Swift: convert already allocated char array to char pointer - c

Question

I am attempting to interface an existing C library to Swift 5.0.1 code on iOS. The C Header files have the following definitions:
char hostname[SFL_MAX_HOSTNAME_CHARS+1];
char os_release[SFL_MAX_OSRELEASE_CHARS+1];
int readHidCounters(HSP *sp, SFLHost_hid_counters *hid, char *hbuf, int hbufLen, char *rbuf, int rbufLen);
typedef struct _HSP {
[Many other elements omitted for brevity]
char hostname[SFL_MAX_HOSTNAME_CHARS+1];
char os_release[SFL_MAX_OSRELEASE_CHARS+1];
} HSP;
readHidCounters has an implementation (edited for brevity) like this:
int readHidCounters(HSP *sp, SFLHost_hid_counters *hid, char *hbuf, int hbufLen, char *rbuf, int rbufLen) {
int gotData = NO;
size_t len = hbufLen;
if(sysctlbyname("kern.hostname", hbuf, &len, NULL, 0) != 0) {
myLog(LOG_ERR, "sysctl(<kern.hostname>) failed : %s", strerror(errno));
}
else {
gotData = YES;
hid->hostname.str = hbuf;
hid->hostname.len = strlen(hbuf);
}
// UUID
memcpy(hid->uuid, sp->uuid, 16);
[...]
}
I have created an HSP struct and attempting to call readHidCounters in Swift like this
var sp = HSP()
[...]
readHidCounters(&sp,
&hidElem.counterBlock.host_hid,
&sp.hostname, // This is the error line
SFL_MAX_HOSTNAME_CHARS,
&sp.os_release,
SFL_MAX_OSRELEASE_CHARS)
I am attempting to pass in &sp.hostname results in the compiler error Cannot convert value of type '(Int8, Int8, Int8, [...], Int8)' to expected argument type 'Int8'. The problem is that hostname is a tuple of Int8's and I can't seem to get it properly converted to char *. I have tried various incarnations of UnsafeMutablePointer, withUnsafeMutablePointer and can't see how to get hostname properly recognized. Any suggestions greatly appreciated!
[Solved]
MartinR pretty much nailed it with his suggestion, but it did have a compiler error: Overlapping accesses to 'sp.hostname', but modification requires exclusive access; consider copying to a local variable. Updated code that compiles is
var myHostName = sp.hostname
var myOsRelease = sp.os_release
let _ = withUnsafeMutablePointer(to: &myHostName) {
$0.withMemoryRebound(to: Int8.self, capacity: MemoryLayout.size(ofValue: sp.hostname)) {
hostNamePtr in
withUnsafeMutablePointer(to: &myOsRelease) {
$0.withMemoryRebound(to: Int8.self, capacity: MemoryLayout.size(ofValue: sp.os_release)) {
osReleasePtr in
readHidCounters(&sp,
&hidElem.counterBlock.host_hid,
hostNamePtr, SFL_MAX_HOSTNAME_CHARS,
osReleasePtr, SFL_MAX_OSRELEASE_CHARS)
}
}
}
}

Answer 1

The “problem” is that C arrays are imported to Swift as tuples, and there is no simple method to treat a tuple as a Swift array, or get a pointer to the element storage (because a tuple can be inhomogeneous).
Similarly as in Converting a C char array to a String one can use the fact that Swift preserves the memory layout of structures imported from C, and with some pointer juggling and rebinding you'll get
let result = withUnsafeMutablePointer(to: &sp.hostname) {
$0.withMemoryRebound(to: Int8.self, capacity: MemoryLayout.size(ofValue: sp.hostname)) {
hostNamePtr in
withUnsafeMutablePointer(to: &sp.os_release) {
$0.withMemoryRebound(to: Int8.self, capacity: MemoryLayout.size(ofValue: sp.os_release)) {
osReleasePtr in
readHidCounters(&sp,
&hidElem.counterBlock.host_hid,
hostNamePtr, SFL_MAX_HOSTNAME_CHARS,
osReleasePtr, SFL_MAX_OSRELEASE_CHARS)
}
}
}
}
Another “trick” is to define C helper functions which return the array addresses as a pointers, and use the
SE-0044 Import as member
feature to make these helper functions available to Swift as computed properties. In the bridging header file you have to add
__attribute__((swift_name("getter:HSP.hostNamePtr(self:)")))
static inline char * _Nonnull hostNamePtr(HSP * _Nonnull hsp)
{
return hsp->hostname;
}
__attribute__((swift_name("getter:HSP.osReleasePtr(self:)")))
static inline char * _Nonnull osReleasePtr(HSP * _Nonnull hsp)
{
return hsp->os_release;
}
and then you can use these easily from Swift:
var sp = HSP()
let result = readHidCounters(&sp,
&hidElem.counterBlock.host_hid,
sp.hostNamePtr, SFL_MAX_HOSTNAME_CHARS,
sp.osReleasePtr, SFL_MAX_OSRELEASE_CHARS)

Answer 2

Try removing the & from the call, sp.hostname is already a pointer because it is an array, if it works you will have to remove the & from &sp.os_release too