glGetProgramIv claims that shader linking failed, but glGetProgramInfoLog is empty - c++

Question

I am currently attempting to write a basic graphics engine with OpenGl and C++. In doing so, OpenGl is behaving in a very strange way with no apparent explanation. It claims that a shader failed to compile, but then refuses to give any sort of error explaining why. Specifically, when I call glGetProgramiv the returned value is 0, but glGetProgramInfoLog gives an empty string¹.
Here is a minimum complete verifiable example:
#include <SDL2/SDL.h>
#include <GL/glew.h>
#include <sstream>
#include <stdexcept>
#include <fstream>
GLuint loadShader(const std::string &path, GLenum type);
int main(int argc, char *argv[]){
//show window & other housekeeping
SDL_Init(SDL_INIT_EVERYTHING);
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_ALPHA_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BUFFER_SIZE, 32);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
SDL_Window *window = SDL_CreateWindow("test window", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 800, 600, SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE);
SDL_GLContext glContext = SDL_GL_CreateContext(window);
SDL_GL_MakeCurrent(window, glContext);
GLenum c = glewInit();
if(c != GLEW_OK){
std::ostringstream sout;
sout << "Failed to initialize OpenGl: " << glewGetErrorString(c);
throw std::runtime_error(sout.str());
}
//**** -- BEGIN IMPORTANT BIT -- ****//
//This is where the shaders are loaded
//load shaders
GLuint vertexShader = loadShader("shader.vert", GL_VERTEX_SHADER);
GLuint fragmentShader = loadShader("shader.frag", GL_FRAGMENT_SHADER);
//create & link program
GLuint program = glCreateProgram();
glAttachShader(program, vertexShader);
glAttachShader(program, fragmentShader);
glLinkProgram(program);
//check for errors
GLint success;
glGetProgramiv(program, GL_LINK_STATUS, &success);
if(!success){
GLchar infoLog[512];
GLint size; //gives 0 when checked in debugger
glGetProgramInfoLog(program, 512, &size, infoLog);
throw std::runtime_error(std::string("Failed to link shader program: ") + infoLog);
}
//cleanup
glDeleteShader(vertexShader);
glDeleteShader(fragmentShader);
//**** -- END IMPORTANT BIT -- ****//
//update loop
bool isClosed = false;
while(!isClosed){
SDL_GL_SwapWindow(window);
SDL_Event e;
while(SDL_PollEvent(&e)){
switch(e.type){
case SDL_QUIT:
isClosed = true;
break;
}
}
}
}
GLuint loadShader(const std::string &path, GLenum type){
//load file
std::ifstream fin(path);
std::string line, total = "";
while(std::getline(fin, line)){
total += (line + "\n");
}
//reformat source into OpenGl's ridiculous format
const char *cArray = total.c_str();
const char **pointerToCArray = &cArray;
//create shader
GLuint shader = glCreateShader(type);
glShaderSource(shader, 1, pointerToCArray, nullptr);
glCompileShader(shader);
//check for erros
GLint success;
glGetShaderiv(shader, GL_COMPILE_STATUS, &success);
if(!success){
GLchar infoLog[512];
glGetShaderInfoLog(shader, 512, nullptr, infoLog);
throw std::runtime_error(std::string("Failed to compile shader: ") + infoLog);
}
}
MCVE testshader.vert:
#version 330 core
layout (location = 0) in vec3 position;
void main()
{
gl_Position = vec4(position.x, position.y, position.z, 1.0);
}
MCVE testshader.frag:
#version 330 core
out vec4 color;
void main()
{
color = vec4(1.0f, 0.5f, 0.2f, 1.0f);
}
When I run this I get the following output in addition to a brief flash of a window:
terminate called after throwing an instance of 'std::runtime_error'
what(): Failed to link shader program:
Aborted (core dumped)
Extra Notes:
I am using Ubuntu 16.10 x64
I am using a Nvidia GTX 1070 with driver version 375.39
The SDL and glew versions are those installed from the libsdl2-dev and libglew-dev packages respectively. I am using pkg-config to link.
During my research to solve this error I mostly found various forum and StackOverflow posts that found specific errors in other people's code. None of these apply to my code, and after finding this error I compared my code to this supposedly working example. This example also compiles and runs correctly on my computer, which rules out any sort of configuration error.
¹: It is not possible that glGetProgramInfoLog is doing nothing with external consequences and that the infoLog array happened to be allocated with a null-terminator character at its first address because the size return of glGetProgramInfoLog returns 0 even if set to a different value before glGetProgramInfoLog is called.

Answer 1

I have found the issue: a missing return statement in the loadShader() method. This, however, as I experienced when attempting to solve this was not helpful. As such, I recommend to any future person reading this that you find a working example (ie. this one) that does something similar as what you are trying to do and copy/paste progressively smaller parts of its code into yours (or visa-versa) to do a kind of binary search to find your error.