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Advanced C 1992

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Part III • Working with Others

Table 11.3. MS-DOS Int 21 function codes.

Function	Function		DOS
(hex)	(decimal)	Description	version
00H	0	Terminate the current	1.0+
		program or process.
01H	1	Read a character from	1.0+
		the console (with echo
		to screen).
02H	2	Write a character to the	1.0+
		screen.

03H	3
04H	4
05H	5
06H	6

Read a character from AUX:.	1.0+
Write a character to AUX:.	1.0+
Send output to the printer.	1.0+
Perform I/O from the	1.0+
console; DOS does not
process the characters.

07H 7

08H 8

09H 9

0AH 10

Get a character from the	1.0+
keyboard, waiting for a
keypress if no character
is available; the character
is not processed by DOS.
Get a character from the	1.0+
keyboard, waiting for a
keypress if no character
is available.
Print a string to the	1.0+
console; the string is
terminated with a dollar
sign.
Read a character string	1.0+
(a line, typed by the
user, up to a carriage
return) from the keyboard.

456

		C and Other Languages		C C C
				11C
				C C C
				C C

Function	Function		DOS
(hex)	(decimal)	Description	version
0BH	11	Test to see whether a	1.0+
		character is available
		from the keyboard.
0CH	12	Discard the contents of	1.0+
		the input buffer, and get
		the input.
0DH	13	Reset the specified disk	1.0+
		drive.
0EH	14	Make the specified drive	1.0+
		the current drive.

0FH	15
10H	16
11H	17

Open a file.	1.0+
Close a file.	1.0+
Find the first file meeting	1.0+
the provided specification.

12H	18	Find the next file after	1.0+
		using Int 0x11.

13H	19
14H	20
15H	21
16H	22
17H	23
18H	24
19H	25
1AH	26
1BH	27

Delete the specified file.	1.0+
Perform a sequential read.	1.0+
Perform a sequential write.	1.0+
Create a new file.	1.0+
Rename a file.	1.0+
Reserved.
Get the current drive.	1.0+
Set the Disk Transfer	1.0+
Address (DTA).
Get the current default	1.0+
drive data.

continues

457

Part III • Working with Others

Table 11.3. continued

Function	Function		DOS
(hex)	(decimal)	Description	version
1CH	28	Get data for the specified	2.0+
		drive.

1DH	29
1EH	30
1FH	31
20H	32
21H	33
22H	34
23H	35

Reserved
Reserved
Reserved
Reserved
Random file read.	1.0+
Random file write.	1.0+
Get the size of the	1.0+
specified file.

24H	36	Set the file’s current	1.0+
		position.

25H	37
26H	38
27H	39
28H	40
29H	41
2AH	42
2BH	43
2CH	44
2DH	45
2EH	46
2FH	47

Set an interrupt vector.	1.0+
Create a new Program	1.0+
Segment Prefix (PSP).
Random block read.
Random block write.	1.0+
Parse a filename into a	1.0+
valid DOS filename
Get the date.	1.0+
Set the date.	1.0+
Get the time.	1.0+
Set the time.	1.0+
Set the write verify flag.	1.0+
Get the Disk Transfer	2.0+
Address (DTA).

458


	C and Other Languages		C C C
			11C
			C C C
			C C

Function	Function	DOS
(hex)	(decimal) Description	version

30H	48
31H	49

Get the DOS version	2.0+
Terminate-and-Stay-Resident	2.0+
(TSR).

32H	50
33H	51

Reserved
Get or set the break flag,	2.0+
and get the boot drive.

34H	52
35H	53
36H	54

Reserved
Get the interrupt vector.	2.0+
Get the drive allocation	2.0+
information.

