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?
The first version is wrong. Arrays are NOT pointers, the declaration extern struct token *id_tokens;
doesn't match the definition type struct token id_tokens[MAX_TOKENS];
.
Reference: C FAQ: I had the definition char a[6] in one source file, and in another I declared extern char *a. Why didn't it work?. Also, see this.
/* 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.
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:
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.
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