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clanguage-lawyerexterndeclaration

extern declaration, T* v/s T[]

I saw following piece of code in a legacy project.

/* token.c */
struct token id_tokens[MAX_TOKENS];

/* analyse.c (v1) */
extern struct token *id_tokens; /* Raised my eyebrow, id_token declares a pointer */

I insisted on changing analyse.c to contain the declaration as below:

/* analyse.c (v2) */
extern struct token id_tokens[]; /* I am happy with this. id_tokens declares array of unspecified size. */

I want v2 because pointer to T is not same as array of T. My friend's counter argumented that behaviour of both are same, so it doesn't matter whether I use v1 and v2.

Question 1: Does array of incomplete type deplete to a pointer?

Question 2: Is my friend right that both versions are behaviorally guaranteed to be equivalent?

Solution 
  • /* token.c */
struct token id_tokens[MAX_TOKENS];

/*
 id_tokens
  +-----+-----+-----+-----+...+-----+
  |     |     |     |     |   |     |
  +-----+-----+-----+-----+...+-----+
    [0]   [1]   [2]   [3]  ...  [MAX_TOKEN-1]

  To access id_tokens[i], add offset of ith element
  i.e. i * sizeof(struct token) to the **address**
  of array token
 */

    So in your analyse.c, following instructions would be generated with this declaration.

    1. extern struct token id_tokens[];
      id_tokens[i]
      a. Address of id_tokens that might be linked from other compilation unit is taken
      b. offset of i is added
      c. Value is referenced
    /* analyse.c (v1) */
extern struct token *id_tokens;

/*
 id_tokens
  +------+           +-----+...
  | addr |---------->|     |
  +------+           +-----+...


  To access id_tokens[i], fetch **contetnts** of pointer
  token, add offset of ith element i.e. i * sizeof(struct token)
  is added to this.
 */

    So in your analyse.c, following instructions would be generated with this declaration:

    1. extern struct token *id_tokens;
      id_tokens[i]
      a. Contents from address of id_tokens that is linked from other compilation unit is taken.
      (Will result in compilation error if present in same compilation unit because of type mismatch)
      b. offset of i is added
      c. Value is referenced

    Let's assume sizeof id_token[0] is 2 byte and sizeof pointer to id_token[0] is 4 byte.

    Your later declaration may (mis)interprete the id_tokens[0] & id_tokens[1] as an address and add some offset to it (which may be an existing or non-existing address, aligned or non-aligned address who knows).

    If it is your good day, program may crash or segfault immediately and you get a chance to fix the bug. If it is your bad day, program may just mess up with some other memory or communicate a wrong state to some module which can result in difficult to track bug and cause a nightmare.

    Now I guess you understand why you got (nil) as output in Mr. 32's answer.