ID Number: Q78542
3.00
WINDOWS
Summary:
The C run-time library function getenv() does not work when called
from a Windows dynamic-link library (DLL). DOS environment variables
can be found by searching the environment block. This article explains
a method to retrieve information from environment variables.
More Information:
In a Windows system, an application's environment block is not
available to a DLL. For this reason, the getenv() C run-time library
function does not work when called from a DLL.
To gain access to environment variables, an application must use the
Windows GetDOSEnvironment() function, which returns a far pointer to
the environment block of the currently running task. Using this
pointer, the application can search the environment block for the
desired variable. The format of the environment block is as follows:
1. Each environment variable is followed by an equal sign (=), the
contents of the variable, and a NULL terminator (\0).
2. The last environment variable in the block is terminated with two
NULL characters.
An environment variable set with the following DOS command
set envvar=c:\this\is\a\test
is stored in memory as follows:
ENVVAR=c:\this\is\a\test\0
DOS converts environment variables to uppercase letters; therefore,
all searches for an environment variable must take this into account.
Below is source code to a function called DLLGetEnv() that acts as a
substitute for the getenv() function in a DLL:
/* forward declaration */
LPSTR FAR PASCAL DLLGetEnv ( LPSTR );
/*********************************************************************
* DLLGetEnv ( lpszVariableName ) *
* *
* Takes a LPSTR to the name of an environment variable and returns *
* the contents of that variable. Returns NULL if the environment *
* variable does not exist. The search for the environment variable *
* is case sensitive. *
* *
*********************************************************************/
LPSTR FAR PASCAL DLLGetEnv ( LPSTR lpszVariableName )
{
LPSTR lpEnvSearch;
LPSTR lpszVarSearch;
if ( !*lpszVariableName ) // Check for a NULL pointer
return NULL;
// Get a pointer to the DOS Environment Block
lpEnvSearch = GetDOSEnvironment ();
while ( *lpEnvSearch ) // While there are strings to parse
{ /*
* Make a copy of the pointer to the name of the
* environment variable to search for.
*/
lpszVarSearch = lpszVariableName;
// Check to see if the variable names match
while ( *lpEnvSearch && * lpszVarSearch &&
*lpEnvSearch == *lpszVarSearch )
{
lpEnvSearch++;
lpszVarSearch++;
}
/*
* If the names match, the lpEnvSearch pointer is on the "="
* character and lpszVarSearch is on a null terminator.
* Increment and return lpszEnvSearch, which will point to the
* environment variable's contents.
*
* If the names do not match, increment lpEnvSearch until it
* reaches the end of the current variable string.
*/
if ( *lpEnvSearch == '=' && *lpszVarSearch == '\0' )
return ( lpEnvSearch + 1 );
else
while ( *lpEnvSearch )
lpEnvSearch++;
/*
* At this point the end of the environment variable's string
* has been reached. Increment lpEnvSearch to move to the
* next variable in the environment block. If it is NULL,
* the end of the environment block has been reached.
*/
lpEnvSearch++;
}
return NULL; /*
* If this section of code is reached, the variable
* was not found.
*/
}
Additional reference words: 3.00