Passing binary data from C to Python using "Extending Python with C"

Question

It is possible to pass binary data from C to Python. For example it is possible to pass the bytes of state to Python?. Maybe using some of these functions https://docs.python.org/3/c-api/bytes.html?

uint32_t state[3] = {0x00000300, 0x00001100, 0x00022200};
unsigned char charstate[1420];
memcpy(charstate, (unsigned char *)state, sizeof(uint32_t)*3);
printf("charstate: %s", charstate);
return PyBytes_FromString(charstate);

I tried PyBytes_FromString without success (maybe because state is not a string)

EDITED

Also, I tried @John Zwinck answer

   uint32_t state[3] = {0x00000300, 0x00001100, 0x00022200};
   char charstate[12];
   memset(charstate, 0, 12);
   memcpy(charstate, state, sizeof(uint32_t)*3);
   return PyBytes_FromStringAndSize(charstate, sizeof(uint32_t)*3);

Now I see in Python [0, 3, 0, 0, 0, 17, 0, 0, 0, 34, 2, 0] which is not equal to state (in C).

How did you try? Using socket?

ghchoi
– ghchoi

2021-01-16 10:08:29 +00:00
Commented Jan 16, 2021 at 10:08 — ghchoi
– ghchoi, Commented Jan 16, 2021 at 10:08

John Zwinck · Accepted Answer · 2021-01-16 10:43:01Z

3

PyBytes_FromString() assumes you pass it a null-terminated string. But you have 00 bytes in the middle of your string, which will make the Python string shorter than you want.

Instead, do this:

PyBytes_FromStringAndSize(charstate, sizeof(uint32_t)*3)

answered Jan 16, 2021 at 10:43

John Zwinck

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4 Comments

Ulrich Eckhardt Over a year ago

The printf() format specifier %s also assumes the same, a NUL-terminated string, so that needs fixing as well.

Juan Over a year ago

Thanks @John Zwinck, I think we are very close to the result. Now I see this in python [0, 3, 0, 0, 0, 17, 0, 0, 0, 34, 2, 0]. It is not equal to {0x00000300, 0x00001100, 0x00022200} (in C). What do you think is happening?

Juan Over a year ago

@Jhon Zwinck I understand most significant bit is left

John Zwinck Over a year ago

OK so it's working now then. The order of the bytes is reversed within each uint32_t because you are on a "little endian" computer, so the number 0x00000300 is stored as 00 03 00 00.

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