Attempt to call dll function from matlab causing crash

I'm trying to use a third-party external DLL (from usbmicro) within matlab, but it keeps crashing matlab. This is from the documentation indicating the syntax of the function call from within a C program:

int USBm_About( char *about );

I tried this matlab script (yes it's very kludgy, I'm a matlab noob):
 

>> loadlibrary('USBm.dll','USBmAPI.h')
>> libfunctions('USBm')
>> s='sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss';
>> st=strcat(s,s,s,s);
>> vp = libpointer('voidPtr',[int8(st) 0]);
>> result=calllib('USBm','USBm_About',vp)

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and this matlab script:
 

>> loadlibrary('USBm.dll','USBmAPI.h')
>> libfunctions('USBm')
>> s='sssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss';
>> st=strcat(s,s,s,s);
>> vp=libpointer('cstring',st);
>> result=calllib('USBm','USBm_About',vp)

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In both cases, the calllib() call causes matlab to crash with a segmentation fault.