Re: Two C code questions

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From	Ian Collins <ian-news@hotmail.com>
Newsgroups	comp.lang.c
Subject	Re: Two C code questions
Date	Tue, 14 Mar 2017 17:16:15 +1300
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On 03/14/17 04:45 PM, MehdiAmini wrote:
> // Clang version 3.8.0
>
> // Question 1:
> #include <stdio.h>
> #include <stdlib.h>
> #include <inttypes.h>
>
> void my_alloc( void *, size_t );
>
> int main(void)
> {
>       unsigned *u_ptr = NULL;
>       //unsigned *u_ptr; // Why using this instead of above line causes
> core dump?
>
>       my_alloc(u_ptr,sizeof(*u_ptr));
>
>       free(u_ptr);
>       return EXIT_SUCCESS;
> }

The answer to why it causes a crash is the value of u_ptr is garbage. 
It is safe to pass NULL to free, but not garbage..

> void my_alloc( void *v_ptr, size_t size )
> {
>       v_ptr = malloc(size);
>       if( v_ptr == NULL )
>       {
>           fprintf(stderr,"Memory allocation failed.\n");
>           exit(EXIT_FAILURE);
>       }
>
>       return ;
> }

The first parameter should b "void**", a pointer to a void pointer. 
Otherwise all you are doing is assigning to the function parameter, with 
is local to the function.

With the correct parameter type, use:

  *v_ptr = malloc(size);

> // Question 2:
> #include <stdio.h>
> #include <stdlib.h>
> #include <inttypes.h>
>
> #define N 6
> //static const long N = 6; // Why using this instead of above line
> causes error?
>
> int main(void)
> {
>       uint64_t ui[N] = {0};

C doesn't have a proper const, so you will be creating a variable length 
array which cannot be initialised.

-- 
Ian

Thread

Two C code questions MehdiAmini <atorrses@gmail.com> - 2017-03-14 07:15 +0330
  Re: Two C code questions Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2017-03-13 22:13 -0600
  Re: Two C code questions Ian Collins <ian-news@hotmail.com> - 2017-03-14 17:16 +1300
  Re: Two C code questions Ike Naar <ike@iceland.freeshell.org> - 2017-03-14 07:20 +0000
    Re: Two C code questions MehdiAmini <atorrses@gmail.com> - 2017-03-14 12:01 +0330
      Re: Two C code questions Ike Naar <ike@iceland.freeshell.org> - 2017-03-14 19:23 +0000
        Re: Two C code questions Keith Thompson <kst-u@mib.org> - 2017-03-14 12:40 -0700
  Re: Two C code questions scott@slp53.sl.home (Scott Lurndal) - 2017-03-14 12:44 +0000
  Re: Two C code questions Keith Thompson <kst-u@mib.org> - 2017-03-14 08:53 -0700
    Re: Two C code questions MehdiAmini <atorrses@gmail.com> - 2017-03-15 14:05 +0330
  Re: Two C code questions John Bode <jfbode1029@gmail.com> - 2017-03-14 10:38 -0700
    Re: Two C code questions Keith Thompson <kst-u@mib.org> - 2017-03-14 11:49 -0700
      Re: Two C code questions John Bode <jfbode1029@gmail.com> - 2017-03-14 14:14 -0700

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