ExternC - Embedding C code in python

Example

import externc

# ExternC modifies builtins, so no `externc.` necessary
fn = c("""
  // remember to properly escape backslashes!
  printf("Hello from C\\n");
  Py_RETURN_NONE;
""")

fn() # "Hello from C"

Benchmark

c.py:

import externc
c(r"""
for(int i = 0; i < 1000000000; i++) {
  if(i % (2 << 16) == 0) {
    printf("%d\n", i);
  }
}
Py_RETURN_NONE;
""")()

python.py:

for i in range(1000000000):
  if(i % (2 << 16) == 0):
    print(i)

$ time python c.py
...

real    0m2.615s
user    0m1.593s
sys     0m0.518s

$ time python python.py
...

real    3m25.695s
user    3m25.423s
sys     0m0.194s

The difference is quite obvious

Use cases

Use ExternC

• for interfacing with C APIs
• for working with raw pointers
• for working with capsules
• when speed is more important than stability

Do not use ExternC

• when your code must be version independent
• when your code must be stable (exception: you have used C a lot before)
• when your code must be distributable
• when your code must run anywhere but the PC you wrote and tested it on
• in production

Technicalities

The C function is registered as METH_VARARGS | METH_KEYWORDS. It is of signature PyObject* (PyObject*, PyObject*, PyObject), which is known in the Python extension environment as PyCFunctionWithKeywords. The arguments have the names self, args and kwargs, while the function is named func and the generated module externc_# where # is a hexadecimal number that is incremented by one with every call to c without an explicit module name. The function name and the module name can be modified by passing the funcname and name keyword arguments to c, respectively. funcname is also used as the name of the function to be exported from the module.

The default compiler used is clang with a few default options. The compilers submodule can be used to configure clang with feature flags, linked files and additional include directories or define another compiler, which can be passed to c via the compiler argument.