Weird issues when passing an array of structs from Swift to C

Question

I'm passing an array of structs from Swift to a C function. The struct looks like this:

struct Struct {
    int a;
    float b;
    float c;
    const char* d;
    const char* e;
    const char* f;
    const char* g;
    int h[4];
};

Function signature of the C function:

void test(struct Struct* structs);

Weirdly, when I print d in the C function, it's often something different than what I set it to in the Swift code: usually an empty string or some garbage. When I set d to a very long string, it works correctly. The other strings are passed correctly too. Is that some struct alignment issue?

If this is an alignment issue, we probably need more details. What are sizeof and alignof of int and float in C? What are the actual pointer values? What compiler are you using, and for what machine? — Daniel H
– Daniel H, Commented Dec 21, 2017 at 17:40
How do you fill the struct in your Swift code? My guess would be that the char * members point to C strings which were temporarily created from Swift strings, and are already deallocated or overwritten when the C function is called. — Martin R
– Martin R, Commented Dec 21, 2017 at 18:15
@DanielH: The struct layout in Swift can be different from C in general, but for structures imported from C, Swift preserves the layout (sizes, alignment, padding) — Martin R
– Martin R, Commented Dec 21, 2017 at 18:43
@MartinR I'm passing string literals to the struct's constructor, i.e. something like Struct("test"). But it seems that your guess was correct, the C strings are overwritten by whatever is written to the stack... — fhucho
– fhucho, Commented Dec 27, 2017 at 14:01

fhucho · Accepted Answer · 2017-12-27 16:14:06Z

0

As @MartinR suggested, when I pass the string to the struct's constructor, Swift creates a temporary C char array on the stack, copies the string's data into it and passes its pointer to the constructor. Immediately after that, the char array is no longer valid. Here's an example code demonstrating this:

let s = Struct(string: "string")
print(s.string) // Prints "string"
print("lol") // Overwrite the stack
print(s.string) // Prints "lol"

See https://stackoverflow.com/a/40121697/243225 for possible solutions.

edited Dec 27, 2017 at 16:14

answered Dec 27, 2017 at 14:18

fhucho

34.6k40 gold badges143 silver badges193 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Weird issues when passing an array of structs from Swift to C

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related