39H	57
3AH	58
3BH	59
3CH	60
3DH	61
3EH	62
3FH	63
40H	64
41H	65
42H	66
43H	67
44H	68
45H	69
46H	70

Create a directory.	2.0+
Delete a directory.	2.0+
Set the current directory.	2.0+
Create a file.	2.0+
Open a file.	2.0+
Close a file.	2.0+
Read from a file (can also	2.0+
read from a device).
Write to a file (can also	2.0+
read from a device).
Delete a file.	2.0+
Set the file pointer.	2.0+
Get (or set) a file’s	2.0+
attributes.
IOCTL processing.	2.0+
Duplicate a file handle.	2.0+
Redirect a file handle.	2.0+

continues

459

Part III • Working with Others

Table 11.3. continued

Function	Function		DOS
(hex)	(decimal)	Description	version

47H	71
48H	72
49H	73
4AH	74
4BH	75

Get the current directory.	2.0+
Allocate a memory block.	2.0+
Release a memory block.	2.0+
Resize a memory block.	2.0+
Execute (run) a program	2.0+
(EXEC).

4CH	76	Terminate a process with	2.0+
		a return code (which can
		be tested in a batch file).
4DH	77	Get the return code from	2.0+
		a child process.

4EH	78
4FH	79

Find the first file.	2.0+
Find the next file, after	2.0+
finding the first file.

50H	80
51H	81
52H	82
53H	83
54H	84
55H	85
56H	86
57H	87
58H	88

Reserved
Reserved
Reserved
Reserved
Get the verify flag.	2.0+
Reserved
Rename a file.	2.0+
Get (or set) the file	2.0+
date and time.
Get (or set) the	3.0+
allocation strategy.

460

		C and Other Languages		C C C
				11C
				C C C
				C C

Function	Function		DOS
(hex)	(decimal)	Description	version
59H	89	Get extended error	3.0+
		information following a
		DOS error.

5AH	90
5BH	91
5CH	92

Create a temporary file.	3.0+
Create a new file.	3.0+
Lock (or unlock) a file	3.0+
region.

5DH	93
5EH	94

Reserved
Get the machine name, get	3.1+
(or set) the printer setup.

5FH	95
60H	96
61H	97
62H	98

Device redirection.	3.1+
Reserved
Reserved
Get the Program Segment	3.0+
Prefix (PSP) address.

63H	99
64H	100
65H	101
66H	102
67H	103
68H	104
69H	105
6AH	106
6BH	107
6CH	108

Get the DBCS lead byte	2.25
only table.
Reserved
Get the extended country	3.3+
information.
Get (or set) the code page.	3.3+
Set the file handle count.	3.3+
Commit a file.	3.3+
Reserved
Reserved
Reserved
Extended open file.	4.0+

461

Part III • Working with Others

Calling C from FORTRAN and Pascal

A C function is called from a FORTRAN or a Pascal program in a manner similar to the way any other type of function is called from FORTRAN or Pascal. The main program must be told that the function is written in C (so that the parameters are passed properly), and C library functions must not be called.

Because C is not the language of the main program, the C initialization code cannot be called, which means the C library functions cannot be called. This restriction can limit the usefulness of a C function; you may decide that it is easier to write the entire program in one language.

All the Things that Can Go Wrong

When you mix languages in your programming, you can easily get things mixed up. Much of the discussion in this section is specific to Microsoft’s C compilers. However, other compilers often behave in a similar manner. Borland’s C compilers, for example, offer similar methods for argument passing.

Following are some of the more common things that can ruin your day:

If you are calling a C function from another language, you must tell the compiler the calling convention. You can tell C that a function will be using FORTRAN calling conventions, then call the function from a FORTRAN program without telling the FORTRAN program that the function is written in C. If the function is written in C with C’s calling conventions, however, the calling program must know this.

When assembly is called from C, the parameters must be read from the stack in the correct order. A C function expects its arguments to be passed in right to left order. A FORTRAN or a Pascal function expects arguments in a left to right order.

A function using the C calling conventions expects its caller to clean the arguments from the stack. A FORTRAN or a Pascal function takes the arguments from the stack itself.

The value being returned must be in the correct place. Generally, AX is used for 2-byte return values, and AX and DX are used for 4-byte return values.

462

C and Other Languages	C C C
	11C
	C C C
	C C

The compiler modifies the function’s name. If it is a C function, an underscore is added (either before or after the name, depending on the compiler) and its case is not changed. If it is a FORTRAN or a Pascal function, an underscore is not added and the name is converted to uppercase. When calling C functions from assembly, the underscore is often forgotten, leading to unresolved references.

