STRUCTURE DEFINITION

typedef struct{
  Uint8 scancode;
  SDLKey sym;
  SDLMod mod;
  Uint16 unicode;
} SDL_keysym;

STRUCTURE DATA

scancode

Hardware specific scancode

sym

SDL virtual keysym

mod

Current key modifiers

unicode

Translated character

DESCRIPTION

The SDL_keysym structure is used by reporting key presses and releases since it is a part of the SDL_KeyboardEvent.

The scancode field should generally be left alone, it is the hardware dependent scancode returned by the keyboard. The sym field is extremely useful. It is the SDL-defined value of the key (see SDL Key Syms. This field is very useful when you are checking for certain key presses, like so:

.
.
while(SDL_PollEvent(&event)){
  switch(event.type){
    case SDL_KEYDOWN:
      if(event.key.keysym.sym==SDLK_LEFT)
        move_left();
      break;
    .
    .
    .
  }
}
.
.

 mod stores the current state of the keyboard modifiers as explained in SDL_GetModState. The unicode is only used when UNICODE translation is enabled with SDL_EnableUNICODE. If unicode is non-zero then this a the UNICODE character corresponding to the keypress. If the high 9 bits of the character are 0, then this maps to the equivalent ASCII character: 

char ch;
if ( (keysym.unicode & 0xFF80) == 0 ) {
  ch = keysym.unicode & 0x7F;
}
else {
  printf("An International Character.
");
}

 UNICODE translation does have a slight overhead so don't enable it unless its needed.