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In today’s C programming language tutorial we take a look at how to use time and date from C programs.
To make use of time and date function you need to include the time.h header file from the standard C library.

This header file contains functions and macros that provide standardized access to time and date. It also contains functions to manipulate and format the time and date output. The time.h reference page contains more information about the different time and date functions.

UNIX Time or POSIX Time

The function time() returns the type time_t. The time that is returned represents the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC. It’s also called UNIX EPOCH time.

It is widely used not only on Unix-like operating systems but also in many other computing systems.

Fun note: On February 13, 2009 at exactly 23:31:30 (UTC), UNIX time was equal to ‘1234567890’.

Let’s take a look at a source code example:

MS Windows and Time

How To Delay In C

Microsoft Windows is also different time for different things. Normally we only place programs that will compile on UNIX, Linux and Windows, but this time we make an exception and show you also a Windows example.

(Note: that the other programs in this tutorial should also work and compile on Windows.)

Windows has it’s own SYSTEMTIME structure which stores information on the date and time. For instance: Year, Month, Day, Hour, Minutes, etc. The format of this structure looks as follows:

Take a look at the following example that uses GetSystemTime function to get the UTC time:

Note: the windows.h include, this program will not work on other platforms. Another thing we need to warn you about is that the program doesn’t respond to daylight saving time issues.

Readable Current Local Time using ctime

The Unix EPOCH time is not readable for humans. To get a human-readable version of the current local time you can use the ctime() function. The function returns a C string containing the date and time information.

This string is followed by a new-line character (‘n’) and it will convert the time_t object pointed by the timer to a C string containing a human-readable version of the corresponding local time and date.

Please note: that the functions ctime() and asctime() share the array which holds the time string.
If either one of these functions is called, the content of the array is overwritten.

An example of ctime() use:

The string that is returned will have the following format:

Manipulating the time structure with mktime()

It is also possible to manipulate the time structure and to create your own time using mktime().

Let’s look at an example (note that the example will not change your system, it only prints a new time.):

Note: that you can set anything you want for the hour, min and sec as long as they don’t cause a new day to occur.

As you can see we can also manipulate the time into the future. If you look at str_time.tm_hour and the output you will see that they don’t correspond (10 and 11). This is because of the daylight saving time set on our computer. If you also have this then you should try the following: set the str_time.tm_mon to 0 to see the effect of daylight saving time. The result will be that the output now will be 10.

C++ Time Delay

Using the function difftime

The function difftime() is a very useful function, because it can be used to measure the performance time of a certain part of code. For instance in our example we measure the time of a loop (that is doing nothing at all.)
Take a look at the example:

Timezones

It is also possible to work with different timeszones by using gmtime() to convert calendar time to UTC.
If you know the UTC time you can add CET, for example Amsterdam +1 hour. Take a look at the following example:

Number of clock ticks

It is also possible to use clock ticks elapsed since the start of the program in your own programs by using the function clock(). For instance you can build a wait function or use it in your frame per second (FPS) function.

Take a look at the following wait example:

In Conclusion

As you can see there are many ways to use time and dates in your programs. You never know when it time to use time functions in your programs, so learn them or at least play with them by making some example programs of your own. Also take a look at our C language calendar tutorial for a more advance use of the things explained in this tutorial.

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2. Using I/O ports in C programs

2.1 The normal method

Routines for accessing I/O ports are in /usr/include/asm/io.h(or linux/include/asm-i386/io.h in the kernel sourcedistribution). The routines there are inline macros, so it is enoughto #include <asm/io.h>; you do not need any additionallibraries.

Because of a limitation in gcc (present in all versions I know of,including egcs), you have to compile any source code that usesthese routines with optimisation turned on (gcc -O1 or higher),or alternatively use #define extern static before you#include <asm/io.h> (remember to #undefexternafterwards).

For debugging, you can use gcc -g -O (at least with modernversions of gcc), though optimisation can sometimes make the debuggerbehave a bit strangely. If this bothers you, put the routines that useI/O port access in a separate source file and compile only that withoptimisation turned on.

Permissions

Before you access any ports, you must give your program permission todo so. This is done by calling the ioperm() function (declaredin unistd.h, and defined in the kernel) somewhere near the startof your program (before any I/O port accesses). The syntax isioperm(from, num, turn_on), where from is the first portnumber to give access to, and num the number of consecutive portsto give access to. For example, ioperm(0x300, 5, 1) would giveaccess to ports 0x300 through 0x304 (a total of 5 ports). The lastargument is a Boolean value specifying whether to give access to theprogram to the ports (true (1)) or to remove access (false (0)). Youcan call ioperm() multiple times to enable multiplenon-consecutive ports. See the ioperm(2) manual page for detailson the syntax.

The ioperm() call requires your program to have root privileges;thus you need to either run it as the root user, or make it setuidroot. You can drop the root privileges after you have calledioperm() to enable the ports you want to use. You are notrequired to explicitly drop your port access privileges withioperm(..., 0) at the end of your program; this is doneautomatically as the process exits.

A setuid() to a non-root user does not disable the port accessgranted by ioperm(), but a fork() does (the child processdoes not get access, but the parent retains it).

ioperm() can only give access to ports 0x000 through 0x3ff; forhigher ports, you need to use iopl() (which gives you access toall ports at once). Use the level argument 3 (i.e., iopl(3)) togive your program access to all I/O ports (so be careful -

Using Delay In Dev C++