cpuid | sysfs example

/* This file is to be compiled only on X86 systems.
	The build system should figure this out for us. 
	reg[4] represent the registers eax, ebx, ecx, and edx respectively. */
#include <stdio.h>
#include <string.h>
#include <errno.h>

#if defined (__clang__) || (__GNUC__)
	
#include <cpuid.h>

static void obs_cpuid(unsigned reg[4])
{
	__get_cpuid(reg[0], &reg[0], &reg[1], &reg[2], &reg[3]);
}

#elif defined (_MSC_VER)
	
#include <intrin.h>

static void obs_cpuid(unsigned reg[4])
{
	__cpuidex(reg, reg[0], reg[3]); /* Output to reg, send eax and ecx */
}

#endif

static void parse_cpufrequency_sysfs(unsigned core)
{
	unsigned frequency = 0;

	/* 10 is maximum width of an unsigned integer.  */
	char buffer[sizeof("/sys/devices/system/cpu/cpu/cpufreq/scaling_cur_freq\0") + 10];
	sprintf(buffer, "/sys/devices/system/cpu/cpu%u/cpufreq/scaling_cur_freq", core);

	{
		FILE* file = fopen(buffer, "r");
		int result;

		if (!file) {
			printf("Unable to open %s for parsing: %s\n", buffer, strerror(errno));
			return;
		}

		result = fscanf(file, "%u", &frequency);

		if (result <= 0) {
			printf("File %s was either empty or an error occured while reading.\n", buffer);
			buffer;
		}

		printf("(CPU #%u) Current Frequency: %fMhz\n", core, frequency * .001);
	} 
}

static void parse_cpucount_sysfs()
{
	const char online_filepath[] = "/sys/devices/system/cpu/online";
	int prev = -1; /* Not 0 as 0 is a valid value. */
	int num_cores = 0;

	/* file_num_online contains the number of cores online (not all possible) across various CPUs. */
	FILE* file = fopen(online_filepath, "r");

	if (!file) {
		printf("You got problems son. Most likely, you're missing an important scheduling module.\n");
		return;
	}

	/* The following is similar to what's found in glibc for fetching number of online processors.  */

	for (;;) {
		char sep;
		int cpu;
		int n = fscanf(file, "%u%c", &cpu, &sep);

		if (n <= 0)
			break;
		
		if (n == 1) /* There was no seperator... meaning eol.*/
			sep = '\n';

		if (prev >= 0) { /* This is the second number in a range, for sure higher than the last. */
			for (int i = prev; i <= cpu; ++i) {
				parse_cpufrequency_sysfs(i);
				++num_cores;
			}

			prev = -1;
		} else if (sep == '-') {
			prev = cpu;
		} else {
			parse_cpufrequency_sysfs(cpu);
			++num_cores;
		}

		if (sep == '\n') break;
	}
}

void print_cpu_vendor()
{
	unsigned reg[4] = { 0, 0, 0, 0 };

	obs_cpuid(reg);

	printf("Vendor: %.4s%.4s%.4s\n", &reg[1], &reg[3], &reg[2]);
}

int main() 
{
	print_cpu_vendor();
	parse_cpucount_sysfs();
}