glShaderSource 中的段错误

Question

Alright I have loaded up a glVertex shader, as a const char string from a hexdump. However I get a compiler error from the following code, and I can't figure out why it refuses to compile this, with an error of:

main.cpp: In function ‘void loadShaders()’:
main.cpp:27:55: error: cannot convert ‘const char (*)[92]’ to ‘const GLchar** {aka const char**}’ in argument passing
     glShaderSource(vertexShader, 1, &vertex_glsl, NULL);                                                           ^

But when I override the error by casting &vertex_glsl to (const char**) , it will compile but segfault on runtime. Why would this segfault?

const char vertex_glsl[] = {
  0x23, 0x76, 0x65, 0x72, 0x73, 0x69, 0x6f, 0x6e, 0x20, 0x31, 0x35, 0x30,
  0x0a, 0x0a, 0x69, 0x6e, 0x20, 0x76, 0x65, 0x63, 0x32, 0x20, 0x70, 0x6f,
  0x73, 0x69, 0x74, 0x69, 0x6f, 0x6e, 0x3b, 0x0a, 0x0a, 0x76, 0x6f, 0x69,
  0x64, 0x20, 0x6d, 0x61, 0x69, 0x6e, 0x28, 0x29, 0x7b, 0x0a, 0x20, 0x20,
  0x20, 0x20, 0x67, 0x6c, 0x5f, 0x50, 0x6f, 0x73, 0x69, 0x74, 0x69, 0x6f,
  0x6e, 0x20, 0x3d, 0x20, 0x76, 0x65, 0x63, 0x34, 0x28, 0x70, 0x6f, 0x73,
  0x69, 0x74, 0x69, 0x6f, 0x6e, 0x2c, 0x20, 0x30, 0x2e, 0x30, 0x2c, 0x20,
  0x31, 0x2e, 0x30, 0x29, 0x3b, 0x0a, 0x7d, 0x00
};
const int vertex_glsl_len = 92;

void loadShaders(){
    GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);

    // here's where it segfaults
    glShaderSource(vertexShader, 1, &vertex_glsl, NULL);
    glCompileShader(vertexShader);
}

Answer 1

While C/C++ pointers and arrays behave the same way in many contexts, they are not the same. What you stumbled over here, with both the initial compile error and with the crash after adding the type cast, is one example of where the two are different:

• The & operator applied to an array gives you the address of the array , which is a pointer to the data stored in the array. The actual address (but not the type) resulting from applying the operator is the same as the address of the first element in the array. So with this declaration:

   char s[] = "abc";

  the following two expressions:

   &s &s[0]

  result in the same address. Again, the type of the two expressions is different, with the first being a pointer to an array, and the second a pointer to a character, but the address is the same.

• The & operator applied to a pointer variable gives you the address of the pointer variable. Following up on the same example:

   char s[] = "abc"; char* t = s;

  the following two expressions:

   &t &t[0]

  result in different addresses. The first one is the address of the pointer variable t , while the second one is again a pointer to the first character, which is the same as &s[0] above.

The type of the 3rd argument to glShaderSource() is const GLchar** , which means that you have to pass the address of a pointer variable. The compile error you got in your first attempt was a serious error, because you really were passing a value of the wrong type (a pointer to an array). Adding the type cast just let the broken code pass through the compiler, and caused a crash at runtime instead.

To make this work, you need to assign the string to a variable of type const GLchar* , and pass the address of that variable:

const GLchar* vertex_glsl_ptr = vertex_glsl;
glShaderSource(vertexShader, 1, &vertex_glsl_ptr, NULL);

This will now compile without a type cast, and will not crash when you run it.

Answer 2

Try:

glShaderSource(vertexShader, 1, &vertex_glsl, &vertex_glsl_len);

The problem might simply be that in this case it should not be driven by null terminated string but a size defined string.

Your char array terminates the string at each line .