Mixing
Inconsistent c_char_p behavior between returning vs pointer assignment
Consider the following C functions:
void AssignPointer(char **p) {
*p = "Test1";
}
char* Return() {
return "Test2";
}
Now consider the following code in Python:
import ctypes
lib = CDLL('LibraryPathHere')
lib.AssignPointer.restype = None
lib.AssignPointer.argtypes = (ctypes.POINTER(ctypes.c_char_p),)
lib.Return.restype = ctypes.c_char_p
lib.Return.argtypes = None
def to_python_string(c_str : ctypes.c_char_p) -> str:
return c_str.value.decode('ascii')
Now the following works:
c_str = ctypes.c_char_p()
lib.AssignPointer(ctypes.byref(c_str))
print(to_python_string(c_str))
However the following gives AttributeError: 'bytes' object has no attribute 'value' :
c_str = lib.Return()
print(to_python_string(c_str))
In the first case the debugger shows c_str as c_char_p(ADDRESS_HERE). In the second case the debugger shows c_str as b'Test2'.
So is this a bug in Python/ctypes or am I doing something wrong?
python c string interop ctypes
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
add a comment |
Consider the following C functions:
void AssignPointer(char **p) {
*p = "Test1";
}
char* Return() {
return "Test2";
}
Now consider the following code in Python:
import ctypes
lib = CDLL('LibraryPathHere')
lib.AssignPointer.restype = None
lib.AssignPointer.argtypes = (ctypes.POINTER(ctypes.c_char_p),)
lib.Return.restype = ctypes.c_char_p
lib.Return.argtypes = None
def to_python_string(c_str : ctypes.c_char_p) -> str:
return c_str.value.decode('ascii')
Now the following works:
c_str = ctypes.c_char_p()
lib.AssignPointer(ctypes.byref(c_str))
print(to_python_string(c_str))
However the following gives AttributeError: 'bytes' object has no attribute 'value' :
c_str = lib.Return()
print(to_python_string(c_str))
In the first case the debugger shows c_str as c_char_p(ADDRESS_HERE). In the second case the debugger shows c_str as b'Test2'.
So is this a bug in Python/ctypes or am I doing something wrong?
python c string interop ctypes
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
add a comment |
Consider the following C functions:
void AssignPointer(char **p) {
*p = "Test1";
}
char* Return() {
return "Test2";
}
Now consider the following code in Python:
import ctypes
lib = CDLL('LibraryPathHere')
lib.AssignPointer.restype = None
lib.AssignPointer.argtypes = (ctypes.POINTER(ctypes.c_char_p),)
lib.Return.restype = ctypes.c_char_p
lib.Return.argtypes = None
def to_python_string(c_str : ctypes.c_char_p) -> str:
return c_str.value.decode('ascii')
Now the following works:
c_str = ctypes.c_char_p()
lib.AssignPointer(ctypes.byref(c_str))
print(to_python_string(c_str))
However the following gives AttributeError: 'bytes' object has no attribute 'value' :
c_str = lib.Return()
print(to_python_string(c_str))
In the first case the debugger shows c_str as c_char_p(ADDRESS_HERE). In the second case the debugger shows c_str as b'Test2'.
So is this a bug in Python/ctypes or am I doing something wrong?
python c string interop ctypes
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
Consider the following C functions:
void AssignPointer(char **p) {
*p = "Test1";
}
char* Return() {
return "Test2";
}
Now consider the following code in Python:
import ctypes
lib = CDLL('LibraryPathHere')
lib.AssignPointer.restype = None
lib.AssignPointer.argtypes = (ctypes.POINTER(ctypes.c_char_p),)
lib.Return.restype = ctypes.c_char_p
lib.Return.argtypes = None
def to_python_string(c_str : ctypes.c_char_p) -> str:
return c_str.value.decode('ascii')
Now the following works:
c_str = ctypes.c_char_p()
lib.AssignPointer(ctypes.byref(c_str))
print(to_python_string(c_str))
However the following gives AttributeError: 'bytes' object has no attribute 'value' :
c_str = lib.Return()
print(to_python_string(c_str))
In the first case the debugger shows c_str as c_char_p(ADDRESS_HERE). In the second case the debugger shows c_str as b'Test2'.
So is this a bug in Python/ctypes or am I doing something wrong?
python c string interop ctypes
python c string interop ctypes
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
edited Jan 1 at 22:34
Serge Rogatch
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
asked Jan 1 at 22:28
Serge RogatchSerge Rogatch
6,42923468
6,42923468
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
ctypes automatically converts c_char_p return values to bytes objects.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python bytes object, not a c_char_p instance.
If you want the actual pointer value, use ctypes.POINTER(ctypes.c_char) as the restype.
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
If I changerestypetoctypes.POINTER(ctypes.c_char_p), I get error:AttributeError: 'LP_c_char_p' object has no attribute 'value'
– Serge Rogatch
Jan 1 at 22:50
@SergeRogatch: First, it'sctypes.POINTER(ctypes.c_char), notctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype.c_char_pis a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.
– user2357112
Jan 1 at 22:53
add a comment |
Finally here is a workaround for this problem:
To avoid automatic conversions of c_char_p to bytes, set C function's restype as c_void_p:
lib.Return.restype = ctypes.c_void_p
Then cast to c_char_p before passing to a function that expects c_char_p as the common case:
void_ptr = lib.Return()
c_str = ctypes.cast(void_ptr, ctypes.c_char_p)
print(to_python_string(c_str))
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
add a comment |
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2 Answers
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votes
2 Answers
2
active
oldest
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active
oldest
votes
active
oldest
votes
ctypes automatically converts c_char_p return values to bytes objects.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python bytes object, not a c_char_p instance.
