NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/2462

Internet Message Format | 1996-08-05 | 2.8 KB

Path: newshost.lanl.gov!tanmoy
From: [e-mail] (Tanmoy Bhattacharya)
Newsgroups: comp.lang.c
Subject: Re: Pointer Conversion
Date: 21 Jan 1996 17:51:30 GMT
Organization: Los Alamos National Laboratory
Distribution: world
Message-ID: <[e-mail]>
References: <4ds4jq$[e-mail]> <[e-mail]>
NNTP-Posting-Host: qcd.lanl.gov
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Content-Type: text
In-reply-to: [e-mail]'s message of 20 Jan 1996 22:46 PST

--text follows this line--
In article <[e-mail]> [e-mail]
(P.Bennett) writes:
<snip>
   >char *my_strcat(const char *a, const char *b)
   >{
   >    char done[1024];
<snip>
   >    return done; /* this is the suspicious pointer conversion error */
   >}

   Your function is declared to return a char pointer, and you are returning an
   array, which is not the same thing.  Sometimes char array names act like char
   pointers (particularly when used as function parameters), but not here.


You are correct in stating that an array is not a pointer, but in this
context, the _value_ of an array is being returned. Except as an
immediate operand of sizeof or &, an array always decays to its value
which is a pointer to its first element. So, at this point, the return
value is of the correct type.


   A more important problem here is that "done" is an automatic array which will
   cease to exist when the function returns, possibly being over-written on the
   next call to another function.

This is the actual problem.

   You could declare "done" as a char pointer, and malloc() some memory for it to
   point to - this would solve both problems, but the calling function would have
   to free() that malloc()ed memory sometime.

Correct.

   You could also declare done as a static array (just add "static" before the
   present declaration), then the array will exist for the life of the program.
   I _think_ (and I know someone will correct me if I'm wrong) that changeing
   "return done;" to "return &done;" will fix the "suspicious pointer conversion"
   error.

Wrong. You can use static arrays, but &done has the type pointer to
character array of length 1024, i.e. char(*)[1024]. This is not the
same as char *.

done itself decays to char *.

(Never take the warnings from an unknown compiler too literally).

Cheers
Tanmoy
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