ctypes pass string by reference: string is not updated

Question

I want to pass the reference to a string from python to a c program. The c program writes something to the string and I want to read it back in python. The c program looks like:

void foo(char *name) {
    name = "Hello World";
}

And the Python script:

>>> import ctypes
>>> lib=ctypes.CDLL('libfoo.so')
>>> name=ctypes.create_string_buffer(32)
>>> print name.value

>>> lib.foo(name)
>>> print name.value

>>>

I'd expect "Hello World" after the second print. I'm sure it's simple, but I can't figure out what's wrong...

David Heffernan · Accepted Answer · 2013-03-27 17:43:56Z

2

Your C code changes the value of the pointer, rather than changing the buffer. Remember that C parameters are passed by value so modifying a parameter can never result in changes that can be seen by the caller.

Anyway, you just need to use strcpy to copy your string into the buffer provided.

void foo(char *name) {
    strcpy(name, "Hello World");
}

Obviously in real code you'll want to pass the buffer length too so that you can avoid buffer overruns.

answered Mar 27, 2013 at 17:43

David Heffernan

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