Use another language to call a function: Difference between revisions

Our embedded python function a) uses information from the main routine in c, and b) determines the information to populate the result returned to the main routine. This, I believe, fulfills the task requirement. The modifications and compilation are shown for Ubuntu linux Autumn 2011 version, with python3. It's easier to call a dynamic library from python using the ctypes module. Consider using <tt>PyRun_SimpleString</tt> to have main.c call python calling back to c.

<lang python>

# store this in file rc_embed.py

def query(buffer_length):

message = b'Here am I'

L = len(message)

n = min(L,buffer_length)

#print(message[:n])

return bytes(message[:n])

</lang>

main.c

<lang c>

#if 0

//I rewrote the driver according to good sense, my style,

//and discussion --Kernigh 15:45, 12 February 2011 (UTC).

#endif

#include<stdio.h>

#include<stdlib.h>

#include<string.h>

int main(int argc,char*argv[]) {

char Buffer[1024], *pc;

unsigned Size = sizeof(Buffer);

if (!Query(Buffer,&Size))

fputs("Failed to call Query",stdout);

else

for (pc = Buffer; Size--; ++pc)

putchar(*pc);

putchar('\n');

return EXIT_SUCCESS;

}

</lang>

In Query.c I don't promise to have tested every case with missing module, missing function, or to have used <tt>Py_DECREF</tt> correctly.

<lang c>

#include<stdio.h>

#include<stdlib.h>

#include<string.h>

#include<Python.h>

int Query(char*Data,unsigned*Length) {

char *module = "rc_embed", *function = "query";

PyObject *pName, *pModule, *pFunc, *pResult, *pArgs, *pLength;

long result = 0;

if (!Py_IsInitialized())

Py_Initialize();

pName = PyUnicode_FromString(module);

pModule = PyImport_Import(pName);

Py_DECREF(pName);

if (NULL == pModule) {

PyErr_Print();

fprintf(stderr,"Failed to load \"%s\"\n",module);

return 0;

}

pFunc = PyObject_GetAttrString(pModule,function);

if ((NULL == pFunc) || (!PyCallable_Check(pFunc))) {

if (PyErr_Occurred())

PyErr_Print();

fprintf(stderr,"Cannot find function \"%s\"\n",function);

if (NULL != pFunc)

Py_DECREF(pFunc);

Py_DECREF(pModule);

return 0;

}

pArgs = PyTuple_New(1);

pLength = PyLong_FromUnsignedLong((unsigned long)(*Length));

if (NULL == pLength) {

Py_DECREF(pArgs);

Py_DECREF(pFunc);

Py_DECREF(pModule);

return 0;

}

PyTuple_SetItem(pArgs,0,pLength);

pResult = PyObject_CallObject(pFunc, pArgs);

if (NULL == pResult)

result = 0;

else if (!PyBytes_Check(pResult)) {

result = 0;

Py_DECREF(pResult);

} else {

*Length = (unsigned)PyBytes_Size(pResult);

strncpy(Data,PyBytes_AsString(pResult),*Length);

Py_DECREF(pResult);

result = 1;

}

Py_DECREF(pArgs);

Py_DECREF(pFunc);

Py_DECREF(pModule);

Py_Finalize();

return result;

}

</lang>

Compilation, linkage, execution. Note the python tools used to extract the correct flags.

<lang bash>

$ make main.o

cc -c -o main.o main.c

$ D=$( dirname $( which python3 ) )

$ gcc $( $D/python3.2-config --cflags ) -c Query.c

In file included from /usr/include/python3.2mu/Python.h:8:0,

from Q.c:18:

/usr/include/python3.2mu/pyconfig.h:1173:0: warning: "_POSIX_C_SOURCE" redefined [enabled by default]

/usr/include/features.h:214:0: note: this is the location of the previous definition

$ gcc -o main main.o Query.o $( $D/python3.2-config --ldflags )

$ ./main

Here am I

$

</lang>