Thread: Need some suggestions for cross platform code

I have multiple functions that could use simplfying but for now I'll start with the file io related functions as I want to get rid of the need for synchronised offsets, MSW uses the OVERLAPPED structure for this, on linux however all I've found is write() and read(), both of which do not support arbitrary offsets, I theorise I could just duplicate the file descriptor but keeping in mind older systems that do not support unlimited file descriptors I decided going that route would be a bad idea, what other means would there be besides just recording the offset and putting it back after?

I think I came up with a workable solution, haven't had the chance to test it yet as I'm still cleaning up stuff that exposed the OVERLAPPED structure to the developer:

Code:

PAW_MIN_API pawzd pawfio_writeat( pawfio fio, pawp src, pawzd cap, pawju pos )
{
	if ( cap > 0 )
	{
		pawzu ps = sysconf( _SC_PAGE_SIZE );
		pawju from = PawToLowerMultiple( pos, ps );
		pawzu size = PawToUpperMultiple( cap, ps );
		void *dest = mmap( NULL, size, PROT_WRITE, MAP_SHARED, fio, from );
		if ( !dest ) return -1;
		paw_cpy( dest + (pos - from), src, cap );
		munmap( dest, size );
		return cap;
	}
	return -(cap < 0);
}

PAW_MIN_API pawzd pawfio_fetchat( pawfio fio, void *dst, pawzd cap, pawju pos )
{
	if ( cap > 0 )
	{
		pawzu ps = sysconf( _SC_PAGE_SIZE );
		pawju from = PawToLowerMultiple( pos, ps );
		pawzu size = PawToUpperMultiple( cap, ps );
		void *data = mmap( NULL, size, PROT_READ, MAP_SHARED, fio, from );
		if ( !data ) return -1;
		paw_cpy( dst, data + (pos - from), cap );
		munmap( data, size );
		return cap;
	}
	return -(cap < 0);
}

PAW_MIN_API pawzd	pawfio_write( pawfio fio, pawp src, pawzd cap )
	{ return pawfio_writeat( fio, src, cap, pawfio_tell( fio ) ); }
PAW_MIN_API pawzd	pawfio_fetch( pawfio fio, void *dst, pawzd cap )
	{ return pawfio_fetchat( fio, dst, cap, pawfio_tell( fio ) ); }

Roughly translated to pure linux code (no typedefs etc for cross platform compatability) it looks like this:

Code:

#define PawToLowerDivision( N, BY ) ((BY) ? (N) / (BY) : 0)
#define PawToUpperDivision( N, BY ) ((BY) ? ((N) / (BY)) + !!((N) % (BY)) : 0)
#define PawToLowerMultiple( N, OF ) ((OF) * PawToLowerDivision(N,OF))
#define PawToUpperMultiple( N, OF ) ((OF) * PawToUpperDivision(N,OF))

ssize_t pawfio_writeat( int fd, void const *src, ssize_t cap, uintmax_t pos )
{
	if ( cap > 0 )
	{
		size_t ps = sysconf( _SC_PAGE_SIZE );
		uintmax_t from = PawToLowerMultiple( pos, ps );
		size_t size = PawToUpperMultiple( cap, ps );
		void *dest = mmap( NULL, size, PROT_WRITE, MAP_SHARED, fd, from );
		if ( !dest ) return -1;
		memcpy( dest + (pos - from), src, cap );
		munmap( dest, size );
		return cap;
	}
	return -(cap < 0);
}

ssize_t pawfio_fetchat( int fd, void *dst, ssize_t cap, uintmax_t pos )
{
	if ( cap > 0 )
	{
		size_t ps = sysconf( _SC_PAGE_SIZE );
		uintmax_t from = PawToLowerMultiple( pos, ps );
		size_t size = PawToUpperMultiple( cap, ps );
		void *data = mmap( NULL, size, PROT_READ, MAP_SHARED, fd, from );
		if ( !data ) return -1;
		memcpy( dst, data + (pos - from), cap );
		munmap( data, size );
		return cap;
	}
	return -(cap < 0);
}

ssize_t pawfio_write( int fd, void const *src, ssize_t cap ) { return pawfio_writeat( fd, src, cap, lseek64( fd, 0, SEEK_CUR ) ); }
ssize_t pawfio_fetch( int fd, void *dst, ssize_t cap ) { return pawfio_fetchat( fd, dst, cap, lseek64( fd, 0, SEEK_CUR ) ); }

If someone's inclined please test it out while I continue with fixing up the windows code in other files that doesn't yet compile

Originally Posted by Vithika

One potential solution is to use the pread and pwrite system calls, which provide the same functionality as read and write, but allow for a file offset to be specified as a parameter. This way, you can avoid all the complexities associated with synchronizing file offsets, and it should be more efficient as well.

Thank you, I'll look into that as soon as I report a bug in mingw, they hid the FileSetPointer & FileSetPointerEx behind this:

Code:

#if WINAPI_FAMILY_PARTITION (WINAPI_PARTITION_DESKTOP) || _WIN32_WINNT >= _WIN32_WINNT_WIN10

The 1st condition is fine but the 2nd is flat out wrong, that function was available way earlier than Windows 10 and I've not been able to compile as a result of it

FYI: "||" means or

Originally Posted by stahta01

FYI: "||" means or

I know, that's why I said the FIRST CONDITION is fine, I tried setting the WINAPI_FAMILY define directly to trigger a "true" result there but that just resulted in nothing being defined, no DWORD, no CreateFile, no nothing, something is abysmally wrong for that to happen

Originally Posted by awsdert

Thank you, I'll look into that as soon as I report a bug in mingw, they hid the FileSetPointer & FileSetPointerEx behind this:

Code:

#if WINAPI_FAMILY_PARTITION (WINAPI_PARTITION_DESKTOP) || _WIN32_WINNT >= _WIN32_WINNT_WIN10

The 1st condition is fine but the 2nd is flat out wrong, that function was available way earlier than Windows 10 and I've not been able to compile as a result of it

I'm a dumbass, I remembered the function names wrong, was supposed to use SetFilePointer instead of FileSetPointer,