difference between time() and gettimeofday() and why does one cause seg fault

Question

I'm trying to measure the amount of time for a system call, and I tried using time(0) and gettimeofday() in this program, but whenever I use gettimeofday() it seg faults. I suppose I can just use time(0) but I'd like to know why this is happening. And I know you guys can just look at it and see the problem. Please don't yell at me!

I want to get the time but not save it anywhere.

I've tried every combination of code I can think of but I pasted the simplest version here. I'm new to C and Linux. I look at the .stackdump file but it's pretty meaningless to me.

GetRDTSC is in util.h and it does rdtsc() , as one might expect. Now it's set to 10 iterations but later the loop will run 1000 times, without printf .

#include <stdio.h>
#include <time.h>
#include "util.h"

int main() {

    int i;
    uint64_t cycles[10];

    for (i = 0; i < 10; ++i) {

         // get initial cycles
         uint64_t init = GetRDTSC();

         gettimeofday(); // <== time(0) will work here without a seg fault.

         // get cycles after
         uint64_t after = GetRDTSC();   

         // save cycles for each operation in an array
         cycles[i] = after - init;

         printf("%i\n", (int)(cycles[i]));
    }  
}

Answer 1

The short version

gettimeofday() requires a pointer to a struct timeval to fill with time data.

So, for example, you'd do something like this:

#include <sys/time.h>
#include <stdio.h>
int main() {  
    struct timeval tv;
    gettimeofday(&tv, NULL); // timezone should be NULL
    printf("%d seconds\n", tv.tv_secs);
    return 0;
}

The long version

The real problem is that gcc is automatically including vdso on your system, which contains a symbol for the syscall gettimeofday . Consider this program (entire file):

int main() {
  gettimeofday();
  return 0;
}

By default, gcc will compile this without warning . If you check the symbols it's linked against, you'll see:

ternus@event-horizon ~> gcc -o foo foo.c
ternus@event-horizon ~> ldd foo
        linux-vdso.so.1 =>  (0x00007ffff33fe000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f56a5255000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f56a562b000)

You just happen to be using a function that has a defined symbol, but without the prototype, there's no way to tell how many command-line arguments it's supposed to have.

If you compile it with -Wall , you'll see:

ternus@event-horizon ~> gcc -Wall -o foo foo.c
foo.c: In function ‘main’:
foo.c:2:3: warning: implicit declaration of function ‘gettimeofday’ [-Wimplicit-function-declaration]

Of course, it'll segfault when you try to run it. Interestingly, it'll segfault in kernel space (this is on MacOS):

cternus@astarael ~/foo> gcc -o foo -g foo.c
cternus@astarael ~/foo> gdb foo
GNU gdb 6.3.50-20050815 (Apple version gdb-1822) (Sun Aug  5 03:00:42 UTC 2012)
[etc]

(gdb) run
Starting program: /Users/cternus/foo/foo
Reading symbols for shared libraries +.............................. done

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000001
0x00007fff87eeab73 in __commpage_gettimeofday ()

Now consider this program (again, no header files):

typedef struct {
  long tv_sec;
  long tv_usec;
} timeval;

int main() {
  timeval tv;
  gettimeofday(&tv, 0);
  return 0;
}

This will compile and run just fine -- no segfault. You've provided it with the memory location it expects, even though there's still no gettimeofday prototype provided.

More information:

Can anyone understand how gettimeofday works?

Is there a faster equivalent of gettimeofday?

The POSIX gettimeofday specification