I am using fopen fseeko64 ftello64 fclose etc. to operating on a file.
How can I truncate a file? I know that there is no standard way to do this in C. All I want is some way that will work on any win32 platform. I'm using mingw gcc to compile.
Please note: I meant truncate the size of the file to a specified size, not make it 0 size. And using a trick like copy part to another file and delete/rename is not appropriate.
If you simply fopen() a file with the "w" argument, it will be truncated.
http://www.cplusplus.com/reference/clibrary/cstdio/fopen.html
You should be able to fopen with the "w" flag:
FILE *fh = fopen("file.txt","w");
if (fh != NULL) fclose(fh);
Following your clarification that you want to truncate the file at a specific point, you can do this with standard C functions such as (add suitable error checking):
FILE *fin = fopen ("input.txt","rb");
FILE *fout = fopen ("output.txt", "wb");
size_t sz = 100000; /* for 100,000 bytes */
char *buff = malloc (sz);
sz = fread (buff, 1, sz, fin);
fwrite (buff, 1, sz, fout);
free (buff);
fclose (fin);
fclose (fout);
Of course, if you have access to the Win32 headers and libraries (and I believe MinGW gives you this), you can use SetEndOfFile() as Adam Davis has suggested, since it does it in place, rather than having to create a new file and then rename it.
Get a handle to the file with write access, set the file pointer, then call SetEndOfFile().
As mentioned already, you can use fopen() with the "w" flag like:
FILE *f = fopen("file.txt", "w");
Also, if you already have the file opened, you can use the function freopen(), again with the "w" flag:
FILE *f = fopen("file.txt", "r"); //initial fopen() call
...
f = freopen("file.txt", "w", f); //reopens "file.txt" and truncates it
http://www.cplusplus.com/reference/clibrary/cstdio/freopen.html
EDIT: After seeing you've edited your OP, I won't repost what Pax and Adam Davis has already put. Also, I'll confirm what Pax said, that the MinGW does give you access to the Win32 headers.