Initializing fields of a structure - c

Question

I'm working on a hash table assignment and I'm getting an unexpected error when trying to initialize my structure variables with a macro value. Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "symTable.h"
#define DEFAULT_TABLE_SIZE 61
#define HASH_MULTIPLIER 65599
typedef struct Node
{
char *key;
int *value;
struct Node *next;
} Node_T;
typedef struct SymTable
{
Node_T **Table;
int tablesize = DEFAULT_TABLE_SIZE; //where I am getting my error
int counter = 1;
} *SymTable_T;
and the error I am getting is
error: expected ':', ',', ';', '}' or '__attribute__' before '=' token
Can someone explain to me why it isn't accepting my macro value?

Answer 1

You try to initialize a member in a struct definition, which is a type1. That's no variable definition and doesn't take up any memory at runtime, mind you. You don't define any variable at all, so there's nothing to initialize.
When actually defining a variable you can do something similar by using a designated initializer list:
struct SymTable table = { .tablesize = DEFAULT_TABLE_SIZE, .counter = 1 };
This feature is only available since C99, however (GCC implements it as a non-conformant extension since C90!). Another, < C99 way, would be the somewhat bulkier
struct SymTable table = { NULL, DEFAULT_TABLE_SIZE, 1 };
where you need to know the order of members and the default values of each member.
1 Thanks to #M.M for that neat wording!

Answer 2

You are trying to use an assignment in the definition of struct SymTable. This is a structure, i.e. a kind of type. You can't assign a value to a type, or to a part of a type: a type isn't a piece of storage, it's a classification of storage.
You can use an assignment when defining an object of type struct SymTable. You can assign all the fields; if you omit some fields, the fields you omit will be initialized to “zero” (meaning 0 for integer fields, 0.0 for floating point fields, NULL for pointer fields).
struct SymTable syms1 = {NULL, DEFAULT_TABLE_SIZE, 1};
struct SymTable syms2 = {.counter = 1, .tablesize = DEFAULT_TABLE_SIZE};
The syntax I used for syms2 was introduced in C99, which is supported by most but not all common compilers). The syntax I used for syms1 has existed since the dawn of time; it has the downside that you need to provide values for the fields in order.
If you were trying to provide default values for any new object of type struct symTable, this is not a feature that C provides natively. A struct symTable declared by just writing struct symTable somename; in a function has uninitialized content, and there's nothing you can do about it. You can do it via a macro, but you'll need to call the macro explicitly when you create an object.
#define STRUCT_SYMTABLE_DEFAULT {NULL, DEFAULT_TABLE_SIZE, 1}
…
struct symTable syms3 = STRUCT_SYMTABLE_DEFAULT;
This approach has some downsides: you can only use this to initialize a struct, you can't use this syntax to assign a value later. Also, there's no type checking. A way to fix these defects is to return a value from a function. Good compilers will optimize it to the same code.
struct symTable inline symTable_default(void) {
struct symTable default = {NULL, DEFAULT_TABLE_SIZE, 1};
return default;
}
…
struct symTable syms4 = symTable_default();
…
/* Reset the table to its default value */
syms4 = symTable_default();