How can I know the bit count of a cpu/os in C

Question

I know how to get the bit count of a cpu or an operation system with shell.

cat /proc/cpuinfo | grep lm #-> get bit count of a cpu
uname -a                    #-> get bit count of an operation system

However, how can we get the bit count of those in a C program. This is an interview question and my solution is as follow:

int *ptr;
printf("%d\n", sizeof(ptr)*8);

But the interviewer said that was wrong. So, what is the correct answer?

While the size of a pointer *often* follow the "size" of the platform, this is not always true. The only way is to query the operating system using OS native functions. — Some programmer dude, Oct 17 '13 at 05:07
@JoachimPileborg er.. I'm sorry. Let's assumed the operation system is Linux. — xianyu1337, Oct 17 '13 at 05:17
possible duplicate of [Detecting 64bit compile in C](http://stackoverflow.com/questions/5272825/detecting-64bit-compile-in-c) — Some programmer dude, Oct 17 '13 at 05:23

score 1 · Answer 1 · answered Oct 17 '13 at 05:21

1

On Linux, a simple way is to do e.g. popen with the uname -m command and parse the output.

Another way is to look at the source for the uname command (as it's readily available) and implement something based on that directly.

answered Oct 17 '13 at 05:21

Some programmer dude

400,186
35
402
621

score 1 · Accepted Answer · answered Oct 17 '13 at 05:32

POSIX provides a C function uname as well. You can get similar result like the shell command uname:

#include <stdio.h>
#include <sys/utsname.h>

int main(){
    struct utsname buf;
    uname(&buf);
    printf("sysname: %s\nversion: %s\nmachine: %s\n ", buf.sysname, buf.version, buf.machine);
    return 0;
}

Output on my machine:

sysname: Linux
version: #1 SMP Tue Oct 2 22:01:37 EDT 2012
machine: i686

How can I know the bit count of a cpu/os in C

2 Answers2