30

I am writing a C library that reads a file into memory. It skips the first 54 bytes of the file (header) and then reads the remainder as data. I use fseek to determine the length of the file, and then use fread to read in the file.

The loop runs once and then ends because the EOF is reached (no errors). At the end, bytesRead = 10624, ftell(stream) = 28726, and the buffer contains 28726 values. I expect fread to read 30,000 bytes and the file position to be 30054 when EOF is reached.

C is not my native language so I suspect I've got a dumb beginner mistake somewhere.

Code is as follows:

const size_t headerLen = 54;

FILE * stream;
errno_t ferrno = fopen_s( &stream, filename.c_str(), "r" );
if(ferrno!=0) {
  return -1;
}

fseek( stream, 0L, SEEK_END );
size_t bytesTotal = (size_t)(ftell( stream )) - headerLen; //number of data bytes to read
size_t bytesRead = 0;
BYTE* localBuffer = new BYTE[bytesTotal];
fseek(stream,headerLen,SEEK_SET);
while(!feof(stream) && !ferror(stream)) {
    size_t result = fread(localBuffer+bytesRead,sizeof(BYTE),bytesTotal-bytesRead,stream);
    bytesRead+=result;
}

Depending on the reference you use, it's quite apparent that adding a "b" to the mode flag is the answer. Seeking nominations for the bonehead-badge. :-)

This reference talks about it in the second paragraph, second sentence (though not in their table).

MSDN doesn't discuss the binary flag until halfway down the page.

OpenGroup mentions the existance of the "b" tag, but states that it "shall have no effect".

Jim Fell
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James Schek
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  • I added the microsoft tag since fopen_s is only in MS's C RTL. – user7116 Oct 02 '08 at 19:09
  • OpenGroup does mention it: r or rb - Open file for reading. ... The character 'b' shall have no effect, but is allowed for ISO C standard conformance. Please fix your edit. – Jonathan Leffler Oct 02 '08 at 20:14
  • Having found the file size, and allocated the memory, You should be able to read the entire file in, with one fread. – EvilTeach Dec 13 '10 at 01:23

3 Answers3

56

perhaps it's a binary mode issue. Try opening the file with "r+b" as the mode.

EDIT: as noted in a comment "rb" is likely a better match to your original intent since "r+b" will open it for read/write and "rb" is read-only.

Evan Teran
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  • +1 for Mike F and myself. Windows bites me with +b all the time. – user7116 Oct 02 '08 at 19:08
  • I would suggest trying "rb" first, as "r+b" opens file file for reading and writing, and if you're not intending to write to the file you should continue to open it as read-only. – Greg Hewgill Oct 02 '08 at 19:08
  • This is the answer. "rb" works. Another case of RTFM. First paragraph says this as well: http://www.cplusplus.com/reference/clibrary/cstdio/fopen.html – James Schek Oct 02 '08 at 19:16
  • Wow, isn't it amazing we still have to deal with these binary vs. text file issues in 2008. I bet if you look in your file at position 10624 you will see 0x1B (decimal 27) which is the end-of-file character. – David Smith Oct 02 '08 at 19:21
  • @David Smith: 0x1B is the Escape character. The old DOS end-of-file character was 0x1A (^Z). – Greg Hewgill Oct 02 '08 at 19:35
  • I agree with Evan that it's probably a binary mode issue. However, I'm pretty sure the C standard does not guarantee that ftell as you're using it will return the actual length of the file. I seem to recall that it is required to return a "token" that will get you back to the same position in the file if you pass it to fseek. – Paul Tomblin Oct 02 '08 at 19:07
  • I had the same thought... According to this reference, it is the actual position: http://www.opengroup.org/onlinepubs/009695399/functions/ftell.html – James Schek Oct 02 '08 at 19:11
  • Thanks, 6.5 years later and this post helped me too. – rsethc Jan 16 '15 at 02:19
  • you sir, just saved me a lot of time! +1 – meetaig Oct 17 '16 at 18:31
0

Also worth noting that simply including binmode.obj into your link command will do this for you for all file opens.

Richard Harrison
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-1

A solution, based on the previous answers:

    size_t bytesRead = 0;
    BYTE* localBuffer = new BYTE[bytesTotal];
    fseek(stream,headerLen,SEEK_SET);
        while(!feof(stream) && !ferror(stream)) {
        size_t result = fread(localBuffer+bytesRead,sizeof(BYTE),bytesTotal-
        bytesRead,stream);
    bytesRead+=result;
}
Brad Larson
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Kumar Pushkar
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