Problem with data from Z axis HMC5883L magnetometer (GY-271)

I plan using HMC5883L to calculate yaw angle but when i run example from library of Jeff Rowberg,why raw data of Z axis is always 0 or -1 ?
I checked I2C adress,it’s exactly GY-271 not a fake one.
Code below:

// I2C device class (I2Cdev) demonstration Arduino sketch for HMC5883L class
// 10/7/2011 by Jeff Rowberg <jeff@rowberg.net>
// Updates should (hopefully) always be available at https://github.com/jrowberg/i2cdevlib
//
// Changelog:
//     2013-05-04 - Added Heading Calculation in degrees
//     2011-10-07 - initial release

/* ============================================
I2Cdev device library code is placed under the MIT license
Copyright (c) 2011 Jeff Rowberg

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
===============================================
*/

// Arduino Wire library is required if I2Cdev I2CDEV_ARDUINO_WIRE implementation
// is used in I2Cdev.h
#include "Wire.h"

// I2Cdev and HMC5883L must be installed as libraries, or else the .cpp/.h files
// for both classes must be in the include path of your project
#include "I2Cdev.h"
#include "HMC5883L.h"

// class default I2C address is 0x1E
// specific I2C addresses may be passed as a parameter here
// this device only supports one I2C address (0x1E)
HMC5883L mag;

int16_t mx, my, mz;

#define LED_PIN 13
bool blinkState = false;

void setup() {
    // join I2C bus (I2Cdev library doesn't do this automatically)
    Wire.begin();

    // initialize serial communication
    // (38400 chosen because it works as well at 8MHz as it does at 16MHz, but
    // it's really up to you depending on your project)
    Serial.begin(38400);

    // initialize device
    Serial.println("Initializing I2C devices...");
    mag.initialize();

    // verify connection
    Serial.println("Testing device connections...");
    Serial.println(mag.testConnection() ? "HMC5883L connection successful" : "HMC5883L connection failed");

    // configure Arduino LED pin for output
    pinMode(LED_PIN, OUTPUT);
}

void loop() {
    // read raw heading measurements from device
    mag.getHeading(&mx, &my, &mz);

    // display tab-separated gyro x/y/z values
    Serial.print("mag:\t");
    Serial.print(mx); Serial.print("\t");
    Serial.print(my); Serial.print("\t");
    Serial.print(mz); Serial.print("\t");
    
// To calculate heading in degrees. 0 degree indicates North
    float heading = atan2(my, mx);
    if(heading < 0)
      heading += 2 * M_PI;
    Serial.print("heading:\t");
    Serial.println(heading * 180/M_PI);

    // blink LED to indicate activity
    blinkState = !blinkState;
    digitalWrite(LED_PIN, blinkState);
}

Why use that library? It is much simpler just to use the Wire library as described in this blog post.

Don't forget that magnetometers must be calibrated to be useful, as described here.

jremington:
Why use that library? It is much simpler just to use the Wire library as described in this blog post.

Don't forget that magnetometers must be calibrated to be useful, as described here.

I use library for testing this module working or not, caliberation has no meanning if it's broken

It would be rather stupid to assume that any problems would be in the device, rather than someone else's code, don't you think?

jremington:
It would be rather stupid to assume that any problems would be in the device, rather than someone else's code, don't you think?

Yes i think it's stupid too but i try so many different code examples even the one you give me,raw data from x,y axis changed only z axis doesn't always is 0 or -1 .I spent few days to searching and checking where is this problem come from and i have no answer.

My question is Why only raw data from z axis not change? Hope some replies from people who have the same problem.

Then there is a good chance that in this case, the chip is defective.

I recommend the much better performing LSM303D magnetometer/accelerometer combination.