Implicit declaration glCreateShader()?

I am getting warning from gcc. Warning is:

warning: implicit declaration of function ‘glCreateShader’ [-Wimplicit-function-declaration]
  137 |  unsigned int vertex_shader = glCreateShader(GL_VERTEX_SHADER);

I included the headers as follows:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

#include <GL/freeglut.h>
#include <GL/gl.h>

And I compile the program as follows:

gcc main.c -lglut -lGL -lGLEW -lGLU -lm -o snakeGame

And My main function code is here:

int main(int argc, char **argv){
    srand(time(NULL));

    for (int i=0; i<snake_size; ++i){
        snake[i] = 0;
    }

//glut set
    glutInit(&argc, argv);

    glutInitWindowSize(window_x, window_y);
    glutInitWindowPosition(window_position_x, window_position_y);
    glutInitDisplayMode(display_mode);

    int window = glutCreateWindow("snake game");

    glutDisplayFunc(display_function);
    glutKeyboardFunc(keyboard_function);
    glutIdleFunc(idle_function);

//opengl set

    unsigned int vertex_shader = glCreateShader(GL_VERTEX_SHADER);

    glutMainLoop();

    glutDestroyWindow(window);
    printf("game over\n");

    return 0;
}

Most likely there is a problem with the headers.

BTW I am using linux.

Per Khronos only GL 1.1 functions are declared in gl.h. GL 1.2 and above live in glext.h and need to be acquired at run-time.

Though since you're using GLEW you should #include <GL/glew.h> instead, since that will have declarations for all GL functions. Don't forget to initialize GLEW.