How do I determine the OS version at runtime in OS X or iOS (without using Gestalt)?

On OS X 10.10 (and iOS 8.0), you can use [[NSProcessInfo processInfo] operatingSystemVersion] which returns a NSOperatingSystemVersion struct, defined as

typedef struct {
    NSInteger majorVersion;
    NSInteger minorVersion;
    NSInteger patchVersion;
} NSOperatingSystemVersion;

There is also a method in NSProcessInfo that will do the comparison for you:

- (BOOL)isOperatingSystemAtLeastVersion:(NSOperatingSystemVersion)version

Beware, although documented to be available in OS X 10.10 and later, both operatingSystemVersion and isOperatingSystemAtLeastVersion: exist on OS X 10.9 (probably 10.9.2) and work as expected. It means that you must not test if NSProcessInfo responds to these selectors to check if you are running on OS X 10.9 or 10.10.

On iOS, these methods are effectively only available since iOS 8.0.

On the command line:

$ sysctl kern.osrelease
kern.osrelease: 12.0.0
$ sysctl kern.osversion
kern.osversion: 12A269

Programmatically:

#include <errno.h>
#include <sys/sysctl.h>

char str[256];
size_t size = sizeof(str);
int ret = sysctlbyname("kern.osrelease", str, &size, NULL, 0);

Darwin version to OS X release:

17.x.x. macOS 10.13.x High Sierra
16.x.x  macOS 10.12.x Sierra
15.x.x  OS X  10.11.x El Capitan
14.x.x  OS X  10.10.x Yosemite
13.x.x  OS X  10.9.x  Mavericks
12.x.x  OS X  10.8.x  Mountain Lion
11.x.x  OS X  10.7.x  Lion
10.x.x  OS X  10.6.x  Snow Leopard
 9.x.x  OS X  10.5.x  Leopard
 8.x.x  OS X  10.4.x  Tiger
 7.x.x  OS X  10.3.x  Panther
 6.x.x  OS X  10.2.x  Jaguar
 5.x    OS X  10.1.x  Puma

A Sample to get and test versions :

#include <string.h>
#include <stdio.h>
#include <sys/sysctl.h>

/* kernel version as major minor component*/
struct kern {
    short int version[3];
};

/* return the kernel version */
void GetKernelVersion(struct kern *k) {
   static short int version_[3] = {0};
   if (!version_[0]) {
      // just in case it fails someday
      version_[0] = version_[1] = version_[2] = -1;
      char str[256] = {0};
      size_t size = sizeof(str);
      int ret = sysctlbyname("kern.osrelease", str, &size, NULL, 0);
      if (ret == 0) sscanf(str, "%hd.%hd.%hd", &version_[0], &version_[1], &version_[2]);
    }
    memcpy(k->version, version_, sizeof(version_));
}

/* compare os version with a specific one
0 is equal
negative value if the installed version is less
positive value if the installed version is more
*/
int CompareKernelVersion(short int major, short int minor, short int component) {
    struct kern k;
    GetKernelVersion(&k);
    if ( k.version[0] !=  major) return major - k.version[0];
    if ( k.version[1] !=  minor) return minor - k.version[1];
    if ( k.version[2] !=  component) return component - k.version[2];
    return 0;
}

int main() {
   struct kern kern;
   GetKernelVersion(&kern);
   printf("%hd %hd %hd\n", kern.version[0], kern.version[1], kern.version[2]);

   printf("up: %d %d eq %d %d low %d %d\n",
        CompareKernelVersion(17, 0, 0), CompareKernelVersion(16, 3, 0),
        CompareKernelVersion(17, 3, 0), CompareKernelVersion(17,3,0),
        CompareKernelVersion(17,5,0), CompareKernelVersion(18,3,0));


}

Result on my machine macOs High Sierra 10.13.2

17 3 0
up: -3 -1 eq 0 0 low 2 1

There is the NSAppKitVersionNumber value which you can use to check the various versions of AppKit, although they don't correspond exactly to OS versions

if (NSAppKitVersionNumber <= NSAppKitVersionNumber10_7_2) {
    NSLog (@"We are not running on Mountain Lion");
}