[c] write binary file - Tilted Forum Project Discussion Community

Silvy · 07-11-2007, 11:22 AM

Hi guys and gals,

I'm trying to write a small tool in C, and while my programming experience is decent in object Pascal, C# and PHP, my C skills are next to nowhere.

Most of the tool is working correctly, but two issues remain:
- writing a fixed binary value to file
- writing a timestamp to file.

I know how to read/write binary streams from a char* type using the function fwrite but I can't figure out how to write a single byte (say: FF) to the same file.

The timestamp is of type time_t and the following is therefore not allowed:

Code:

fwrite(timestamp, 1, sizeof(timestamp), fp)

I'm creating the tool for logging a stream, and while I got that part working, it is adding timestamps that's puzzling me. (the FF is for differentiating between timestamp and actual data).

Any help or pointers (*ouch*) is greatly appreciated.

xepherys · 07-11-2007, 03:43 PM

For reference:

Quote:

size_t fwrite(
const void *buffer,
size_t size,
size_t count,
FILE *stream
);

You can frontload your buffer with all of the data then call it as your first parameter in fwrite.

Silvy · 07-11-2007, 04:31 PM

I see where you're getting at.

So I'm looking for a way to get byte(FF)+timestamp+byte(FF) into a buffer so I can use fwrite to get that buffer into the file.

Trying to create a buffer for the FF byte should be something like this I suppose:

Code:

	CHAR byteBuffer[1];
	byteBuffer[0] = 255;
	printf("byteBuffer: %X \n", byteBuffer);

But it is not working as expected:
Output: "byteBuffer: BFFFF48F"
Expected: "byteBuffer: FF"

How do I tell C that 255 is a single byte value?

Pragma · 07-11-2007, 05:46 PM

That's because you're referencing it wrong.

Code:

printf("byteBuffer: %X \n", byteBuffer[0] );

Try that instead - I think what's happening instead is that you're printing a the address of the char* byteBuffer pointer.

Silvy · 07-11-2007, 06:01 PM

Thanks a lot! That is indeed the solution.

I've been at this for quite a while trying to get to grips with pointers and casts and streams and such, and sometimes the logical solution eludes

I think I've also figured out how to write a timestamp to the file, but I'm gonna toy a bit with it for now.

Thanks again for the help. If I get stuck again I'll know where to find ya ;-)

07-11-2007, 11:22 AM	#1 (permalink)
Silvy paranoid Location: The Netherlands	[c] write binary file Hi guys and gals, I'm trying to write a small tool in C, and while my programming experience is decent in object Pascal, C# and PHP, my C skills are next to nowhere. Most of the tool is working correctly, but two issues remain: - writing a fixed binary value to file - writing a timestamp to file. I know how to read/write binary streams from a char* type using the function fwrite but I can't figure out how to write a single byte (say: FF) to the same file. The timestamp is of type time_t and the following is therefore not allowed: Code: fwrite(timestamp, 1, sizeof(timestamp), fp) I'm creating the tool for logging a stream, and while I got that part working, it is adding timestamps that's puzzling me. (the FF is for differentiating between timestamp and actual data). Any help or pointers (ouch) is greatly appreciated. __________________ "Do not kill. Do not rape. Do not steal. These are principles which every man of every faith can embrace. " - Murphy MacManus (Boondock Saints)

07-11-2007, 04:31 PM	#3 (permalink)
Silvy paranoid Location: The Netherlands	I see where you're getting at. So I'm looking for a way to get byte(FF)+timestamp+byte(FF) into a buffer so I can use fwrite to get that buffer into the file. Trying to create a buffer for the FF byte should be something like this I suppose: Code: CHAR byteBuffer[1]; byteBuffer[0] = 255; printf("byteBuffer: %X \n", byteBuffer); But it is not working as expected: Output: "byteBuffer: BFFFF48F" Expected: "byteBuffer: FF" How do I tell C that 255 is a single byte value? __________________ "Do not kill. Do not rape. Do not steal. These are principles which every man of every faith can embrace. " - Murphy MacManus (Boondock Saints)

07-11-2007, 05:46 PM	#4 (permalink)
Pragma I am Winter Born Location: Alexandria, VA	That's because you're referencing it wrong. Code: printf("byteBuffer: %X \n", byteBuffer[0] ); Try that instead - I think what's happening instead is that you're printing a the address of the char* byteBuffer pointer. __________________ Eat antimatter, Posleen-boy!

07-11-2007, 06:01 PM	#5 (permalink)
Silvy paranoid Location: The Netherlands	Thanks a lot! That is indeed the solution. I've been at this for quite a while trying to get to grips with pointers and casts and streams and such, and sometimes the logical solution eludes I think I've also figured out how to write a timestamp to the file, but I'm gonna toy a bit with it for now. Thanks again for the help. If I get stuck again I'll know where to find ya ;-) __________________ "Do not kill. Do not rape. Do not steal. These are principles which every man of every faith can embrace. " - Murphy MacManus (Boondock Saints)