Truncate file

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.

From stackoverflow

• 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.

  Chris Lutz : Why are you using a pointer to a single character? You can use fgetc() and fputc() and just use a non-pointer (or not even a variable): for(int i = 0; i < 100000; i++) fputc(fgetc(fin), fout);

  paxdiablo : Which would result in 100,000 times as many function calls :-)

  Chris Lutz : Ah, there was some misreading. Sorry. (However, I don't think 100000 fgetc()s and fputc()s really take that long.)

  paxdiablo : Interesting, I was going to prove you wrong by writing sample programs but it appears you are right. Transfer of a 34M file actually takes longer (twice as long) with malloc and repeated fread(100k)/fwrite(100K) than with a repeated fgetc/fputc. So I'll wisely keep my mouth shut :-)

• SetEndOfFile()

  Get a handle to the file with write access, set the file pointer, then call SetEndOfFile().

  paxdiablo : I think we have a winner here, @myforwik.

• 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.