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From: Lew Pitcher <lew.pitcher@digitalfreehold.ca>
Newsgroups: comp.lang.c
Subject: Re: Whaddaya think?
Date: Tue, 25 Jun 2024 17:37:24 -0000 (UTC)
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On Sun, 16 Jun 2024 15:52:16 +0000, Lew Pitcher wrote:

> On Sat, 15 Jun 2024 15:36:22 -0400, DFS wrote:
> 
>> I want to read numbers in from a file, say:
>> 
>> 47 185 99 74 202 118 78 203 264 207 19 17 34 167 148 54 297 271 118 245 
>> 294 188 140 134 251 188 236 160 48 189 228 94 74 27 168 275 144 245 178 
>> 108 152 197 125 185 63 272 239 60 242 56 4 235 244 144 69 195 32 4 54 79 
>> 193 282 173 267 8 40 241 152 285 119 259 136 15 83 21 78 55 259 137 297 
>> 15 141 232 259 285 300 153 16 4 207 95 197 188 267 164 195 7 104 47 291
>> 
>> 
>> This code:
>> 1 opens the file
>> 2 fscanf thru the file to count the number of data points
>> 3 allocate memory
>> 4 rewind and fscanf again to add the data to the int array
>> 
>> 
[snip]
> You /could/ create a temporary, binary, file, and write the fscanf()'ed
> values to it as part of the first loop. Once the first loop completes,
> you rewind this temporary file, and load your integer array by reading
> the (now converted to native integer format) values from that file.
> 
> Still two passes, but using fscanf() in only one of those passes.
[snip]

For what it's worth, here's an example of what I suggest:

/*
The following code provides two examples of the approach I suggested.

Example 1: while counting input numbers, write temp file with int values
	   malloc() a buffer big enough for that count of int values
	   fread() the temp file into the malloc()'ed buffer
	   Note: conformant to ISO Standard C.

Example 2: while counting input numbers, write temp file with int values
	   mmap() the temp file, starting at the beginning, and sized to
	   include all the int values in the file.
	   Note: conformant to POSIX C extensions to ISO Standard C.

Note: compile with -DUSE_MMAP to obtain mmap() variant, otherwise
      this will compile the malloc()/fread() variant
*/

#include <stdio.h>
#include <stdlib.h>

#ifdef USE_MMAP
  #include <sys/mman.h>
  #define BANNER	"Example of array loading using mmap()"
  #define FREEALLOC(x)
#else
  #define BANNER	"Example of array loading using malloc() and fread()"
  #define FREEALLOC(x) free((x))
#endif

static int *LoadIntArray(FILE *fp, size_t *Count);

int main(void)
{
  int status = EXIT_FAILURE, *array;
  size_t count;

  puts(BANNER);

  if ((array = LoadIntArray(stdin,&count)))
  {
    printf("%zu elements loaded\n",count);
    for (size_t index = 0; index < count; ++index)
      printf("array[%3zu] == %d\n",index,array[index]);

    FREEALLOC(array);	/* if necessary, free() the malloc()'ed array */
    status = EXIT_SUCCESS;
  }
  return status;
}

static int *LoadIntArray(FILE *fp,size_t *Count)
{
  FILE *tmp;
  int *array = NULL;
  size_t count = 0;

  if ((tmp = tmpfile()))
  {
    int buffer;

    for (count = 0; fscanf(fp,"%d",&buffer) == 1; ++count)
      fwrite(&buffer,sizeof buffer, 1,tmp);

    if (count)
    {
#ifdef USE_MMAP
      /*
      ** USE mmap() to map temp_file data into process memory
      */
      array = mmap(NULL,
                   count * sizeof *array,
                   PROT_READ,MAP_PRIVATE,
                   fileno(tmp),
                   0);
      if (array == MAP_FAILED)
      {
	array = NULL;
	fprintf(stderr,"FAIL: Cannot mmap %zu element array\n",count);
      }
#else
      /*
      ** USE malloc() to reserve a big enough heap-space buffer,
      ** then fread() the temp_file data into that buffer
      */
      if ((array = malloc(count * sizeof *array)))
      {
	rewind(tmp);
	if (fread(array,sizeof *array,count,tmp) != count)
	{
	  free(array);
	  array = NULL;
	  fprintf(stderr,"FAIL: Cannot load %zu element array\n",count);
	}
      }
      else fprintf(stderr,"FAIL: Cant malloc() %zu element array\n",count);
#endif
    }
    fclose(tmp);

  }
  else fprintf(stderr,"FAIL: Cannot allocate temporary work file\n");

  *Count = count;	/* byproduct value that caller might find useful */
  return array;		/* either NULL (on failure) or pointer to array */
}



-- 
Lew Pitcher
"In Skills We Trust"