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According to the Microsoft Visual Studio 2005/.NET Framework 2.0 help pages http://msdn2.microsoft.com/en-us/library/tehxacec(VS.80).aspx:
The getenv function searches the list of environment variables for varname. getenv is not case sensitive in the Windows operating system. getenv and _putenv use the copy of the environment pointed to by the global variable _environ to access the environment. getenv operates only on the data structures accessible to the run-time library and not on the environment "segment" created for the process by the operating system. Therefore, programs that use the envp argument to main or wmain may retrieve invalid information.
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char *pValue; size_t len; errno_t err = _dupenv_s( &pValue, &len, "pathext" ); if ( err ) return -1; printf( "pathext = %s\n", pValue ); free( pValue ); err = _dupenv_s( &pValue, &len, "nonexistentvariable" ); if ( err ) return -1; printf( "nonexistentvariable = %s\n", pValue ); free( pValue ); // It's OK to call free with NULL |
Compliant Solution (Windows)
Microsoft Visual Studio 2005 provides provides the ((getenv_s()}} and _wgetenv_s() functions for getting a value from the current environment.
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#include <stdlib.h>
#include <stdio.h>
int main( void ) {
char* libvar;
size_t requiredSize;
getenv_s( &requiredSize, NULL, 0, "LIB");
libvar = (char*) malloc(requiredSize * sizeof(char));
if (!libvar)
{
printf("Failed to allocate memory!\n");
exit(1);
}
// Get the value of the LIB environment variable.
getenv_s( &requiredSize, libvar, requiredSize, "LIB" );
if( libvar != NULL )
printf( "Original LIB variable is: %s\n", libvar );
// Attempt to change path. Note that this only affects
// the environment variable of the current process. The command
// processor's environment is not changed.
_putenv_s( "LIB", "c:\\mylib;c:\\yourlib" );
getenv_s( &requiredSize, NULL, 0, "LIB");
libvar = (char*) malloc(requiredSize * sizeof(char));
if (!libvar) {
printf("Failed to allocate memory!\n");
exit(1);
}
// Get the new value of the LIB environment variable.
getenv_s( &requiredSize, libvar, requiredSize, "LIB" );
if( libvar != NULL )
printf( "New LIB variable is: %s\n", libvar );
}
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There is a race condition here even after you call getenv() and before you copy. Be careful to only manipulate the process environment from a single thread at a time.
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