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@ -4,22 +4,26 @@ Timers and time management in the Linux kernel. Part 7.
Time related system calls in the Linux kernel
--------------------------------------------------------------------------------
This is the seventh and last part [chapter](https://0xax.gitbooks.io/linux-insides/content/Timers/index.html) which describes timers and time management related stuff in the Linux kernel. In the previous [part](https://0xax.gitbooks.io/linux-insides/content/Timers/timers-6.html) we saw some [x86_64](https://en.wikipedia.org/wiki/X86-64) like [High Precision Event Timer](https://en.wikipedia.org/wiki/High_Precision_Event_Timer) and [Time Stamp Counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter). Internal time management is interesting part of the Linux kernel, but of course not only the kernel needs in the `time` concept. Our programs need to know time too. In this part, we will consider implementation of some time management related [system calls](https://en.wikipedia.org/wiki/System_call). These system calls are:
This is the seventh and last part [chapter](https://0xax.gitbooks.io/linux-insides/content/Timers/index.html), which describes timers and time management related stuff in the Linux kernel. In the previous [part](https://0xax.gitbooks.io/linux-insides/content/Timers/timers-6.html), we discussed timers in the context of [x86_64](https://en.wikipedia.org/wiki/X86-64): [High Precision Event Timer](https://en.wikipedia.org/wiki/High_Precision_Event_Timer) and [Time Stamp Counter](https://en.wikipedia.org/wiki/Time_Stamp_Counter). Internal time management is an interesting part of the Linux kernel, but of course not only the kernel needs the `time` concept. Our programs also need to know time. In this part, we will consider implementation of some time management related [system calls](https://en.wikipedia.org/wiki/System_call). These system calls are:
* `clock_gettime`;
* `gettimeofday`;
* `nanosleep`.
<<<<<<< HEAD
We will start from simple userspace [C](https://en.wikipedia.org/wiki/C_%28programming_language%29) program and see all way from the call of the [standard library](https://en.wikipedia.org/wiki/Standard_library) function to the implementation of certain system call. As each [architecture](https://github.com/torvalds/linux/tree/master/arch) provides its own implementation of certain system call, we will consider only [x86_64](https://en.wikipedia.org/wiki/X86-64) specific implementations of system calls, as this book is related to this architecture.
=======
We will start from a simple userspace [C](https://en.wikipedia.org/wiki/C_%28programming_language%29) program and see all way from the call of the [standard library](https://en.wikipedia.org/wiki/Standard_library) function to the implementation of certain system calls. As each [architecture](https://github.com/torvalds/linux/tree/master/arch) provides its own implementation of certain system calls, we will consider only [x86_64](https://en.wikipedia.org/wiki/X86-64) specific implementations of system calls, as this book is related to this architecture.
>>>>>>> b9d8ea78bb41df5ffd2ea94b745298c91c8d171c
Additionally we will not consider concept of system calls in this part, but only implementations of these three system calls in the Linux kernel. If you are interested in what is it a `system call`, there is special [chapter](https://0xax.gitbooks.io/linux-insides/content/SysCall/index.html) about this.
Additionally, we will not consider the concept of system calls in this part, but only implementations of these three system calls in the Linux kernel. If you are interested in what is a `system call`, there is a special [chapter](https://0xax.gitbooks.io/linux-insides/content/SysCall/index.html) about this.
So, let's from the `gettimeofday` system call.
So, let's start from the `gettimeofday` system call.
