c88ee54d-2edd-4b67-ba1b-3bb18bc351ff
commented
15 years ago We have an application that calls a 3rd party library that returns a structure (by value) from a callback. I'm including some sample code that duplicates the behavior. The problem is that return types from callbacks cannot be anything other than simple datatypes (c_int, c_float, ..., c_void_p). For other datatypes (STRUCTURE, POINTER, ...), ctypes returns the following error:
"invalid result type for callback function"
The error message comes from callback.c, in function AllocFunctionCallback.
I think this may be a duplicate of issue bpo-1574584.
I've tested this on Windows and linux; I'm including a Makefile and source code that works on Windows/cygwin.
------ file: Makefile ----
all: hello.dll hello.exe hello_dll.exe clean: rm *.exe *.dll
hello.exe: hello.h hello.c hello_main.c gcc hello.c hello_main.c -o hello.exe --save-temps
hello.dll: hello.h hello.c gcc -mno-cygwin -shared hello.c -o hello.dll --save-temps
hello_dll.exe: hello.h hello.c gcc hello_main.c -L. -lhello -o hello_main.exe
------ file: hello.h ----
struct helloStruct {
int i;
float f;
int i2;
int i3;
};
float fxn (struct helloStruct callback());------ file: hello.c ----
#include <stdio.h>
#include "hello.h"
float fxn (struct helloStruct callback()) {
struct helloStruct result = callback();printf ("i: %d\n", result.i); printf ("f: %f\n", result.f); return result.f * result.i; }
------ file: hello_main.c ----
#include <stdio.h>
#include "hello.h"
struct helloStruct callback();
int main (int argc, char **argv) {
float f;
struct helloStruct result;
printf ("Hello world\n");
f = fxn (callback);
printf ("Callback result: %f\n", f);
}
struct helloStruct callback () {
struct helloStruct result;
result.i = 10;
result.f = 3.14159;
return result;
}
int int_callback () {
return 42;
}------ file: hello.py ----
from ctypes import cdll, c_char, c_int, c_float, Structure, CFUNCTYPE,
POINTER, c_char_p
class helloStruct (Structure):
pass
helloStruct._fields_ = [
('i', c_int),
('f', c_float) ]
def callback():
print ("callback()")
hs = helloStruct()
hs.i = 10
hs.f = 25.5
return hs
libc = cdll.msvcrt
#helloLib = libc.load_library("hello")
#helloLib = libc.hello
helloLib = cdll.hello
helloLib.fxn.restype = helloStruct
# It looks like only simple return types are supported for
# callback functions. simple = c_int, c_float, ...
# Python bug # 1574584 - status: closed.
# Suggests posting to ctypes-users, but I don't see any recent activity.
#
TMP_FCN = CFUNCTYPE (POINTER(c_char)) # Error message
#TMP_FCN = CFUNCTYPE (c_char_p) # Runs, but invalid result
#TMP_FCN = CFUNCTYPE (c_void_p) # Runs, but invalid result
#TMP_FCN = CFUNCTYPE (c_int) # Runs, but invalid result
#TMP_FCN = CFUNCTYPE (POINTER(c_int)) # Error message
#TMP_FCN = CFUNCTYPE (POINTER(helloStruct)) # Error message
#TMP_FCN = CFUNCTYPE (helloStruct) # Error message
callback_fcn = TMP_FCN (callback)
result = helloLib.fxn (callback_fcn)# 2.5
print "result: ", result
# 3.0 print ("result: ", result)
theller
amauryfa
1cbd51ed-0c85-4ad8-9beb-1b25f971f6ac
bf28a09e-6d72-4c82-a4b1-8cc669e32358
bebc4138-1558-4d27-8ecf-b8b86bdcdb67
12cd5ddb-1b49-4d91-ac6c-8fce79b8b268
freakboy3742
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields: ```python assignee = None closed_at = None created_at =
labels = ['ctypes', 'type-feature']
title = 'ctypes should return composite types from callbacks'
updated_at =
user = 'https://bugs.python.org/gholling'
```
bugs.python.org fields:
```python
activity =
actor = 'freakboy3742'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation =
creator = 'gholling'
dependencies = []
files = []
hgrepos = []
issue_num = 5710
keywords = []
message_count = 8.0
messages = ['85654', '85731', '85749', '198501', '255582', '306730', '306731', '306778']
nosy_count = 7.0
nosy_names = ['theller', 'amaury.forgeotdarc', 'gholling', 'freakboy3742', 'Albert.Zeyer', 'Pam.McANulty', 'Mason.Bially']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue5710'
versions = ['Python 2.7', 'Python 3.4']
```