Re: [PATCH 14/17] Add ftruncate implementation for MSVC

[Junio C Hamano <gitster@pobox.com>]

Marius Storm-Olsen <mstormo@gmail.com> writes:

> The MSVC Posix implementation doesn't contain ftruncate, so add our own
> which can handle large files (64bit offsets).
>
> Signed-off-by: Marius Storm-Olsen <mstormo@gmail.com>
> ---
>  compat/msvc.c |    8 ++++++++
>  compat/msvc.h |    2 ++
>  2 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/compat/msvc.c b/compat/msvc.c
> index ac04a4c..b96b045 100644
> --- a/compat/msvc.c
> +++ b/compat/msvc.c
> @@ -32,4 +32,12 @@ int closedir(DIR *dir)
>  	return 0;
>  }
>  
> +int ftruncate(int fd, __int64 length)
> +{
> +	HANDLE fh = (HANDLE)_get_osfhandle(fd);
> +	if (!fh || _lseeki64(fd, length, SEEK_SET))
> +		return -1;
> +	return SetEndOfFile(fh) ? 0 : -1;
> +}
> +


Wouldn't it break this toy program?  ftruncate() should preserve the file
offset and use of the seek in the above feels iffy.

#include <stdlib.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>

int main(int ac, char **av)
{
        int fd = open("testfile", O_CREAT|O_WRONLY, 0666);
        if (fd < 0)
                exit(1);
        write(fd, "abcdefghijklmnopqrstuvwxyz\n", 27);
        lseek(fd, 7, SEEK_SET);
        write(fd, "H", 1);
        ftruncate(fd, 24);
        write(fd, "IJ", 2);
        close(fd);
        exit(0);
}

The only two in-tree users of ftruncate() do not care about this, though.
They both seek to the beginning and then truncate to zero length.