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Jim's Depository

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Little-endian to host conversion

Sometimes you will find data structures to read that are encoded little-endian. You may find these functions useful. They aren't as nightmarish to read as macros and presumably your compiler will do right by them.
#include <stdio.h>
#include <arpa/inet.h>
/*
** Little endian to host, short
*/
static unsigned short ltohs( unsigned short v)
{
    if ( htons(1) == 1) {
	return ((v>>8)&0xff) | ((v<<8)&0xff00);
    } else return v;
}
/*
** Little endian to host, Long
*/
static unsigned long ltohl( unsigned long v)
{
    if ( htons(1) == 1) {
	return ((v>>24)&0xff) | ((v>>8)&0xff00) | 
               ((v<<8)&0xff0000) | ((v << 24)&0xff000000);
    } else return v;
}
/*
** Little endian to host, "double" i.e. long long
*/
static unsigned long long ltohd( unsigned long long v)
{
    if ( htons(1) == 1) {
	return (unsigned long long)ltohl( v&0x00000000ffffffff) << 32 | 
	    (unsigned long long)ltohl( (v>>32)&0x00000000ffffffff);
    } else return v;
}
int main( int argc, char **argv)
{
    unsigned char t1[] = { 0x01, 0x02, 0x03, 0x04, 
                           0x05, 0x06, 0x07, 0x08 };
    unsigned char t2[] = { 0xff, 0xfe, 0xfd, 0xfc, 
                           0xfb, 0xfa, 0xf9, 0xf8 };
    printf("ltohs(01,02) = %04x\n", ltohs(*(unsigned short *)t1));
    printf("ltohs(ff,fe) = %04x\n", ltohs(*(unsigned short *)t2));
    printf("ltohl(01,02,03,04) = %08x\n", ltohl(*(unsigned long *)t1));
    printf("ltohl(ff,fe,fd,fc) = %08x\n", ltohl(*(unsigned long *)t2));
    printf("ltohd(01,02,03,04,05,06,07,08) = %016llx\n", 
           ltohd(*(unsigned long long *)t1));
    printf("ltohd(ff,fe,fd,fc,fb,fa,f9,f8) = %016llx\n", 
           ltohd(*(unsigned long long *)t2));
}
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Avoiding Hard Disk Failures

Temperature is the enemy of hard disks. Hot disks fail sooner.

If you have a well designed server with lightly loaded disks in a cool room your drives are probably running around 25°C(77°F). If you have a 4U box that you chucked full of drives you may find some of your drives running much hotter. For instance, I just found two running at 41°C(105°F) in a box with otherwise cool drives.

Drives are typically specified to operate up to 55°C(131°F), but their lives are shortened. Estimating from Google data it looks like your failure rate within three years is about triple for 45°C versus 25°C. In the third year, about 1 in 6 of the hot drives will fail.

So protect your servers:
  1. Know the problem: In debian land, install hddtemp and run it to see which drives are hot. Windows users might use DTemp.
    vev# for v in a b c d ;do hddtemp /dev/sd$v ;done
    /dev/sda: ST3750640AS: 34°C
    /dev/sdb: ST3750640AS: 27°C
    /dev/sdc: ST3750640AS: 41°C
    /dev/sdd: ST3750640AS: 40°C
    vev# # I have two hot drives.
  2. Mitigate: If you have hot drives, move them around or adjust airflow, perhaps by making little cardstock air dams inside the server to cool the drives. Add a fan blowing on them if you have a large case.
  3. Monitor: Record their temperatures and check in on them once in a while to make sure things aren't going badly.
Debian users might also want to install smartmontools which will track your S.M.A.R.T. data and notify you of problems.
Note: for SATA drives you will need a "-d ata" or it will misaddress them as SCSI drives and you need a "-m foo@example.com" if you want to be email notified.

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A new wrinkle for printf()

Somewhere along the line the GNU C library and the BSD folks added variants of sprintf() that malloc() the resulting string. Very handy for avoiding an overflow situation and also makes for cleaner reading code.

int asprintf( char **ret, const char *format, ...);
int vasprintf( char **ret, const char *format, va_list ap);

Before:
char buf[32];
sprintf(buf, "s:%d m:%s", code, msg);
// stack overflow when msg is too long
… or …
char buf[1024];
snprintf( buf, sizeof(buf)-1, "s:%d m:%s", code, msg);
// fail oddly if we guessed wrong on the maximum buf size
After:
char *buf;
asprintf( &buf, "s:%d m:%s", code, msg);
…
free(buf);
Danger danger DANGER: This will not work on the tiny libc in OpenWRT and similar embedded systems. But at least you will get a link error during the build.

Now if only we had a scanf() that would do a realloc() on its pointers.
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comment by jim, 2 years ago
Oh look, there is a similar function for scanf().  You can do something like...

char *adj = 0;
sscanf(somestuff,"Some %as stuff", &adj);

... but only if you are using GNU libc. I got burned when I used this in a daemon and then moved it to OpenWRT where they uses a different libc.

First 100 users

Femtoblogger has just been rolled out to a corporate community of 150 users. Let the bug reports and feature requests roll.
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