Obtain a pointer to a C char array in Swift

Question

A have a structure like this (defined in bson.h of mongodb c driver):

typedef struct
{
  uint32_t domain;
  uint32_t code;
  char message[504];
} bson_error_         


        

Answer 1

It's not pretty, not intuitive, but it's doable. Purely in Swift, no C glue code needed. A minimal demo:

b.h

typedef struct {
    int n;
    char s[8];
} Bridged;

Bridged *make_b(void);

b.c

#include 
#include 
#include "b.h"

Bridged *make_b(void)
{
    Bridged *p = calloc(sizeof(*p), 1);
    memcpy(p->s, "foobarz", 8);
    return p;
}

b.swift:

// half compile-time, half run-time black magic
func toCharArray(t: T) -> [CChar] {
     var a: [CChar] = []
     let mirror = reflect(t)
     for i in 0 ..< mirror.count {
         a.append(mirror[i].1.value as CChar)
     }
     return a
}

let b = make_b().memory.s     // bridged tuple of 8 chars
let a = toCharArray(b)        // Swift array of (8) CChars
let s = String.fromCString(a) // proper Swift string

println(s)

Compile:

$ xcrun swiftc -O -c b.swift -import-objc-header b.h
$ clang -O2 -c b.c -o b.c.o
$ xcrun swiftc b.o b.c.o -o b

Run:

$ ./b
Optional("foobarz")