Implementation of the `gettimeofday` system call
--------------------------------------------------------------------------------
As we can understand from the name of the `gettimeofday`, this function returns current time. First of all, let's look on the following simple example:
As we can understand from the name `gettimeofday`, this function returns the current time. First of all, let's look at the following simple example:
```C
#include <time.h>
@ -40,7 +44,7 @@ int main(int argc, char **argv)
}
```
As you can see, here we call the `gettimeofday` function which takes two parameters: pointer to the `timeval` structure which represents an elapsed tim:
As you can see, here we call the `gettimeofday` function, which takes two parameters. The first parameter is a pointer to the `timeval` structure, which represents an elapsed time:
```C
struct timeval {
@ -49,7 +53,7 @@ struct timeval {
};
```
The second parameter of the `gettimeofday` function is pointer to the `timezone` structure which represents a timezone. In our example, we pass address of the `timeval time` to the `gettimeofday` function, the Linux kernel fills the given `timeval` structure and returns it back to us. Additionally, we format the time with the `strftime` function to get something more human readable than elapsed microseconds. Let's see on result:
The second parameter of the `gettimeofday` function is a pointer to the `timezone` structure which represents a timezone. In our example, we pass address of the `timeval time` to the `gettimeofday` function, the Linux kernel fills the given `timeval` structure and returns it back to us. Additionally, we format the time with the `strftime` function to get something more human readable than elapsed microseconds. Let's see the result:
```C
~$ gcc date.c -o date
@ -57,16 +61,16 @@ The second parameter of the `gettimeofday` function is pointer to the `timezone`
Current date/time: 03-26-2016/16:42:02
```
As you already may know, an userspace application does not call a system call directly from the kernel space. Before the actual system call entry will be called, we call a function from the standard library. In my case it is [glibc](https://en.wikipedia.org/wiki/GNU_C_Library), so I will consider this case. The implementation of the `gettimeofday` function is located in the [sysdeps/unix/sysv/linux/x86/gettimeofday.c](https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86/gettimeofday.c;h=36f7c26ffb0e818709d032c605fec8c4bd22a14e;hb=HEAD) source code file. As you already may know, the `gettimeofday` is not usual system call. It is located in the special area which is called `vDSO` (you can read more about it in the [part](https://0xax.gitbooks.io/linux-insides/content/SysCall/syscall-3.html) which describes this concept).
As you may already know, a userspace application does not call a system call directly from the kernel space. Before the actual system call entry will be called, we call a function from the standard library. In my case it is [glibc](https://en.wikipedia.org/wiki/GNU_C_Library), so I will consider this case. The implementation of the `gettimeofday` function is located in the [sysdeps/unix/sysv/linux/x86/gettimeofday.c](https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/x86/gettimeofday.c;h=36f7c26ffb0e818709d032c605fec8c4bd22a14e;hb=HEAD) source code file. As you already may know, the `gettimeofday` is not a usual system call. It is located in the special area which is called `vDSO` (you can read more about it in the [part](https://0xax.gitbooks.io/linux-insides/content/SysCall/syscall-3.html), which describes this concept).
The `glibc` implementation of the `gettimeofday` tries to resolve the given symbol, in our case this symbol is `__vdso_gettimeofday` by the call of the `_dl_vdso_vsym` internal function. If the symbol will not be resolved, it returns `NULL` and we fallback to the call of the usual system call:
The `glibc` implementation of `gettimeofday` tries to resolve the given symbol; in our case this symbol is `__vdso_gettimeofday` by the call of the `_dl_vdso_vsym` internal function. If the symbol cannot be resolved, it returns `NULL` and we fallback to the call of the usual system call:
```C
return (_dl_vdso_vsym ("__vdso_gettimeofday", &linux26)
?: (void*) (&__gettimeofday_syscall));
```
The `gettimeofday` entry is located in the [arch/x86/entry/vdso/vclock_gettime.c](https://github.com/torvalds/linux/blob/master/arch/x86/entry/vdso/vclock_gettime.c) source code file. As we can see the `gettimeofday` is weak alias of the `__vdso_gettimeofday`:
The `gettimeofday` entry is located in the [arch/x86/entry/vdso/vclock_gettime.c](https://github.com/torvalds/linux/blob/master/arch/x86/entry/vdso/vclock_gettime.c) source code file. As we can see the `gettimeofday` is a weak alias of the `__vdso_gettimeofday`:
```C
int gettimeofday(struct timeval *, struct timezone *)