NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/815

Internet Message Format | 1996-08-05 | 2.6 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: A problem with fprintf
Date: Tue, 09 Jan 96 14:34:26 GMT
Organization: none
Distribution: world
Message-ID: <[e-mail]>
References: <[e-mail]>
Reply-To: [e-mail]
X-NNTP-Posting-Host: genesis.demon.co.uk
X-Newsreader: Demon Internet Simple News v1.27
X-Mail2News-Path: genesis.demon.co.uk

In article <[e-mail]>
           [e-mail] "Ray Robert" writes:

>[e-mail] writes:
>
>> I am experiencing a peculiar problem with fprintf.
>
>> I have a piece of code where the following statement fits in:
>
>fprintf(fp, "%s\n", src);
>>
>>fp is a file-pointer and src is a static array of characters. When the
>> code is executed repeatedly, it fails at the above statement. The error
>> message from the debugger is as follows:
>
>> signal SEGV (segmentation violation) in malloc at 0xef778634
>> malloc+0x150:   st      %o0, [%l5]

You have probably corrupted your heap before the fprintf call.

>> fp is pointing to a file that has been opened and is not the problem.
>> Does somebody have a clue?
>
>Apparently your system's fprintf is doing an malloc() to set up a space in
>which to construct the formatted string (wow! what an inefficiency there).

Unlikely. More likely it allocates the stream buffer the first time the
stream is read/written. It didn't do it when the stream was opened because
at that point the program has the opportunity to set the buffering type
and buffer size with setbuf/setvbuf.

>It is probably incorrectly determining the size of "src", which it expects to
>be a (pointer to a) null-terminated string.  One of 2 things is happening (I
>leave it to the academics out there to tell us which is ANSI C):
>
>(1) The array is being passed by value, i.e., the literal value of the first
>element is being passed to fprintf and hence to malloc, but it's being
>interpretted as if it's a pointer.

The 'value' of an array rvalue is a pointer to its first element. This isn't
something that a compiler is likely to get wrong.

>(2) The last element of the array is not a null ('\0' in the old style, 0 in
>the new).

What styles are you referring to? '\0' and 0 both represent an int constant
with value 0. Stylistically is it sensible to use '\0' to represent a
value that is conceptually a character.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------