OpenGL ES2.0: What's wrong with these shaders? [closed] - opengl-es

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I'm trying to follow a FreeType2 tutorial here but I cannot get the shaders to link.
Is there something wrong with these shaders?
Vertex shader:
attribute vec4 coord;
varying vec2 texcoord;
void main()
{
gl_Position = vec4( coord.xy, 0, 1 );
texcoord = coord.zw;
}
Fragment shader:
varying vec2 texcoord;
uniform sampler2D tex;
uniform vec4 color;
void main()
{
gl_FragColor = vec4(1, 1, 1, texture2D( tex, texcoord ).r ) * color;
}
I have successfully uses the below function calls with my own shaders for doing simple solid filling and simple textures. I have no idea why I can't use the above shaders.
Here's the code which compilers and links the shaders into the program:
GLuint BuildShader( char *pszSource, GLenum shaderType )
{
GLuint hShader = glCreateShader( shaderType );
glShaderSource(hShader, 1, &pszSource, 0 );
glCompileShader( hShader );
GLint compileSuccess = GL_FALSE;
glGetShaderv( hShader, GL_COMPILE_STATUS, &compileSuccess );
if( compileSuccess == GL_FALSE )
{
GLchar message[ 256 ];
glGetShaderInfoLog( hShader, sizeof( message ), 0, &message[ 0 ] );
printf( "SHADER (%s) %s\n", pszSource, message );
exit( 1 );
}
return hShader;
}
GLuint BuildProgram( char *pszVertexShaderSource, char *pszFragmentShaderSource )
{
GLuint vShader = BuildShader( pszVertexShaderSource, GL_VERTEX_SHADER );
GLuint fShader = BuildShader( pszFragmentShaderSource, GL_FRAGMENT_SHADER );
GLuint hProgram = glCreateProgram();
glAttachShader( hProgram, vShader );
glAttachShader( hProgram, fShader );
glLinkProgram( hProgram );
GLint linkSuccess;
glGetProgramiv( hProgram, GL_LINK_STATUS, &linkSuccess );
if( linkSuccess == GL_FALSE )
{
GLchar message[ 256 ];
glGetProgramInfoLog( hProgram, sizeof( message ), 0, &message[ 0 ] );
printf( "BUILD %s\n", message );
exit( 1 );
}
return hProgram;
}
...
g_FontProgram = BuildProgram( vsFont, fsFont );
...
I get the below output at the linking phase:
BUILD ERROR: One or more attached shaders not successfully compiled
UPDATE: I have fixed the shader compilation check. Now I get the following error:
SHADER (varying vec2 texcoord; uniform sampler2D tex; uniform vec4 color; void main() { gl_FragColor = vec4(1, 1, 1, texture2D( tex, texcoord ).r ) * color; }) ERROR: 0:1: 'vec2' : declaration must include a precision qualifier for type
ERROR: 0:1: 'vec4' : declaration must include a precision qualifier for type
ERROR: 0:1: Use of undeclared identifier 'texcoord'
ERROR: 0:1: Use of undeclared identifier 'color'

Answer 1

The shaders failed to compile because the following line was needed:
precision mediump float;
For anyone else trying to follow the same tutorial, the following fragment shader will work under OpenGL ES2.0:
precision mediump float;
varying vec2 texcoord;
uniform sampler2D tex;
uniform vec4 color;
void main()
{
gl_FragColor = vec4(1, 1, 1, texture2D( tex, texcoord ).r ) * color;
}