Thread: fputc inserting extra bytes

  1. #1
    Registered User
    Join Date
    Sep 2009

    fputc inserting extra bytes

    As part of a project I'm writing a small program to write 24-bit bitmap (BMP) files and am having a quite confusing problem. The file header includes the size of the file with the least significant byte first, and the next 3 bytes in order of increasing significance - so if the size was 0xAFCB6, for example, that would be in the header as "B6 FC 0A 00."

    To construct the header I made a char array with an entry for every byte of the header, pretty straightforward. My code to write the bytes of the filesize in the correct order looks like:
    char header[] = { ... filesize&0xFF,(filesize>>8)&0xFF,(filesize>>16)&0xFF,(filesize>>24)&0xFF, ... }
    Which should (and usually does) work.
    To write the header to the file I wrote the following:

    void fputchars(const char *str, FILE *out, int len) {
      int i;
      for(i = 0; i < len; i++) {
        fputc(str[i], out);
    Which also should (and generally does) work.

    However, with certain filesizes, an extra byte shows up in the file - not the incorrect byte, a complete extra one that shifts everything over and just generally messes everything up. The filesize I mentioned above is one such number - 0xAFCB6 results in "B6 FC 0D 0A 00" to be written to the file - so in the end I have more bytes written to the file than there are in the header array, fputc seems to be sneaking them in.
    I believe the problem occurs when the least significant byte of the filesize is >= 128, which leads me to believe it has something to do with the chars in question being signed or unsigned, but I can't figure out why additional bytes are being written.

    Any thoughts? Am I misusing fputc?

  2. #2
    Guest Sebastiani's Avatar
    Join Date
    Aug 2001
    Waterloo, Texas
    Did you open the file in binary mode?

  3. #3
    Registered User
    Join Date
    Sep 2009
    Well, I feel very stupid.

    Thanks much

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