getnstimeofday is a front-end for __get_realtime_clock_ts but also works if no high-resolution clocks are available in the system. In this case, getnstimeofday as defined in kernel/time.c (instead of kernel/time/timekeeping.c) is used to provide a timespec that fulfills only low-resolution requirements.
The linux kernel provides a number of interfaces to manage time. getnstimeofday is one of them, which gives the time in seconds and nanoseconds. The function is implemented in “timekeeping32.h” and returns a structure of the type timespec which has two members.
struct timespec64 {
time64_t tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
To print the time, we only need to print the values of tv_sec and tv_nsec which gets filled by the call to the function getnstimeofday. In the following example code, we have created a proc entry called gettime, which prints out the values of seconds and nanoseconds when read.
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/slab.h>
int len;
char *msg;
ssize_t size;
struct timespec ts;
ssize_t read_proc(struct file *filp,char *buf,size_t count,loff_t *offp )
{
struct timespec ts;
char *temp;
temp=kmalloc(50*sizeof(char),GFP_KERNEL);
getnstimeofday(&ts);
sprintf(temp,"%ld seconds \n%ld nanoseconds\n",ts.tv_sec, ts.tv_nsec);
len=strlen(temp);
size=sizeof(char)*len;
return simple_read_from_buffer(buf,count,offp,temp,size);
}
struct file_operations proc_fops = {
read: read_proc
};
void create_new_proc_entry(void)
{
proc_create("gettime",0,NULL,&proc_fops);
}
int proc_init (void) {
create_new_proc_entry();
return 0;
}
void proc_cleanup(void) {
remove_proc_entry("gettime",NULL);
}
MODULE_LICENSE("GPL");
module_init(proc_init);
module_exit(proc_cleanup);
Save the above code as proc_read_gettimeofday.c and compile the code using the following makefile.
ifneq ($(KERNELRELEASE),) obj-m := proc_read_gettimeofday.o else KERNELDIR ?= /lib/modules/$(shell uname -r)/build PWD := $(shell pwd) default: $(MAKE) -C $(KERNELDIR) M=$(PWD) modules clean: $(MAKE) -C $(KERNELDIR) M=$(PWD) clean endif
Compile and insert the module using:
$ make $ sudo insmod proc_read_getnstimeofday.ko
To see the output, just read the proc entry gettime, using the cat command.
# cat /proc/gettime 1584690328 seconds 290430470 nanoseconds