
| Using <code>putenv</code> and <code>setenv</code> new entries can be added again |
| later. |
| |
| <p>If the function is successful it returns <code>0</code>. Otherwise the return |
| value is nonzero. |
| </p></blockquote></div> |
| |
| <p>You can deal directly with the underlying representation of environment |
| objects to add more variables to the environment (for example, to |
| communicate with another program you are about to execute; |
| see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>). |
| |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Variable: char ** <b>environ</b><var><a name="index-environ-3108"></a></var><br> |
| <blockquote><p>The environment is represented as an array of strings. Each string is |
| of the format ‘<samp><var>name</var><span class="samp">=</span><var>value</var></samp>’. The order in which |
| strings appear in the environment is not significant, but the same |
| <var>name</var> must not appear more than once. The last element of the |
| array is a null pointer. |
| |
| <p>This variable is declared in the header file <samp><span class="file">unistd.h</span></samp>. |
| |
| <p>If you just want to get the value of an environment variable, use |
| <code>getenv</code>. |
| </p></blockquote></div> |
| |
| <p>Unix systems, and the GNU system, pass the initial value of |
| <code>environ</code> as the third argument to <code>main</code>. |
| See <a href="Program-Arguments.html#Program-Arguments">Program Arguments</a>. |
| |
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