Access to the windows api gives vb6 allot of power.

Normally we just pull these api declarations from the api viewer,
but understanding exactly how it works and all of its nuances
becomes important when you start writing your own C dlls.

the first thing you will want to learn is how to pass simple types
such as numbers and strings between C and VB.

In the case of strings this actually isnt that simple!

Rather than include pages and pages of information here, I am going
to link you to various articles that will be helpful.



while this topic is allot to digest, it is the gateway for
taking your VB6 coding to the next level.

Through the windows Api you have the same low level access
to the system that MFC and C apps use. You can even write your
own x86 debuggerin VB6.

In this section its also important to know the byte size and representation of internal data types:

The following table shows the supported VB6 data types, including storage sizes and ranges.

Data type Storage size Range
Byte 1 byte 0 to 255
Boolean 2 bytes True or False
Integer 2 bytes -32,768 to 32,767
Long
(long integer)		 4 bytes -2,147,483,648 to 2,147,483,647
Single
(single-precision floating-point)		 4 bytes -3.402823E38 to -1.401298E-45 for negative values; 1.401298E-45 to 3.402823E38 for positive values
Double
(double-precision floating-point)		 8 bytes -1.79769313486231E308 to
-4.94065645841247E-324 for negative values; 4.94065645841247E-324 to 1.79769313486232E308 for positive values
Currency
(scaled integer)		 8 bytes -922,337,203,685,477.5808 to 922,337,203,685,477.5807
Decimal 14 bytes +/-79,228,162,514,264,337,593,543,950,335 with no decimal point;
+/-7.9228162514264337593543950335 with 28 places to the right of the decimal; smallest non-zero number is
+/-0.0000000000000000000000000001
Date 8 bytes January 1, 100 to December 31, 9999
Object 4 bytes Any Object reference
String
(variable-length)		 10 bytes + string length 0 to approximately 2 billion
String
(fixed-length)		 Length of string 1 to approximately 65,400
Variant
(with numbers)		 16 bytes Any numeric value up to the range of a Double
Variant
(with characters)		 22 bytes + string length Same range as for variable-length String
User-defined
(using Type)		 Number required by elements The range of each element is the same as the range of its data type.

*Note* Arrays of any data type require 20 bytes of memory plus 4 bytes for each array dimension plus the number of bytes occupied by the data itself. The memory occupied by the data can be calculated by multiplying the number of data elements by the size of each element. For example, the data in a single-dimension array consisting of 4 Integer data elements of 2 bytes each occupies 8 bytes. The 8 bytes required for the data plus the 24 bytes of overhead brings the total memory requirement for the array to 32 bytes.

A Variant containing an array requires 12 bytes more than the array alone.

*Note* Use the StrConv function to convert one type of string data to another.



You can compare these to the C data types and sizes to determine which to use (strings require special consideration as detailed in articles above)

Type Storage size Value range
char 1 byte -128 to 127 or 0 to 255
unsigned char 1 byte 0 to 255
signed char 1 byte -128 to 127
int 2 bytes -32,768 to 32,767
unsigned int 2 bytes 0 to 65,535
short 2 bytes -32,768 to 32,767
long 4 bytes -2,147,483,648 to 2,147,483,647
unsigned long 4 bytes 0 to 4,294,967,295
float 4 bytes 1.2E-38 to 3.4E+38 (6 decimal places)
double 8 bytes 2.3E-308 to 1.7E+308 (15 decimal places)
Long double 10 bytes 3.4E-4932 to 1.1E+4932 (19 decimal places) 

Microsoft compilers also have some extra definitions built in such as:

__int8 nSmall;      // Declares 8-bit integer
__int16 nMedium;    // Declares 16-bit integer
__int32 nLarge;     // Declares 32-bit integer
__int64 nHuge;      // Declares 64-bit integer

stdint.h also has its own similar naming conventions.