CODEWITHSUNDEEP
gocgomach
user2308381
user2308381

Reputation: 31

GoLang / CGO: Problems calling Mach API host_statistics() from Go

I use the following C code fragment to get the CPU load on OS X:

    #include <mach/message.h> 
    #include <mach/mach_host.h>
    #include <mach/host_info.h>

    [...]

    mach_msg_type_number_t  count = HOST_CPU_LOAD_INFO_COUNT;
    kern_return_t error;
    host_cpu_load_info_data_t r_load;

    mach_port_t host_port = mach_host_self();
    error = host_statistics(host_port, HOST_CPU_LOAD_INFO, (host_info_t)&r_load, &count);

I have tried to port this piece of code to Go after reading the cgo tutorial. The resulting code looks like this:

package main

/*
#include <stdlib.h>
#include <mach/message.h>
#include <mach/mach_host.h>
#include <mach/host_info.h>
*/
import "C"

func main() {
    var err C.kern_return_t
    var host_info_out C.host_info_t
    var host_port C.mach_port_t = C.mach_host_self()

    count := C.mach_msg_type_number_t(C.HOST_CPU_LOAD_INFO_COUNT)

    err = C.host_statistics(C.host_t(host_port), C.HOST_CPU_LOAD_INFO, &host_info_out, &count)
}

However, when I try to build the code, I end up with the following error message

go build cputimes.go 
# command-line-arguments
cputimes.go:33: cannot use &host_info_out (type *_Ctype_host_info_t) as type *_Ctype_integer_t in function argument

I do not udnerstand why cgo complains about the type. The signature of host_statistics() is defined in the mach headers as:

 kern_return_t host_statistics
 (
      host_t host_priv,
      host_flavor_t flavor,
      host_info_t host_info_out,
      mach_msg_type_number_t *host_info_outCnt
 );

Upvotes: 3

Views: 564

Answers (1)

James Henstridge
James Henstridge

Reputation: 43949

The function prototype says that the third argument to host_statistics is a host_info_t variable, while you are passing a pointer to a host_info_t variable in your sample program.

Looking at the mach/host_info.h header file, we can see that host_info_t is a pointer type:

typedef integer_t   *host_info_t;       /* varying array of int. */

That explains why you got the error message about a type mismatch with integer_t.

Your Go code doesn't really look equivalent to the C code when handling that argument. You probably want something more like this:

...
var r_load C.host_cpu_load_info_data_t
...
err = C.host_statistics(C.host_t(host_port), C.HOST_CPU_LOAD_INFO, C.host_info_t(unsafe.Pointer(&r_load)), &count)
...

(you need to use the unsafe package to cast between the incompatible pointer types).

Upvotes: 1

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