The Definitive Guide to gettimeofday in C

The gettimeofday() system call is a staple C function for fetching timestamps, benchmarking code, and measuring elapsed time with microsecond precision. First introduced in early UNIX systems, it continues to serve as a portable, high-resolution timer for time-sensitive applications today.

In this comprehensive guide, we‘ll cover everything you need to effectively use this function – including history, internals, usage best practices, alternatives, and more.

A Cornerstone UNIX Timing Function

To understand gettimeofday(), we must go back to UNIX V6 in the mid-1970s. The original C library (then called the Standard I/O Package) included the time() call for getting seconds since the epoch, but lacked precision below integral seconds.

Requests came for expanding resolution to support profiling code performance across shorter durations – crucial for optimizing early microcomputers.

Thus gettimeofday() was born to provide microsecond granularity by utilizing the underlying hardware clock cycles to interpolate between integral seconds. This fulfilled the need for accurately benchmarking programs and log file continuity.

Over 40+ years later, gettimeofday() remains a ubiquitous timing function – now specified by POSIX and implemented across all major operating systems. It has proven versatile enough to stand the test of time.

Function Signature

Here is the function prototype as defined in <sys/time.h>:

int gettimeofday(struct timeval *tv, struct timezone *tz);

It takes two struct pointers:

• timeval*: Holds elapsed time since the UNIX epoch – Jan 1, 1970 UTC.
• timezone*: Information about the local timezone. Largely obsolete today.

And returns an int signaling success (0) or failure (-1).

The key data structure is:

struct timeval {
  time_t      tv_sec;   // whole seconds 
  suseconds_t tv_usec;  // microseconds  
};

This splits our elapsed time into integral seconds + additional microseconds.

The maximum representable duration is over 290 years with microsecond accuracy – sufficient for most use cases.

Usage Walkthrough

Let‘s explore a basic example fetching the current UNIX timestamp:

#include <sys/time.h>

int main() {

  struct timeval tv;

  if (gettimeofday(&tv, NULL) == -1) {
     perror("gettimeofday failed");
     exit(1);
  }

  printf("Seconds since epoch: %ld\n", tv.tv_sec);  
  printf("Microseconds: %ld\n", tv.tv_usec);

  return 0;
}

Here we:

1. Declare a timeval instance.
2. Call gettimeofday() to populate it – passing NULL for the obsolete timezone parameter.
3. On error, print failure and exit.
4. Otherwise, access tv_sec and tv_usec for our program‘s logic.

When run, this prints out seconds and microseconds elapsed since January 1, 1970 UTC per the POSIX specification.

Easy enough! Now let‘s explore more advanced usage…

Benchmarking Performance

One of the most common applications is benchmarking code performance:

struct timeval start, end;

gettimeofday(&start, NULL);

// Code block to measure
calculatePrimes(1000);

gettimeofday(&end, NULL);  

long secs = end.tv_sec - start.tv_sec;  
long micros = end.tv_usec - start.tv_usec;

double elapsed = secs + micros/1e6;  

By calling gettimeofday() before and after our function, we can calculate total elapsed wall clock time. This shines a light on optimizations to improve speed.

Just be mindful that the separate second + microsecond fields can overflow if durations exceed storage capacity. Proper variable types help mitigate.

Converting Formats

While the raw UNIX timestamp suits internal usage, user output demands proper date/time formatting:

struct tm *info;
char buffer[80];    

time_t secs; 
struct timeval tv;

gettimeofday(&tv, NULL);

secs = tv.tv_sec;  

info = localtime(&secs);
strftime(buffer, sizeof(buffer), "%c", info);

puts(buffer); // Formatted string

Here localtime and strftime format our timestamp into a human readable string. Customize to local conventions with strftime specifiers.

Threading Considerations

Since gettimeofday() utilizes shared system state, we must take care when calling from multi-threaded code.

While many implementations are thread-safe nowadays, to avoid any potential race conditions, consider using a mutex lock around calls:

pthread_mutex_t lock;

// Lock to ensure only one thread accessing
pthread_mutex_lock(&lock); 

struct timeval tv;
gettimeofday(&tv, NULL);

// Unlock mutex  
pthread_mutex_unlock(&lock);

This guarantees serialized access if needed for your use case.

Alternatives to Explore

While versatile, gettimeofday() is not without limitations on certain platforms. Precision can vary wildly depending on hardware capabilities.

For benchmarks needing maximal accuracy, consider clock_gettime() instead:

struct timespec ts;

clock_gettime(CLOCK_MONOTONIC, &ts);

This provides nanosecond resolution where supported. Additionally, the timespec epoch cannot be adjusted like standard UNIX time.

For low-latency event handling, Linux offers timerfd for receiving callbacks after defined intervals. This avoids polling.

Other options like mach_absolute_time() (MacOS) and QueryPerformanceCounter() (Windows) yield high-resolution timestamps tailored for their environments.

Understand tradeoffs to pick the optimal timer.

Conclusion

We‘ve covered the key aspects for effectively applying gettimeofday() within C programs – from origin stories to usage tips. Though a legacy function, its portability and microsecond precision retain usefulness for tasks like:

• Benchmarking code blocks
• Adding timestamps to persistent logs
• Measuring transport latency
• Ensuring output continuity

Combined with conversion utilities like localtime and strftime, it returns a versatile building block for any project requiring accurate system time.

When used properly, gettimeofday() serves as a reliable inspector – shedding light on the passing milliseconds with snapshots from your system clock.