what is typedef?

A typedef declaration introduces a name that, within its scope, becomes   a   synonym for the type given by the decl-specifiers portion of the declaration.   In   contrast   to   the   class,   struct,   union,   and   enum   declarations,   typedef   declarations   do   not   introduce   new   types   —   they   introduce   new   names   for   existing   types.

Example

//   Example   of   the   typedef   keyword
typedef   unsigned   long   ulong;

ulong   ul;           //   Equivalent   to   "unsigned   long   ul; "

typedef   struct   mystructtag
{
      int       i;
      float   f;
      char     c;
}   mystruct;

mystruct   ms;       //   Equivalent   to   "struct   mystructtag   ms; "

typedef   int   (*funcptr)();     //   funcptr   is   synonym   for   "pointer
                                                      //         to   function   returning   int "

funcptr   table[10];       //   Equivalent   to   "int   (*table[10])(); "