Appending to files and getting file size

[mike_g]

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.

[dwks]

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.

[mike_g]

Ok cool so ftell will work when appending.

Cheers.

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

[dwks]

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.

[mike_g]

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