Appending to files and getting file size

I have a function that saves to a file. If it is set to append I will need to check if the file has any data in it; if not then it will need to write the header info out. My problem is, how would I get the filesize? If i open it to append wouldent the file pointer would be set to the end of the file?

Edit: Actually it would also have to check the first four bytes of the file to make sure its the valid file format as well.

Yes. Try ftell() or (for C99) fgetpos(). It should return the position in the file, which should be nonzero if the file has some data in it.

Ok cool so ftell will work when appending.

Cheers.

Edit: I'm compiling as C99 so should I use fgetpos() instead?

Yeah, I think it would. I've never tried it.

Edit: Actually it would also have to check the first four bytes of the file to make sure its the valid file format as well.

Sounds like you want the "a+" fopen mode. It lets you read from the file and append to it too. Just make sure you fflush()/rewind() the file before switching from reading to writing or vise versa.

Ah, what do I know. http://msdn.microsoft.com/en-us/libr...0b(VS.71).aspx

Note that when a file is opened for appending data, the current file position is determined by the last I/O operation, not by where the next write would occur. For example, if a file is opened for an append and the last operation was a read, the file position is the point where the next read operation would start, not where the next write would start. (When a file is opened for appending, the file position is moved to end of file before any write operation.) If no I/O operation has yet occurred on a file opened for appending, the file position is the beginning of the file.

So I'd open the file for a+ mode, fseek() to the end of the file, and ftell() from there. Or you could just fgets() the first few bytes and see if it's a valid file.

So I'd open the file for a+ mode, fseek() to the end of the file, and ftell() from there. Or you could just fgets() the first few bytes and see if it's a valid file.

Sounds good to me. I'll do that