argtypes and restype
- Python course
- Expert
- Lesson 21/47
- 5 min
Learn to specify signatures with argtypes and restype.
What are argtypes and restype?
To call a C function correctly you must declare its signature. argtypes lists the argument types and restype sets the return type.
For example libc.strlen.argtypes = [c_char_p] specifies a single char* argument. libc.strlen.restype = c_size_t says the return is size_t. Without these Python defaults to c_int and a size_t return may be truncated. Type declaration is mandatory for both safety and portability.
Type safety
When argtypes and restype are set Python converts arguments automatically and warns on wrong types. This prevents silent errors.
For example if argtypes = [c_char_p] is set and you write libc.strlen(42) Python raises a TypeError. Without type declaration this would silently misbehave. That is why production code always declares signatures. Also without restype large return values can be truncated.
Practice Write a signature for strlen
Import the module with import ctypes. Load libc with ctypes.CDLL(None). Set libc.strlen.argtypes = [ctypes.c_char_p] and libc.strlen.restype = ctypes.c_size_t. Print the result of libc.strlen(b"merhaba").
Starter code
# pick a function from the library
# set its signature and call itSample solution
import ctypes
libc = ctypes.CDLL(None)
libc.strlen.argtypes = [ctypes.c_char_p]
libc.strlen.restype = ctypes.c_size_t
print(libc.strlen(b"merhaba"))Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does restype specify?
libc.strlen.restype = ctypes.c_size_t- The return type Correct answer
- The library name
- The argument types
- The function name
Explanation: restype sets the C type of the function's return value.
2 Match signature concepts
| Term | Meaning |
|---|---|
argtypes |
Argument types |
restype |
Return type |
c_char_p |
char* argument |
c_size_t |
size_t return |
3 Complete the argtypes assignment
libc.strlen.____ = [ctypes.c_char_p]libc.strlen.argtypes = [ctypes.c_char_p]Explanation: argtypes takes a list of argument types.