[Mono-dev] How to write files > 4GB with pwrite in Mono C# in 32-bit OS?

pedrom71 pedrosanzm71 at gmail.com
Sat Mar 18 21:47:57 UTC 2017

I'm trying to write a new file with size > 64 gb using mono c# and Mono.Posix
library, with the Syscall.pwrite() function. But in a 32-bit OS, I get error
EOVERFLOW when the offset reach 4GB.

This is the example code I'm using:

    public unsafe static void Main (string[] args)
        int fd = Syscall.open("/dev/null", OpenFlags.O_WRONLY);
        long bLen = 4096;
        long fLen = 5368709120;
        byte[] buffer = new byte[bLen];
        long c = 0;
        long res = -1;
        if (fd == -1)
            goto Finish;

            fixed (byte *pb = buffer)

                res = Syscall.pwrite(fd, pb, (ulong)bLen, c);
                if (res == -1)
                c+= bLen;
        while (c < fLen);

        if (res == -1)
            Console.WriteLine("Error in offset " + c.ToString());
            Console.WriteLine("Error: " + Stdlib.GetLastError());
            Console.WriteLine("All OK");

In C, the solution is compiling the code with gcc with parameter
-D_FILE_OFFSET_BITS=64. But in mono I haven't found the solution yet.

*Note: This error only happens in a 32-bit system. In a 64-bit OS works

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