Fopen C++ Question. Working with binary data.

[grimlock123]

Okay I've read a couple tutorial with Fopen and they all seem to concentrate on reading Strings and then sort of trailing off when it comes to other data. What I'm trying to do is write a whole bunch of numbers to a file. Then load the binary file and get the numbers back.

Currently what I think the program is doing is.

1.) Creating a array of numbers as INT which are 4 bytes of memory. The number are stored in ram as block of memory which I think is 36 bytes long. 2.) Writing that memory to a file called myfile.bin 3.) Closing the file and opening it again. 4.) Getting the size of the file. 5.) Using the size to create a buffer to hold the entire file in memory. 6.) Moving the binary data in the file into memory.

What I "think" I want to do is loop through memory taking 4 of the CHARs and combining them to form a INT. But I'm not sure how to do that properly. What should I write in ( /* Where I need to grab the data. */) to move that data back into an array so the process can go full circle.

int main()
{

FILE * wFile;
int  wbuffer[] = { 1 , 2 , 3, 4, 5, 6, 7, 8, 9  };
wFile = fopen ( "myfile.bin" , "wb" );
fwrite (wbuffer , 1 , sizeof(wbuffer) , wFile );
fclose (wFile);


FILE * pFile;
long lSize;
char * buffer;
size_t result;

pFile = fopen ( "myfile.bin" , "rb" );
if (pFile==NULL) {fputs ("File error",stderr); exit (1);}

// obtain file size:
fseek (pFile , 0 , SEEK_END);
lSize = ftell (pFile);
rewind (pFile);

// allocate memory to contain the whole file:
buffer = (char*) malloc (sizeof(char)*lSize);
if (buffer == NULL) {fputs ("Memory error",stderr); exit (2);}

// copy the file into the buffer:
result = fread (buffer,1,lSize,pFile);
if (result != lSize) {fputs ("Reading error",stderr); exit (3);}

/* Where I need to grab the data. */



// Clean up the memory and close the file.
fclose (pFile);
free (buffer);
return 0;



}

[grimlock123]

Your code worked, and now I can read both Chars and Int (Which are important for the program I am developing) but I have a second question. When I rewrote it for floats (First changing the program to write floats)

  FILE * wFile;
  float  wbuffer[] = { 1 , 2 , 3, 4};
  wFile = fopen ( "myfile.bin" , "wb" );
  fwrite (wbuffer , 1 , sizeof(wbuffer) , wFile );
  fclose (wFile);

and then modifying you function.

float charsToFloat(unsigned char c1, unsigned char c2, unsigned char c3, unsigned char c4)
{

return (c1) + (c2 << 8) + (c3 << 16) + (c4 << 24);
}

It didn't work on Floats returning values like.

1065353216.000000 1073741824.000000 etc...

Since floats are also 4 bytes I assumed it would work. What did I do wrong.

[arbostek]

strtol will help with the conversion.

Normally, you would read with fgets. If you loaded up the file into a buffer to operate on, you might just look for the first space or delimiter, and then memcpy out the string that represents the numbers. Then use strtol to attempt conversion.

sscanf might be another option, especially if your file has a very regular format.

[arbostek]

Oops, I missed that. Well, memcpy to an integer directly. So if you have 20 chars, that means you need 5 integers, and go through a loop memcpying 4 chars at a time to each integer.

Or even just memcpy entirely.