If you want the actual pointer value, use ctypes.POINTER(ctypes.c_char) as the restype.
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
If I changerestypetoctypes.POINTER(ctypes.c_char_p), I get error:AttributeError: 'LP_c_char_p' object has no attribute 'value'
– Serge Rogatch
Jan 1 at 22:50
@SergeRogatch: First, it'sctypes.POINTER(ctypes.c_char), notctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype.c_char_pis a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.
– user2357112
Jan 1 at 22:53
add a comment |
ctypes automatically converts c_char_p return values to bytes objects.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python bytes object, not a c_char_p instance.
If you want the actual pointer value, use ctypes.POINTER(ctypes.c_char) as the restype.
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
If I changerestypetoctypes.POINTER(ctypes.c_char_p), I get error:AttributeError: 'LP_c_char_p' object has no attribute 'value'
– Serge Rogatch
Jan 1 at 22:50
@SergeRogatch: First, it'sctypes.POINTER(ctypes.c_char), notctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype.c_char_pis a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.
– user2357112
Jan 1 at 22:53
add a comment |
ctypes automatically converts c_char_p return values to bytes objects.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python bytes object, not a c_char_p instance.
If you want the actual pointer value, use ctypes.POINTER(ctypes.c_char) as the restype.
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
ctypes automatically converts c_char_p return values to bytes objects.
Fundamental data types, when returned as foreign function call results, or, for example, by retrieving structure field members or array items, are transparently converted to native Python types. In other words, if a foreign function has a restype of c_char_p, you will always receive a Python bytes object, not a c_char_p instance.
If you want the actual pointer value, use ctypes.POINTER(ctypes.c_char) as the restype.
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
edited Jan 1 at 22:38
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
answered Jan 1 at 22:34
user2357112user2357112
156k12170264
156k12170264
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
If I changerestypetoctypes.POINTER(ctypes.c_char_p), I get error:AttributeError: 'LP_c_char_p' object has no attribute 'value'
– Serge Rogatch
Jan 1 at 22:50
@SergeRogatch: First, it'sctypes.POINTER(ctypes.c_char), notctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype.c_char_pis a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.
– user2357112
Jan 1 at 22:53
add a comment |
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
If I changerestypetoctypes.POINTER(ctypes.c_char_p), I get error:AttributeError: 'LP_c_char_p' object has no attribute 'value'
– Serge Rogatch
Jan 1 at 22:50
@SergeRogatch: First, it'sctypes.POINTER(ctypes.c_char), notctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype.c_char_pis a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.
– user2357112
Jan 1 at 22:53
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
If this functions as designed, could you elaborate on how to workaround this?
– Serge Rogatch
Jan 1 at 22:35
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
@SergeRogatch: Answer expanded.
– user2357112
Jan 1 at 22:39
If I change
restype to ctypes.POINTER(ctypes.c_char_p), I get error: AttributeError: 'LP_c_char_p' object has no attribute 'value'– Serge Rogatch
Jan 1 at 22:50
If I change
restype to ctypes.POINTER(ctypes.c_char_p), I get error: AttributeError: 'LP_c_char_p' object has no attribute 'value'– Serge Rogatch
Jan 1 at 22:50
@SergeRogatch: First, it's
ctypes.POINTER(ctypes.c_char), not ctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype. c_char_p is a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.– user2357112
Jan 1 at 22:53
@SergeRogatch: First, it's
ctypes.POINTER(ctypes.c_char), not ctypes.POINTER(ctypes.c_char_p). Second, you're going to need more adjustment than just swapping out the restype. c_char_p is a "pseudo-primitive" type that doesn't behave like normal pointers. You're going to have to do pointer handling yourself, or cast back to c_char_p.– user2357112
Jan 1 at 22:53
add a comment |
Finally here is a workaround for this problem:
To avoid automatic conversions of c_char_p to bytes, set C function's restype as c_void_p:
lib.Return.restype = ctypes.c_void_p
Then cast to c_char_p before passing to a function that expects c_char_p as the common case:
void_ptr = lib.Return()
c_str = ctypes.cast(void_ptr, ctypes.c_char_p)
print(to_python_string(c_str))
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
add a comment |
Finally here is a workaround for this problem:
To avoid automatic conversions of c_char_p to bytes, set C function's restype as c_void_p:
lib.Return.restype = ctypes.c_void_p
Then cast to c_char_p before passing to a function that expects c_char_p as the common case:
void_ptr = lib.Return()
c_str = ctypes.cast(void_ptr, ctypes.c_char_p)
print(to_python_string(c_str))
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
add a comment |
Finally here is a workaround for this problem:
To avoid automatic conversions of c_char_p to bytes, set C function's restype as c_void_p:
lib.Return.restype = ctypes.c_void_p
Then cast to c_char_p before passing to a function that expects c_char_p as the common case:
void_ptr = lib.Return()
c_str = ctypes.cast(void_ptr, ctypes.c_char_p)
print(to_python_string(c_str))
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
Finally here is a workaround for this problem:
To avoid automatic conversions of c_char_p to bytes, set C function's restype as c_void_p:
lib.Return.restype = ctypes.c_void_p
Then cast to c_char_p before passing to a function that expects c_char_p as the common case:
void_ptr = lib.Return()
c_str = ctypes.cast(void_ptr, ctypes.c_char_p)
print(to_python_string(c_str))
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
answered Jan 1 at 23:09
Serge RogatchSerge Rogatch
6,42923468
6,42923468
add a comment |
add a comment |
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