Swift string literal assignment to C variable

Question

In Swift, when calling C functions Swift strings are auto-coerced to CString when passed as parameters. However, I don't get the same behavior when filling out C structs or globals.

strlen(swiftString)                     //Works!
CGlobalStruct.stringPtr = swiftString   //Doesn't work!

First, Why do they behave different when they are both "const *char". Second, what is the cleanest way to fill out C Structs?

Martin R · Accepted Answer · 2016-10-19 03:49:00Z

5

A Swift String can be passed as an argument to a function taking a UnsafePointer<Int8> parameter. The compiler generates a temporary representation of the Swift string as a NUL-terminated sequence of char and passes a pointer to that C String to the function. The representation and the pointer is only valid during the function call. That is what happens at

strlen(swiftString)

When assigning a global C string pointer then you have to take the lifetime of this pointer into account. One possibility is

let swiftString = "Hello world"

swiftString.withCString {
    CGlobalStruct.stringPtr = $0

    // Pointer valid inside this closure ...
}
// Pointer not valid anymore ...

where a pointer to a C string representation is passed to the closure. This pointer is (only) valid during the execution of the closure.

For longer-lived usage you could allocate a C string in memory, but then you are responsible for freeing that memory eventually:

guard let cStringPtr = strdup(swiftString) else {
    fatalError("strdup failed")
}
CGlobalStruct.stringPtr = UnsafePointer(cStringPtr)

// ...

free(cStringPtr)

edited Oct 19, 2016 at 3:49

answered Oct 19, 2016 at 3:38

Martin R

541k98 gold badges1.3k silver badges1.4k bronze badges

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

2 Comments

Troy Harvey Over a year ago

Very thoughtful response. The bridging of swift to c is quite good in most places, but if this is as good as it gets for assigning swift string literals to C structs... its kind of gross (not to mention obscure). Compare to the simplicity of bridging other types like CGlobalStruct.integer = 45

Martin R Over a year ago

@TroyHarvey: Integers are "simple" types and have the same memory representation in C and Swift. A Swift String is a struct with opaque pointers to some backing store which can hold bytes (for pure ASCII strings) or UTF-16 code units, and is memory managed by Swift, whereas C strings are NUL-terminated sequences of chars. There is no simple bridging between them.

Collectives™ on Stack Overflow

Swift string literal assignment to C variable

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related