When accessing a multidimensional array, C and Pascal vary the subscripts in row-major order. FORTRAN and BASIC vary them in column-major order.

The C array nArray[2][10][20] is typically nArray(20,10,2) in

FORTRAN.

In C, arrays are indexed from zero. In FORTRAN, arrays typically are indexed from one. In Pascal, the lower bound is specified by the programmer. Exceeding the bounds of an array is common in mixed C/FORTRAN/ Pascal code because the initial starting points for the bounds differ.

Do not call C library code from a program that has a main procedure written in FORTRAN or Pascal.

Do not call library functions from a function written in a language different from the language of the main program. It may be possible to call the main program language’s library functions, but make sure that you use the correct calling conventions.

In all, mixed language programming is not used frequently. It is complex and prone to subtle failures that may be difficult to find and correct. Mixed language programming should be used as a last resort.

Looking at Data

Each programming language views data a little differently. Table 11.4 is a crossreference of simple data types used in C, BASIC, FORTRAN, and Pascal. The similarity in the data types of FORTRAN and Pascal shows the common roots between those two languages.

463

Part III • Working with Others

Table 11.4. Data types.

C/C++	BASIC	FORTRAN	Pascal
short	variable%	INTEGER*2	INTEGER2
int	INTEGER		INTEGER
unsigned short	(unsigned not	(unsigned not	WORD
	supported)	supported)
unsigned (see
unsigned long
and unsigned
short)
long	variable&	INTEGER*4	INTEGER4
	LONG	INTEGER (default)
unsigned long	(unsigned not	(unsigned not	(unsigned not
	supported)	supported)	supported)
float	variable!	REAL*4	REAL4
	variable	REAL	REAL
	SINGLE
double	variable#	REAL*8	REAL8
	DOUBLE	DOUBLE PRECISION
long double
unsigned char		CHARACTER*1	CHAR
		(not the same
		as LOGICAL)
		LOGICAL *2
		LOGICAL *4

Because BASIC does not predeclare scalar variables, a variable’s type is indicated by a suffix code (%, &, !, or #). If the suffix is missing, the variable is assumed to be a real (floating-point) variable.

464

C and Other Languages	C C C
	11C
	C C C
	C C

Variables are classified according to

•Type (what is stored in them), such as integer, character, or floating point

•Size (how large they are)

If you picture a variable as a certain number of bytes holding a certain type of data, you can easily convert from one language’s variable types to another.

Names and Limits

C allows a longer name length (the number of characters in a name that are significant) than FORTRAN. In C, the limit is 32 characters for internal names and 6 characters for external names. With some versions of FORTRAN, the limit is 6 characters for internal or external names.

When an external name is processed by C compilers, an underscore is usually added before or after the name. When programming in languages that do not use this convention, you must explicitly add the underscore. In Listing 11.9, for example, the printf() library function is called. The referenced name is _printf because the compiler prefixes the name with an underscore, and you must do the same. A FORTRAN or Pascal identifier does not have an added underscore.

When an external identifier is used in FORTRAN, the case of the name will have been changed to uppercase. This can create problems when linking the program, depending on the linker’s options. If you use the /NOIGNORECASE option (which tells the linker that the case of the identifiers must match) when you create a mixed language program, the linker may generate unresolved reference errors for names in the wrong case.

Summary

In this chapter, you learned about mixed language programming.

•Programs written in C can call functions written in assembly, FORTRAN, Pascal, and BASIC, provided the compiler supports mixed language calls to the language to be called.

465

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39H	57
3AH	58
3BH	59
3CH	60
3DH	61
3EH	62
3FH	63
40H	64
41H	65
42H	66
43H	67
44H	68
45H	69
46H	70

39H	57
3AH	58
3BH	59
3CH	60
3DH	61
3EH	62
3FH	63
40H	64
41H	65
42H	66
43H	67
44H	68
45H	69
46H	70

39H	57
3AH	58
3BH	59
3CH	60
3DH	61
3EH	62
3FH	63
40H	64
41H	65
42H	66
43H	67
44H	68
45H	69
46H	70