How to get network adapter stats in linux/Mac OSX?

The Darwin netstat source code uses sysctl.
Here’s some code that prints the number of bytes in and out on OSX:

#import <Foundation/Foundation.h>
#include <sys/sysctl.h>
#include <netinet/in.h>
#include <net/if.h>
#include <net/route.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    int mib[] = {
        CTL_NET,
        PF_ROUTE,
        0,
        0,
        NET_RT_IFLIST2,
        0
    };
    size_t len;
    if (sysctl(mib, 6, NULL, &len, NULL, 0) < 0) {
        fprintf(stderr, "sysctl: %s\n", strerror(errno));
        exit(1);
    }
    char *buf = (char *)malloc(len);
    if (sysctl(mib, 6, buf, &len, NULL, 0) < 0) {
        fprintf(stderr, "sysctl: %s\n", strerror(errno));
        exit(1);
    }
    char *lim = buf + len;
    char *next = NULL;
    u_int64_t totalibytes = 0;
    u_int64_t totalobytes = 0;
    for (next = buf; next < lim; ) {
        struct if_msghdr *ifm = (struct if_msghdr *)next;
        next += ifm->ifm_msglen;
        if (ifm->ifm_type == RTM_IFINFO2) {
            struct if_msghdr2 *if2m = (struct if_msghdr2 *)ifm;
            totalibytes += if2m->ifm_data.ifi_ibytes;
            totalobytes += if2m->ifm_data.ifi_obytes;
        }
    }
    printf("total ibytes %qu\tobytes %qu\n", totalibytes, totalobytes);
    [pool drain];
    return 0;
}