argtypes and restype

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.

Exercise 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").

Show example 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"))

Test yourself

What does restype specify?
```
libc.strlen.restype = ctypes.c_size_t
```
- The return type
- The library name
- The argument types
- The function name
Show answer

The return type — restype sets the C type of the function's return value.
Match signature concepts

argtypes

Argument types
restype

Return type
c_char_p

char* argument
c_size_t

size_t return

Complete the argtypes assignment

libc.strlen.____ = [ctypes.c_char_p]

Show answer

libc.strlen.argtypes = [ctypes.c_char_p]

Next lesson: cffi

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