Python ctypes how to read string modified by C code

Question

After following this answer, I wrote a C library which I call from python using ctypes. The C library contains a function which takes a char* and modifies its contents. I would like to read the contents in Python, however, after calling the library function in Python, I get an empty string even though I see the correct output in the terminal if I include a printf statement in the C code. What am I doing wrong?

C code:

void somefunction(char * pythonbuffer)
{
    /* do something */
    printf("%s", pythonbuffer);
}

Python code:

lib.somefunction.argtypes = [ctypes.c_char_p] 
lib.somefunction.restype = ctypes.c_void_p 

buffer = ctypes.create_string_buffer(upper_limit)

lib.somefunction(buffer)

print(buffer.value)

Output: b''

How exactly does somefunction modify its parameter pythonbuffer? What did you expect to see happen? — iRove
– iRove, Commented Jun 21, 2017 at 17:57
It calls another library which stores a long string in pythonbuffer. If I then print the content in C I get what I expect, but I can't get the same output in Python. — fbartolic
– fbartolic, Commented Jun 21, 2017 at 18:06
@Coldspeed, POINTER(buffer) makes no sense, and pointer(buffer) would be redundant. — Eryk Sun
– Eryk Sun, Commented Jun 21, 2017 at 19:14
@Coldspeed , if you look at this the buffer should be passed as is. — fbartolic
– fbartolic, Commented Jun 22, 2017 at 8:48

fbartolic · Accepted Answer · 2017-06-22 10:04:55Z

1

Found the problem. Turns out that the code in the /* do something */ part somehow changed the adress of pythonbuffer. I solved it by using a temporary char * and then copying it to pythonbuffer with strcpy.

answered Jun 22, 2017 at 10:04

fbartolic

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2 Comments

Eryk Sun Over a year ago

Remember to fix restype to be None instead of returning a garbage c_void_p pointer.

fbartolic Over a year ago

Yep, I did that.

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