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Posted 29 Jan 2005

Bypassing ftell/fseek's 2gb limitation

, 29 Jan 2005
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Instructions on how to implement _fseeki64 and _ftelli64.


Recently, I was working on completing a port of a Linux program to Windows. It's called mpgtx, and is a command line MPEG editing program. The port was pretty easy, since it didn't use many Linux-specific functions (and someone had already done part of the port), but I did run across one big problem: it would fail on any long MPEG-2 clip. I quickly realized that it was because the fseek/ftell functions use the data type long for file offsets, which is a 32-bit signed integer on most systems. Therefore, those functions don't work on files that are bigger than 2048 megabytes. I then searched around for information on getting around that problem, and the only info I found was on Linux-specific functions (fseeko/ftello) or a non-buffered function (_lseeki64). I didn't want to use Cygwin, and I really didn't want to convert all the fopen/fread/fseek/etc... function calls to _open/_read/_lseeki64/etc... and still maintain code compatibility with Linux, so I had to find another way. I thought that because there was a _lseeki64 function there might be a fseeki64 function, but nothing was mentioned about that function in the Help system and hardly anything was listed when searching Google for it. One thing Google did find, though, was a page that said there was a file named "fseeki64.c" in "crt/src". Sure enough, both the fseeki64.c and ftelli64.c files existed in my "Microsoft Visual Studio .NET 2003\Vc7\crt\src" folder. Unfortunately, neither function is mentioned in stdio.h, so it's going to take a little extra work to use those functions...

Using the code

First, you'll have to add the following imports into your code:

extern "C" int __cdecl _fseeki64(FILE *, __int64, int);
extern "C" __int64 __cdecl _ftelli64(FILE *);

Note that they use __in64 for the file offset data type, instead of long.

Next, replace all calls to fseek and ftell with _fseeki64 and _ftelli64, respectively. Finally, change all the variables that you use to hold file offsets from long to __int64. That's it; now you should be good to go!

Points of Interest

Does anyone know why _fseeki64 and _ftelli64 are completely, mysteriously undocumented?


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About the Author

Web Developer
United States United States
I started programming by getting books on BASIC programs from the local library and entering them into QBASIC, and when I was 10 I bought a copy of Visual Basic 5 and started visual development on my 33mhz (talk about long compile times!). After a couple years I decided to learn the language that most professional programs and games were made in: C++. As soon as I learned about the power of pointers, I was addicted to programming for life Wink | ;) .

I've worked on a number of programs since then, including IGCutter, Pockulous, and Freelancer Mod Manager. I've also had the great opportunity to intern at Adobe over a summer; definitely a dream come true Smile | :) .

I'm currently a freshman/sophomore in college, trying to stay awake in boring required classes on Java...

-Matthew Lieder

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