On ESXi hosts, virtual machine disk (VMDK) files are usually located under one of the /vmfs/volumes, perhaps on shared storage. Storage volumes are visible from the vSphere Client, in the inventory of hosts and clusters. Typical names are datastore1 and datastore2. To see a VMDK file, click Summary > Resources > Datastore, right-click Browse Datastore, and select a virtual machine.
On Workstation, VMDK files are stored in the same directory with virtual machine configuration (VMX) files, for example, /path/to/disk on Linux or C:\My Documents\My Virtual Machines on Windows.
VMDK files store data representing a virtual machine’s hard disk drive. Almost the entire portion of a VMDK file is the virtual machine’s data, with a small portion allotted to overhead.

Initialize the Library:
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VixDiskLib_Init() initializes the old virtual disk library. The arguments majorVersion and minorVersion represent the VDDK library’s release number and dot-release number. The optional third, fourth, and fifth arguments specify log, warning, and panic handlers. DLLs and shared objects may be located in libDir.
VixError vixError = VixDiskLib_Init(majorVer, minorVer, &logFunc, &warnFunc, &panicFunc, libDir);
You should call VixDiskLib_Init() only once per process because of internationalization restrictions, at the beginning of your program. You should call VixDiskLib_Exit() at the end of your program for cleanup. For multithreaded programs you should write your own logFunc because the default function is not thread safe.
In most cases you should replace VixDiskLib_Init() with VixDiskLib_InitEx(), which allows you to specify a configuration file.

Virtual Disk Types:
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The following disk types are defined in the virtual disk library:
>>>
VIXDISKLIB_DISK_MONOLITHIC_SPARSE – Growable virtual disk contained in a single virtual disk file. This is the default type for hosted disk, and the only setting in the Virtual Disk API .
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VIXDISKLIB_DISK_MONOLITHIC_FLAT – Preallocated virtual disk contained in a single virtual disk file. This takes time to create and occupies a lot of space, but might perform better than sparse.
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VIXDISKLIB_DISK_SPLIT_SPARSE – Growable virtual disk split into 2GB extents (s sequence). These files can to 2GB, then continue growing in a new extent. This type works on older file systems.
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VIXDISKLIB_DISK_SPLIT_FLAT – Preallocated virtual disk split into 2GB extents (f sequence). These files start at 2GB, so they take a while to create, but available space can grow in 2GB increments.
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VIXDISKLIB_DISK_VMFS_FLAT – Preallocated virtual disk compatible with ESX 3 and later. Also known as thick disk. This managed disk type is discussed in Managed Disk and Hosted Disk.
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VIXDISKLIB_DISK_VMFS_SPARSE – Employs a copy-on-write (COW) mechanism to save storage space.
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VIXDISKLIB_DISK_VMFS_THIN – A Growable virtual disk that consumes only as much space as needed, compatible with ESX 3 or later, supported by VDDK 1.1 or later, and highly recommended.
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VIXDISKLIB_DISK_STREAM_OPTIMIZED – Monolithic sparse format compressed for streaming. Stream optimized format does not support random reads or writes.

Check the sample programs on :

http://pubs.vmware.com/vsphere-65/index.jsp#com.vmware.vddk.pg.doc/vddkSample.7.2.html#995259