Re: [ctypes-users] calling function with a string

[Mark T. <met...@gm...>]

<jas...@ma...> wrote in message 
news:8CC...@we......
> Hi Users

> Am trying out the ctypes module.
> I have a dll with a function expecting the following.
> double priceFromYield(
>       double Yield,                 // number of percentage points
>       double Coupon,                // assumed face value = 100
>       double compfreq,        // compounding frequency of yield
>       int settleDays,               // number of settlement days
>       string TodaysDate,            // today's date - trade date
>       string DatedDate,       // dated date - issue date
>       string FirstCpnDate,    // first coupon date
>       string LastCpnDate,           // last coupon date prior to maturity
>       string MaturityDate,    // maturity date
>       string Tenor,                 // coupon tenor: "6m", "1y"
>       string DayCount,        // day count conventions
>       string RollConv,        // roll conventions of business days
>       string Calendar,        // holiday calendar
>       string CouponTpye       // coupon type
>       );

"string" is a C++ object.  ctypes knows nothing about it.  Also, a C++ 
function looks "different"...declare it as a "C" function.

If you control the source code, use "const char *" instead of "string" and 
make sure to declare the function extern "C", e.g.:

extern "C" __declspec(dllexport) double priceFromYield(
      double Yield,                 // number of percentage points
      double Coupon,                // assumed face value = 100
      double compfreq,        // compounding frequency of yield
      int settleDays,               // number of settlement days
      const char* TodaysDate,            // today's date - trade date
      const char* DatedDate,       // dated date - issue date
      const char* FirstCpnDate,    // first coupon date
      const char* LastCpnDate,           // last coupon date prior to 
maturity
      const char* MaturityDate,    // maturity date
      const char* Tenor,                 // coupon tenor: "6m", "1y"
      const char* DayCount,        // day count conventions
      const char* RollConv,        // roll conventions of business days
      const char* Calendar,        // holiday calendar
      const char* CouponTpye       // coupon type
      )
 {
  ...code...
 }

The Python code needs to declare the result type and arguments:

from ctypes import *
dll=CDLL('mydll')
dll.priceFromYield.restype = c_double
dll.priceFromYield.argtypes = [
    c_double,c_double,c_double,c_int,
    c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,
    c_char_p,c_char_p,c_char_p,c_char_p,c_char_p
]
dll.priceFromYield(1,1,1,1,'a','b','c','d','e','f','g','h','i','j')

I posted this quickly...ask if anything is unclear.

-Mark