A series of functions exist to convert VARIABLEs to specific types:

IDL_VPTR IDL_CvtByte(int argc, IDL_VPTR argv[])

IDL_VPTR IDL_CvtBytscl(int argc, IDL_VPTR argv[], char *argk) 

IDL_VPTR IDL_CvtFix(int argc, IDL_VPTR argv[])

IDL_VPTR IDL_CvtUInt(int argc, IDL_VPTR argv[])

IDL_VPTR IDL_CvtLng(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtULng(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtLng64(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtULng64(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtFlt(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtDbl(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtComplex(int argc, IDL_VPTR argv[]) 

IDL_VPTR IDL_CvtDComplex(int argc, IDL_VPTR argv[])

IDL_VPTR IDL_CvtString(int argc, IDL_VPTR argv[], char *argk)

When calling these functions, you should set the argk argument to NULL. These functions are the direct implementations of the IDL commands: BYTE, BYTSCL, FIX, UINT, LONG, ULONG, LONG64, ULONG64, FLOAT, DOUBLE, COMPLEX, DCOMPLEX, and STRING.

The behavior of these functions is the same as IDL_BasicTypeConversion() except when converting between bytes and strings. Calling IDL_CvtByte() with a single argument of string type causes each string to be converted to a byte vector of the same length as the string. Each array element is the character code of the corresponding character in the string. Calling IDL_CvtString() with a single argument of IDL_TYP_BYTE has the opposite effect.