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DBJDBJ/dbj_defer_begin_end.h

Last active July 16, 2021 09:07
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Defer and Begin/End. For any C version. With Gobolt demo and proof of concept.
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dbj_defer_begin_end.h
#ifndef DBJ_DEFER_BEGIN_END_H
#define DBJ_DEFER_BEGIN_END_H
/*
https://godbolt.org/z/zsYre5Kfj
first seen it here https://youtu.be/QpAhX-gsHMs
2021-APR dbj@dbj.org added macro defer()
Two Macros. Usage:
// these two footprints are totaly whatever you need!
// Q: when are the macro arguments executed?
// A: be sure to understand ;)
void on_scope_begin(void) ;
void on_scope_end(void) ;
beginend( on_scope_begin(), on_scope_end() ) { }
defer( on_scope_end() ) { }
See the ad-hoc-demo bellow.
Q: What might be also good about this two macros?
A: They work on any C version, using any compiler.
There is no trick: Both macros are for(){ } that loops once
I see no reason for this not to work on any C
*/
#define _POSIX_C_SOURCE 200112L
#include <stdlib.h>
#define macro_concat_(a,b) a ## b
#define macro_concat(a,b) macro_concat_(a,b)
#define macro_var(name) macro_concat( name, __LINE__)
#define beginend( start,end) \
for ( int macro_var(_i_) = ( start, 0 ) ; ! macro_var(_i_) ; \
( macro_var(_i_) += 1, end) )
static inline void do_nothing_ (void) {}
// #define defer( at_scope_end ) beginend( __LINE__, at_scope_end )
#define defer( at_scope_end) for ( int macro_var(_i_) = 0;\
! macro_var(_i_) ; ( macro_var(_i_) += 1, at_scope_end) )
/*
Ad-hoc demo
*/
#if DBJ_DEFER_BEGIN_END_DEMO
#include <string.h>
#include <stdio.h>
#include <stdbool.h>
// NOTE: F must be a string literal
#define B(X_) (X_) ? "true" : "false"
#define P(F,X_) fprintf( stdout, "\n%04d : %16s :\t" F, __LINE__, #X_, (X_))
#define M(S) fprintf( stdout, "\n%04d : %16s :\t%s", __LINE__, " ", S)
#ifdef __linux__
#include <unistd.h> // readlink
#endif
static void here_ () { M("Begin!"); }
static void there_ () { M("End!"); }
static void make_ ( char ** ptr, size_t sze_ ) { *ptr= calloc(sze_, sizeof(char)); printf("\nmade (%p) char [%zu]", *ptr , sze_); }
static void free_ ( void * ptr ) {printf("\nfreed (%p)", ptr ); free(ptr); ptr = NULL; }
static void close_file ( FILE * fp_ ) {
if ( fp_ == NULL) { M("close_file(): Avoiding NULL file pointer");
return ; }
if ( ferror(fp_)) perror("I/O error");
P("closing FILE * (%p)", (void*)fp_);
fclose(fp_ ) ;
}
int main (void)
{
beginend( here_(), there_() )
{
M("Work.");
}
char * bufy_ = 0 ;
printf("\n");
beginend( make_( &bufy_, 0xFF ), free_( bufy_ ) )
{
memcpy( bufy_, "DATA", 5 );
printf("\nusing (%p) : \"%s\"", (void*)bufy_, bufy_);
}
printf("\n");
FILE * dummsy_ = tmpfile();
defer( close_file( dummsy_ )) {
P("temp FILE * (%p)", (void*)dummsy_);
#ifdef __linux__
// Linux-specific
char fname[FILENAME_MAX] = {0}, link[FILENAME_MAX] = {0};
sprintf(fname, "/proc/self/fd/%d", fileno(dummsy_));
P("temp FILE name: %s", fname);
if(readlink(fname, link, sizeof link - 1) > 0)
P("temp FILE link: %s", link);
#endif // __linux__
}
return 42 ;
}
#undef B
#undef P
#undef M
#endif // DBJ_DEFER_BEGIN_END_DEMO
#undef macro_concat_
#undef macro_concat
#undef macro_var
#endif // DBJ_DEFER_BEGIN_END_H
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DBJDBJ commented Jul 16, 2021 •
edited

Once more: arguments are function calls. But: not executed because they are macro arguments.

   // make() and free_() are *not* executed on this line
    beginend( make_( &bufy_, 0xFF ), free_( bufy_ ) )
    {
         memcpy( bufy_, "DATA", 5 );
         printf("\nusing (%p) : \"%s\"", (void*)bufy_, bufy_);
    }

That is because beginend is a macro, not a function. And macro is just a text replacement mechanism. Ditto, the above is translated into this:

// __LINE__ was 123
for ( int _i_123 = ( make_( &bufy_, 0xFF ) , 0 ) ; ! _i_123 ; ( _i_123 += 1,  free_( bufy_ )) )
{
         memcpy( bufy_, "DATA", 5 );
         printf("\nusing (%p) : \"%s\"", (void*)bufy_, bufy_);
}

Notice the comma operators in the for() above 😉

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