Noob: needs help with file I/O

[crws416]

Hi all,

I'm having a prob with loading a file into an array.

I'm fine with opening the file but when it come to the fread command i seem to be failing as the document size will be variable, so how can i state a file size as "num_of_elements".

if this is not clear, then my basic idea is:

i want to open a file in 128 bit chunks and load it into an array so i can perform function on the array. Thats about it.

I would provide code, but i've butchered it that much it aint much use.

any help would be greatly appreciated.

cheers.

[Narue]

>so how can i state a file size as "num_of_elements"
There are basically two ways: 1) read the file and dynamically resize an array accordingly:

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

int main ( void )
{
  FILE *in = fopen ( "test.txt", "r" );

  if ( in != NULL ) {
    char *myarray = NULL;
    char buffer[5];
    size_t i, n, curr = 0, size = 0;

    while ( ( n = fread ( buffer, 1, 5, in ) ) > 0 ) {
      char *save;

      size += n;
      save = realloc ( myarray, size );

      if ( save == NULL )
        break;

      myarray = save;

      for ( i = 0; i < n; i++ )
        myarray[curr++] = buffer[i];
    }

    for ( i = 0; i < size; i++ )
      printf ( "%c", myarray[i] );

    free ( myarray );
  }

  return 0;
}

and 2) gather the number of items in the array and do a one time allocation:

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

int main ( void )
{
  FILE *in = fopen ( "test.txt", "r" );

  if ( in != NULL ) {
    char *myarray = NULL;
    size_t i, size = 0;
    int ch;

    while ( ( ch = fgetc ( in ) ) != EOF )
      ++size;

    rewind ( in );
    myarray = malloc ( size );

    if ( fread ( myarray, 1, size, in ) == size ) {
      for ( i = 0; i < size; i++ )
        printf ( "%c", myarray[i] );
    }

    free ( myarray );
  }

  return 0;
}

Any solution will be a variation of that, such as processing the file in smaller chunks individually rather than growing an array, or getting the size of the file with some non-portable method such as fstat.

[crws416]

cheers for the prompt reply,

will this still work when dealing with binary files, as i've noted within your code you only use the 'r' identifier

many thanks

[Narue]

>will this still work when dealing with binary files
It should, though a little more detail in what kind of data you're working with and what you're doing would help me to give you a more definite answer.

[crws416]

sorry about that.. basically i will be attempting to load a number of different files and there is no pre-defined structure to them.. the over-all code will go to form part of a basic form of encryption, where by the file will e loaded in binary. Block of the data will then be transformed and output into a file.

Hope this is of more help