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What is a subroutine in C language?

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subdural

subdural

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Hi

I'm newbie. I need to know what is the subroutine in C language?
For example I have C source code name main1.c as below;

main()

{

int a = 0x12345678, b;
b = endswap(a);
printf("Existing value in hex: %X\n", a);
printf("Endian-swapped value in hex: %X\n", b);

}

How to link the subroutine with other C source code named endswap.c

Thanks in addvanced
 

Re: subroutine in C

subroutines are called functions in C language.

in your code "endswap" & "printf" are functions (same as subroutines)

"printf" is a standard library function.

if you are using a any GUI including TC, you have to create a project and add all C files to the project. for example main.c and endswap.c

If you are using a commnadline compiler like gcc, you have to supply all files as arguments like

gcc main.c endswap.c "or"

gcc -test main.c endswap.c (here output exe/out file name will be test)


Cheers
idlebrain
 

    subdural

    subdural

    Points: 2
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Re: subroutine in C

You can also put multiple C functions into one source file, like this:

Code: 
#include <stdio.h>

int endswap(int x)
{
  return x*x;     /* or whatever you want to do */
}

int main(void)
{
  int a = 0x12345678, b;

  b = endswap(a);
  printf("Existing value in hex: %X\n", a);
  printf("Endian-swapped value in hex: %X\n", b);
  return 0;
}
 

    subdural

    subdural

    Points: 2
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Re: subroutine in C

Hi Idlebrain,

Excellent information.
Since the subroutine is function in C, in main.c source code assume that a=0x12345678 is little-endian (memory) and how to program the subroutine in endswap.c source code so that the output b = 78563412 in big-endian.
Should I write as below; (I'm using arm-elf-gcc compiler)

endswap()

{

int a = 0x78563412, b;
b = endswap(a);
printf("Existing value in hex: %X\n", a);
printf("Endian-swapped value in hex: %X\n", b);

}
 

subroutine in C

I think you can put it by using include.

#include <endswap.c>
main()

{

int a = 0x12345678, b;
b = endswap(a);
printf("Existing value in hex: %X\n", a);
printf("Endian-swapped value in hex: %X\n", b);

}
 

    subdural

    subdural

    Points: 2
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Re: subroutine in C

here is the code. the endswap function converts little endian to big endian and vice versa.
It is very simple for understanding.

in endswap.c
Code: 
int endswap(int number)
{
	unsigned char *bp1, *bp2;
	int rev;

	bp1 = (char *)&number;
	bp2 = (char *)&rev;

	bp2[0] = bp1[3];
	bp2[1] = bp1[2];
	bp2[2] = bp1[1];
	bp2[3] = bp1[0];

	return rev;

}

in main.c
Code: 
#include <stdio.h>

int endswap(int); /* function prototype */

void main(void)
{
	int no = 0x12345678;
	printf("before = %X\n",no);
	no = endswap(no);
	printf("After  = %X\n",no);
}

pico said:
I think you can put it by using include.

#include <endswap.c>
Click to expand...
Offcourse Yes, But it is not recomonded to include c files.

Cheers
idlebrain
 

    subdural

    subdural

    Points: 2
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Re: subroutine in C

Hi Idle,

Appriecite you endswap function. However when I compile and link between main.c and endswap.c I got these errors. Could help me to solved it.

$ arm-elf-gcc main.c endswap.c -o assn3a.exe

endswap.c: In function `main':
endswap.c:20: warning: return type of 'main' is not `int'
/cygdrive/c/DOCUME~1/NAZRI~1.WAR/LOCALS~1/Temp/ccSgudxp.o(.text+0xa4): In functi
on `main':
: multiple definition of `main'
/cygdrive/c/DOCUME~1/NAZRI~1.WAR/LOCALS~1/Temp/ccswBXjg.o(.text+0x0): first defi
ned here
/cygdrive/c/program files/gnuarm/bin/../lib/gcc/arm-elf/3.4.3/../../../../arm-el
f/bin/ld: Warning: size of symbol `main' changed from 88 in /cygdrive/c/DOCUME~1
/NAZRI~1.WAR/LOCALS~1/Temp/ccswBXjg.o to 80 in /cygdrive/c/DOCUME~1/NAZRI~1.WAR/
LOCALS~1/Temp/ccSgudxp.o
collect2: ld returned 1 exit status
 

Re: subroutine in C

May be the return type of main startup code is defined as int.
Try this in main.c file.
Code: 
#include <stdio.h> 

int endswap(int); /* function prototype */ 

int main(void) 
{ 
   int no = 0x12345678; 
   printf("before = %X\n",no); 
   no = endswap(no); 
   printf("After  = %X\n",no); 

   return 0;
}
If it is not working first try a simple hello world
program.
 

    subdural

    subdural

    Points: 2
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Re: subroutine in C

Hi Idlebrain,

Good job! I managed to compile and get the expected output. However, could you explain how this function operate/work? Thanks
 

Re: subroutine in C

The Basics

"Little Endian" means that the low-order byte of the number is stored in memory at the lowest address, and the high-order byte at the highest address. (The little end comes first.) For example, a 4 byte LongInt

Byte3 Byte2 Byte1 Byte0

will be arranged in memory as follows:
Base Address+0 Byte0
Base Address+1 Byte1
Base Address+2 Byte2
Base Address+3 Byte3

Intel processors (those used in PC's) use "Little Endian" byte order.

"Big Endian" means that the high-order byte of the number is stored in memory at the lowest address, and the low-order byte at the highest address. (The big end comes first.) Our LongInt, would then be stored as:

Base Address+0 Byte3
Base Address+1 Byte2
Base Address+2 Byte1
Base Address+3 Byte0

Motorola processors (those used in Mac's) use "Big Endian" byte order.

The Algorithm
as per the above expl, we need to swap the bytes as follows
BA+0 to BA+3,BA+1 to BA+2,BA+2 to BA+1,BA+3 to BA+0.
This is done by taking two char(byte) pointers bp1 = BA of actual endian, bp2 = BA of other endian. Now byte locations are swaped as required.

I hope it is clear.
idlebrain
 

    subdural

    subdural

    Points: 2
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Re: subroutine in C

Hi

Good information and self explanation. However the endswap function seem like array.
 

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