There are many commonly asked questions regarding C programming language. Below are some collected such question-answer examples. The questions are usually related with Turbo C IDE in windows or GCC under Linux environment [not always].
For more such examples, click C_Q&A label.
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Graphics
Building Mouse Cursors...
In text mode the mouse cursor appears as a block, whereas in graphics mode it appears as an arrow. If we wish we can change the graphics cursor to any other shape the way Windows does. The mouse cursor in graphics mode occupies a 16 by 16 pixel box. By highlighting or dehighlighting some of the pixels in this box we can get the desired shape. For example, the following bit-pattern can be used to generate the cursor which looks like an hour-glass.
1111111111111111 0000000000000000
1000000000000001 0000000000000000
1111111111111111 0000000000000000
1000000000000001 0000000000000000
0100000000000010 1000000000000001
0010000000000100 1100000000000011
0000100000010000 1111000000001111
0000001001000000 1111110000111111
0000001001000000 1111110000111111
0000100000010000 1111000000001111
0010000000000100 1100000000000011
0100000000000010 1000000000000001
1000000000000001 0000000000000000
1111111111111111 0000000000000000
1000000000000001 0000000000000000
1111111111111111 0000000000000000
Mouse pointer bitmap Screen Mask the one's in the mouse pointer bitmap indicate that the pixel would be drawn whereas the zeros indicate that the pixel would stand erased. It is important to note that the mouse pointer bit pattern is 32 bytes long. However, while actually writing a program to change the pointer shape we need a 64 byte bit-map. This provision is made to ensure that when the cursor reaches a position on the screen where something is already written or drawn only that portion should get overwritten which is to be occupied by the mouse cursor. Of the 64 bytes the first 32 bytes contain a bit mask which is first ANDed with the screen image, and then the second 32 bytes bit mask is XORed with the screen image.
The following program changes the mouse cursor in graphics mode to resemble an hour
glass.
# include "graphics.h"
# include "dos.h"
union REGS i, o ;
struct SREGS s ;
int cursor[32] =
{ /* Hour-glass screen mask */
0x0000, 0x0000, 0x0000, 0x0000,
0x8001, 0xc003, 0xf00f, 0xfc3f,
0xfc3f, 0xf00f, 0xc003, 0x8001,
0x0000, 0x0000, 0x0000, 0x0000,
/* The mouse pointer bitmap */
0xffff, 0x8001, 0xffff, 0x8001,
0x4002, 0x2004, 0x1008, 0x0240,
0x0240, 0x0810, 0x2004, 0x4002,
0x8001, 0xffff, 0x8001, 0xffff,
} ;
main( )
{
int gd = DETECT, gm ;
initgraph ( &gd, &gm, "c:\\tc\\bgi" ) ;
if ( initmouse( ) == -1 )
{
closegraph( ) ;
printf ( "\n Mouse not installed!" ) ;
exit( ) ;
}
gotoxy ( 10, 1 ) ; printf ( "Press any key to exit..." ) ;
changecursor ( cursor ) ; showmouseptr( ) ;
getch( ) ;
}
initmouse( )
{
i.x.ax = 0 ; int86 ( 0x33, &i, &o ) ;
return ( o.x.ax == 0 ? -1 : 0 ) ;
}
showmouseptr( )
{
i.x.ax = 1 ; int86 ( 0x33, &i, &o ) ;
}
changecursor ( int *shape )
{
i.x.ax = 9 ; /* service number */
i.x.bx = 0 ; /* actual cursor position from left */
i.x.cx = 0 ; /* actual cursor position from top */
i.x.dx = ( unsigned ) shape ; /* offset address of pointer image*/
segread ( &s ) ;
s.es = s.ds ; /* segment address of pointer */
int86x ( 0x33, &i, &i, &s ) ;
}
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The access( ) function...
The access( ) function checks for the existence of a file and also determines whether it can be read, written to or executed. This function takes two arguments the filename and an integer indicating the access mode. The values 6, 4, 2, and 1 checks for read/write, read, write and execute permission of a given file, whereas value 0 checks whether the file exists or not.
Following program demonstrates how we can use access( ) function to check if a given file exists.
#include <io.h>
main( )
{
char fname[67] ;
printf ( "\nEnter name of file to open" ) ;
gets ( fname ) ;
if ( access ( fname, 0 ) != 0 )
{
printf ( "\nFile does not exist." ) ;
return ;
}
}
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How do I convert a floating-point number to a string?
Ans:
Use function gcvt( ) to convert a floating-point number to a string. Following program
demonstrates the use of this function.
#include <stdlib.h>
main( )
{
char str[25] ;
float no ;
int dg = 5 ; /* significant digits */
no = 14.3216 ;
gcvt ( no, dg, str ) ;
printf ( "String: %s\n", str ) ;
}
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What is a stack ?
Ans:
The stack is a region of memory within which our programs temporarily store data as they execute. For example, when a program passes parameters to functions, C places the parameters on the stack. When the function completes, C removes the items from the stack.
Similarly, when a function declares local variables, C stores the variable's values on the stack during the function's execution. Depending on the program's use of functions and parameters, the amount of stack space that a program requires will differ.
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…till next post, bye-bye & take care.
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