Re: Two macros for resource management
From: Stefan Ram (ram_at_zedat.fu-berlin.de)
Date: 08/01/04
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Date: 1 Aug 2004 14:11:49 GMT
ram@zedat.fu-berlin.de (Stefan Ram) writes:
> TRY( source = fopen( "source", "r" ))
> PUSH( fclose( source ))
> TRY( target = fopen( "target", "w" ))
> PUSH( fclose( target ))
> TRY( buffer = malloc( 1024 ))
> PUSH( free( buffer ))
> result = buffcopy( target, source, buffer );
> return result; }
In the above posting, I raised the question how to define
"TRY" and "PUSH" and I now will reply to some
Followup-postings and give my own solution.
Richard Bos writes:
>This is not a good idea at all. Every single maintenance
>programmer who reviews this code will put semicolons after
>these "statements", thus breaking your macros.
This should be taken into account, when deciding whether to
use this approach.
These two macros are of limited use for production code.
Their main goal is to show a way to look a C code and resource
management that might not be obvious.
Peter Nilsson writes:
> int stack[3], *sptr = &stack[sizeof stack / sizeof *stack];
>
> #define TRY(alloc) \
> for (*--sptr = 0; \
> *sptr != 0 \
> || (*sptr == 0 && (alloc) != 0) \
> || (*sptr == 0 && (sptr++, 0)) \
> ; )
>
> #define PUSH(dealloc) \
> if (++*sptr == 2) { (dealloc); sptr++; break; } else
This is indeed a working solution, which differs from mine,
although both use a for-loop.
> If there's more than one statement to be performed, it needs
> to be enclosed in a block.
Yes.
>I'd prefer a solution where TRY and PUSH statements are ;
>terminated. You also lose control over fclose return values.
>Regaining control using these macros would be more trouble than
>it's worth. Even if it wasn't, I still wouldn't use this. ;-)
The approach taken is indeed of limited flexibility.
I do not believe that I would use this approach often in
production code, but still like the idea to abstract from the
details of resource management.
**
Now, it will be shown how to arrive at my own solution. The
first program is doing what is required (except for possible
inspection of the results of "fclose"), but without those two
macros.
.-----------------------------------------------------------.
| Warning: When the following programs are executed, they |
| might overwrite (delete) an existing file named "target". |
'-----------------------------------------------------------'
/* < filename = [twomac0.c] > */
#include <stdlib.h> /* malloc, free, EXIT_FAILURE */
#include <stdio.h> /* FILE, fopen, fclose */
#include "example.h" /* buffcopy */
int main( void )
{ int result = EXIT_FAILURE;
FILE * source;
FILE * target;
char * buffer;
if( source = fopen( "source", "r" ))
{ if( target = fopen( "target", "w" ))
{ if( buffer = malloc( 1024 ))
{ result = buffcopy( target, source, buffer );
free( buffer ); }
fclose( target ); }
fclose( source ); }
return result; }
The code is rewritten to use a while-statement:
/* < filename = [twomac1.c] > */
#include <stdlib.h> /* malloc, free, EXIT_FAILURE */
#include <stdio.h> /* FILE, fopen, fclose */
#include "example.h" /* buffcopy */
int main( void )
{ int result = EXIT_FAILURE;
FILE * source;
FILE * target;
char * buffer;
int try = 1;
{ while( try &&( source = fopen( "source", "r" )))
{ while( try &&( target = fopen( "target", "w" )))
{ while( try &&( buffer = malloc( 1024 )))
{ result = buffcopy( target, source, buffer );
try = 0; free( buffer ); }
try = 0; fclose( target ); }
try = 0; fclose( source ); }}
return result; }
The variable "try" was used to have each loop be executed at
most once. Thus, it helps "emulating" the if-statements using
the while-statements. Why was the while-loop introduced above
at all? Because the code now can be rewritten to use
for-statements:
/* < filename = [twomac2.c] > */
#include <stdlib.h> /* malloc, free, EXIT_FAILURE */
#include <stdio.h> /* FILE, fopen, fclose */
#include "example.h" /* buffcopy */
int main( void )
{ int result = EXIT_FAILURE;
FILE * source;
FILE * target;
char * buffer;
for
( int try = 1;
try &&( source = fopen( "source", "r" ));
try = 0, fclose( source ))
for
( try = 1;
try &&( target = fopen( "target", "w" ));
try = 0, fclose( target ))
for
( try = 1;
try &&( buffer = malloc( 1024 ));
try = 0, free( buffer ))
result = buffcopy( target, source, buffer );
return result; }
Now, one can see how the "magic" works. If one does not like
macros, the for-statements can be used as they are given above
to be able to write close-statements near to their
corresponding open statement. Otherwise, they might be packed
into macros:
/* < filename = [twomac.c] > */
#include <stdlib.h> /* malloc, free, EXIT_FAILURE */
#include <stdio.h> /* FILE, fopen, fclose */
#include "example.h" /* buffcopy */
#define TRY(x) for(int try=1;(try&&(x));
#define PUSH(y) ((try=0),(y)))
int main( void )
{ int result = EXIT_FAILURE;
FILE * source;
FILE * target;
char * buffer;
TRY( source = fopen( "source", "r" ))
PUSH( fclose( source ))
TRY( target = fopen( "target", "w" ))
PUSH( fclose( target ))
TRY( buffer = malloc( 1024 ))
PUSH( free( buffer ))
result = buffcopy( target, source, buffer );
return result; }
The next programm features a debug-version of the code.
It can be executed as it is, because it does not contain
a reference to the undefined function "buffcopy".
The comparison in the function "mymalloc" might be modified to
make the function "mymalloc" return 0 (by changing the value
"2" to "1", "0" or "-1"). This might be used to test all
possible branches of control flow and to observe that the
function "myfree" only is called for those calls of the
function "mymalloc", which did not return 0; and that the
function "printf" only is called if all attempts did succeed.
/* < filename = [twomacd.c] > */
#include <stdlib.h> /* EXIT_SUCCESS, EXIT_FAILURE */
#include <stdio.h> /* fprintf, printf */
#define TRY(x) for( int try=1; (try&&(x));
#define PUSH(y) ((try=0),(y)))
void *
mymalloc
( size_t const s )
{
static int i = 0;
void * const result = i > 2 ? 0 : malloc( s );
fprintf( stderr, "%p = mymalloc( %zu );\n", result, s );
++i;
return result; }
void
myfree
( char * const p )
{
fprintf( stderr, "free( %p );\n", p );
free( p ); }
int
main
( void )
{
int result = EXIT_FAILURE;
char *a;
int *b;
double *c;
TRY( a = mymalloc( 2 * sizeof( *a )))
PUSH( myfree( a ))
TRY( b = mymalloc( 3 * sizeof( *b )))
PUSH( myfree(( char * )b ))
TRY( c = mymalloc( 4 * sizeof( *c )))
PUSH( myfree(( char * )c ))
{ printf( "%p %p %p\n",( void * )a, ( void * )b, ( void * )c );
result = EXIT_SUCCESS; }
return result; }
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