convert a 4 bytes to four 1 byte variables in c

[Xiaoming Zhang]

I ran following c program:

#include
main()
{
FILE *ftmp;
int index;
unsigned char aryIp[4];
unsigned long int lgIp;

lgIp =2559841793;

ftmp=fopen("ipaddr","w+b");
fwrite(&lgIp,4,1,ftmp);
fclose(ftmp);

ftmp=fopen("ipaddr","r+b");
for(index=0; index < 4; index++)
printf("%d\n",fgetc(ftmp));
}

and got the output as I expected:
152
148
22
1

Is there any simple way to split a 4 bytes long int variable to four one byte char?

Thanks

Xiaoming

[Francisco Gazapo]

Hi,

look at this...


#include
main()
{
FILE *ftmp;
int index;
unsigned char *b1;
unsigned long int lgIp;

lgIp=2559841793;
b1=(char *)&lgIp;
printf ("%d,%d,%d,%d",*(b1+3),*(b1+2),*(b1+1),*b1);
}

[Felix J. Liu]

How about this:

#include

main() {
unsigned long lgIp;
unsigned long byte=255;

lgIp = 2559841793;

printf("%d\n", (lgIp>>24)&byte);
printf("%d\n", (lgIp>>16)&byte);
printf("%d\n", (lgIp>>8)&byte);
printf("%d\n", lgIp&byte);
}

[Xiaoming Zhang]

Your codes worked and my code became#include
main()
{
unsigned char *b1, aryChar[4], index;
unsigned long int lgIp;

lgIp=2559841793;
b1=(short int *)&lgIp;
for (index =0; index < 4; index++)
{
aryChar[index]=*(b1+index);
printf("%d.", aryChar[index]);
}
}

which actually produces a IP address from the unsigned long int.

Thanks.

Xiaoming

[Gregory Fruth · Accepted solution]

You should be using inet_ntoa() and friends to work with
IP addresses. Any other method may not be portable (due to
byte ordering) and will break totally when the format of IP
addresses changes (IP4 -> IP6).

Even code using inet_ntoa() may have byte ordering problems
(depending on where the IP address came from) and problems
converting to IP6, but the problems will be smaller than those you'll
have if you do the conversion manually.

If you *must* do the conversion manually, a union is a better way
do it. The following code does the conversion using both
inet_ntoa() and a union.

(For a good discussion of how to work with IP addresses, refer
to the book "Unix Network Programming" by W. Richard Stevens.)

Good luck!

#include
#include
#include
#include

int main() {
unsigned long int lgIp;
struct in_addr addr;
char *aryIp;

union {
unsigned long int ulval;
unsigned char cval[4];
} uIp;

lgIp = 2559841793L;

/* convert using inet_ntoa() */
addr.s_addr = lgIp;
aryIp = inet_ntoa(addr);
printf("by inet_ntoa(): %s\n", aryIp);

/* note that the string returned by inet_ntoa() is in a static buffer,
so that the next call to inet_ntoa() will overwrite its contents.
therefore, you must copy it somewhere (via strdup() or
somesuch) if you want to use it later */

/* convert using union */
uIp.ulval = lgIp;
printf(" by union: %d.%d.%d.%d\n",
uIp.cval[0], uIp.cval[1], uIp.cval[2], uIp.cval[3]);

return(0);
}

[Xiaoming Zhang]

The conversion by union is great!.

Thanks.