Reading glsl code from file won't work - opengl

Question

I start to create a Qt OpenGL application. And I followed the tutorial from Learnopengl In this tutorial, it gives the a static C-String corresponding to the vertex and fragment shader. When I give to my application a static C-String shader there is no error and my triangle appears in the screen.
But when I try to process all shader code to separate files, I have an error during the glLinkProgram command such as must write to gl_Position
Here is my C++ code:
#include "widget.h"
Widget::Widget(QOpenGLWidget *parent)
: QOpenGLWidget(parent)
{
this->setWindowTitle("Learning OpenGL");
QString pathV="shaders/main.vert";
QString pathF="shaders/main.frag";
vertexShaderSource=loadShader(pathV);
fragmentShaderSource=loadShader(pathF);
}
Widget::~Widget()
{
}
void Widget::initializeGL()
{
this->initializeOpenGLFunctions();
qDebug()<<"Vous travaillez sur une OpenGL version:"<<QString((char*)glGetString(GL_VERSION));
compileShaders();
glClearColor(0.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
}
void Widget::paintGL()
{
GLfloat vertices[] = {
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f,
0.0f, 1.0f, 0.0f
};
GLuint VBO,VAO;
glGenVertexArrays(1, &VAO);
glGenBuffers(1, &VBO);
// Bind the Vertex Array Object first, then bind and set vertex buffer(s) and attribute pointer(s).
glBindVertexArray(VAO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (GLvoid*)0);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0); // Note that this is allowed, the call to glVertexAttribPointer registered VBO as the currently bound vertex buffer object so afterwards we can safely unbind
glBindVertexArray(0); // Unbind VAO (it's always a good thing to unbind any buffer/array to prevent strange bugs)
// Draw our first triangle
glUseProgram(shaderProgram);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES, 0, 3);
glBindVertexArray(0);
}
QString Widget::loadShader(QString &path)
{
QFile file(path);
file.open(QIODevice::ReadOnly);
QTextStream stream(&file);
QString lines;
while(!stream.atEnd())
{
lines.append(stream.readLine()+"\n");
}
//lines.append('\0');
file.close();
return lines;
}
void Widget::compileShaders()
{
/* //Static shaders
// Shaders
vs = "#version 330 core\n"
"layout (location = 0) in vec3 position;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(position.x, position.y, position.z, 1.0);\n"
"}\0";
fs = "#version 330 core\n"
"out vec4 color;\n"
"void main()\n"
"{\n"
"color = vec4(0.0f, 1.0f, 0.0f, 1.0f);\n"
"}\n\0";
//*/
//Vertex shader
GLuint vertexShader;
vertexShader = glCreateShader(GL_VERTEX_SHADER);
vs=vertexShaderSource.toStdString().c_str();
glShaderSource(vertexShader, 1,(const GLchar **) &vs, 0);
glCompileShader(vertexShader);
//check if any error during compilation
GLint success;
GLchar infoLog[512];
glGetShaderiv(vertexShader, GL_COMPILE_STATUS, &success);
if(!success)
{
glGetShaderInfoLog(vertexShader, 512, NULL, infoLog);
qDebug()<< "ERROR::SHADER::VERTEX::COMPILATION_FAILED\n" << infoLog ;
}
else qDebug()<<"compile vertex ok";
//Fragment shader
GLuint fragmentShader;
fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
fs=fragmentShaderSource.toStdString().c_str();
glShaderSource(fragmentShader, 1,(const GLchar **) &fs, 0);
glCompileShader(fragmentShader);
//Check if any error during compilation
glGetShaderiv(fragmentShader, GL_COMPILE_STATUS, &success);
if(!success)
{
glGetShaderInfoLog(fragmentShader, 512, NULL, infoLog);
qDebug()<< "ERROR::SHADER::FRAGMENT::COMPILATION_FAILED\n" << infoLog ;
}
else qDebug()<<"compile fragment ok";
shaderProgram = glCreateProgram();
glAttachShader(shaderProgram, vertexShader);
glAttachShader(shaderProgram, fragmentShader);
glLinkProgram(shaderProgram);
glGetProgramiv(shaderProgram, GL_LINK_STATUS, &success);
if(!success) {
glGetProgramInfoLog(shaderProgram, 512, NULL, infoLog);
qDebug()<< "Linkage error\n" << infoLog;
}
}
When I display on the console the data I read from shader files it is the same as the static C-String shader code.
So I don't see why my shader files crush.

Answer 1

This code:
fragmentShaderSource.toStdString()
Results in a temporary copy of the string. That temporary will be destroyed after the expression that generates it. So when you call c_str() on that string, you're getting a pointer to memory that is about to be deleted.
Instead, your fragmentShaderSource and similar vertex shader types should just be std::string itself. As such, loadShader should return a std::string (returning lines.toStdString() instead of just lines).
Once done, you can take fragmentShaderSource.c_str() and freely pass that to OpenGL.

Answer 2

Your shader source seems to be empty or invalid. Most probably its due to the fact that you're assigning a temporary copy a buffer to vs/fs variables:
vs=vertexShaderSource.toStdString().c_str();
fs=fragmentShaderSource.toStdString().c_str();
These pointers will probably be invalid right after these lines, so passing them to glShaderSource function might not be the best idea.
You have to copy buffer contents. Try something like this:
char* vs = new char[vertexShaderSource.size() + 1];
strcpy(vs, vertexShaderSource.toStdString().c_str());