First make sure you link against the logging library in your Android.mk file:

LOCAL_LDLIBS := -llog   

Then use one of the following __android_log_print() calls:

#include <android/log.h>    
#define TAG "MY LOG"

__android_log_print(ANDROID_LOG_VERBOSE,    TAG, "The value of 1 + 1 is %d", 1 + 1)
__android_log_print(ANDROID_LOG_WARN,       TAG, "The value of 1 + 1 is %d", 1 + 1)
__android_log_print(ANDROID_LOG_DEBUG,      TAG, "The value of 1 + 1 is %d", 1 + 1)
__android_log_print(ANDROID_LOG_INFO,       TAG, "The value of 1 + 1 is %d", 1 + 1)
__android_log_print(ANDROID_LOG_ERROR,      TAG, "The value of 1 + 1 is %d", 1 + 1)

Or use those in a more convenient way by defining corresponding macros:

#define  LOGV(...)  __android_log_print(ANDROID_LOG_VERBOSE,    TAG, __VA_ARGS__)
#define  LOGW(...)  __android_log_print(ANDROID_LOG_WARN,       TAG, __VA_ARGS__)
#define  LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,      TAG, __VA_ARGS__)
#define  LOGI(...)  __android_log_print(ANDROID_LOG_INFO,       TAG, __VA_ARGS__)
#define  LOGE(...)  __android_log_print(ANDROID_LOG_ERROR,      TAG, __VA_ARGS__)

Example:

int x = 42;
LOGD("The value of x is %d", x);

project/jni/main.c

#include <stdio.h>
#include <unistd.h>

int main(void) {
  printf("Hello world!\n");
  return 0;
}

project/jni/Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := hello_world
LOCAL_SRC_FILES := main.c
include $(BUILD_EXECUTABLE)

project/jni/Application.mk

APP_ABI := all
APP_PLATFORM := android-21

If you want to support devices running Android versions lower than 5.0 (API 21), you need to compile your binary with APP_PLATFORM set to an older API, e.g. android-8. This is a consequence of Android 5.0 enforcing Position Independent Binaries (PIE), whereas older devices do not necessarily support PIEs. Therefore, you need to use either the PIE or the non-PIE, depending on the device version. If you want to use the binary from within your Android application, you need to check the API level and extract the correct binary.

APP_ABI can be changed to specific platforms such as armeabi to build the binary for those architectures only.

In the worst case, you will have both a PIE and a non-PIE binary for each architecture (about 14 different binaries using ndk-r10e).

To build the executable:

cd project
ndk-build

You will find the binaries at project/libs/<architecture>/hello_world. You can use them via ADB (push and chmod it with executable permission) or from your application (extract and chmod it with executable permission).

To determine the architecture of the CPU, retrieve the build property ro.product.cpu.abi for the primary architecture or ro.product.cpu.abilist (on newer devices) for a complete list of supported architectures. You can do this using the android.os.Build class from within your application or using getprop <name> via ADB.

If you need to clean the build:

ndk-build clean

ndk-build NDK_PROJECT_PATH=PROJECT_PATH APP_BUILD_SCRIPT=MyAndroid.mk