Bonus: `clock_gettime` and `time`¶

Tools: GCC, Make, strace

Goal¶

Implement clock_gettime(2) and build a libc-style time() on top of it, this time spread across a proper header and implementation file rather than a single main.c.

Background¶

Same three-layer pattern as the previous exercises:

  my_time()   -->   my_clock_gettime()   -->   my_syscall()   -->   kernel

clock_gettime reads a high-resolution clock into a timespec (see man 2 clock_gettime):

int clock_gettime(clockid_t clkid, struct timespec *tp);

clkid — which clock to read. CLOCK_REALTIME is wall-clock time since the Unix Epoch; CLOCK_MONOTONIC only ever increases and is the right choice for measuring durations.
tp — an output parameter: the kernel fills it in.

time(2) is the coarser call, returning only whole seconds of CLOCK_REALTIME:

time_t time(time_t *tloc);

It also has the unusual habit of returning its result and optionally storing it through a pointer.

Your Task¶

my_time.h — add declarations for my_clock_gettime() and my_time() where the TODO comments are. Note that this file is included by both main.c and my_time.c, so the declarations must match the definitions exactly.
my_time.c — complete the three TODOs:
1. SYS_clock_gettime — define the correct x86-64 syscall number.
2. my_clock_gettime() — the syscall wrapper: clkid first, tp second, 0 for the rest.
3. my_time() — read CLOCK_REALTIME, store the seconds through tloc if it is not NULL, and return them.

Build & Run¶

make
./clock_gettime_demo

Check Your Work¶

The two values printed should agree, or differ by at most 1 if the second ticked over between the calls.
Sanity-check the absolute value against the system clock:
```
date +%s
```
Your seconds should match to within a second or two. A value near zero means the kernel never filled the structure; a small negative value is an error code being returned rather than a time.
Confirm the calls reach the kernel:
```
strace -e trace=clock_gettime ./clock_gettime_demo
```
You may see fewer clock_gettime lines than you expect, or none at all — that is not necessarily a bug in your code. Compare with a program that calls libc's clock_gettime() and discuss the difference with the teaching assistant.
Passing NULL for tloc must work and must not crash. Try a non-NULL pointer too, and check both paths give the same value.

Bonus: clock_gettime and time¶