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Date and Time

This chapter describes functions for manipulating dates and times, including functions for determining what the current time is and conversion between different time representations.

The time functions fall into three main categories:

Processor Time

If you're trying to optimize your program or measure its efficiency, it's very useful to be able to know how much processor time or CPU time it has used at any given point. Processor time is different from actual wall clock time because it doesn't include any time spent waiting for I/O or when some other process is running. Processor time is represented by the data type clock_t, and is given as a number of clock ticks relative to an arbitrary base time marking the beginning of a single program invocation.

Basic CPU Time Inquiry

To get the elapsed CPU time used by a process, you can use the clock function. This facility is declared in the header file `time.h'.

In typical usage, you call the clock function at the beginning and end of the interval you want to time, subtract the values, and then divide by CLOCKS_PER_SEC (the number of clock ticks per second), like this:

#include <time.h>

clock_t start, end;
double elapsed;

start = clock();
... /* Do the work. */
end = clock();
elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;

Different computers and operating systems vary wildly in how they keep track of processor time. It's common for the internal processor clock to have a resolution somewhere between hundredths and millionths of a second.

In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value. But in other systems, both clock_t and the type of the macro CLOCKS_PER_SEC can be either integer or floating-point types. Casting processor time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

Macro: int CLOCKS_PER_SEC
The value of this macro is the number of clock ticks per second measured by the clock function.

Macro: int CLK_TCK
This is an obsolete name for CLOCKS_PER_SEC.

Data Type: clock_t
This is the type of the value returned by the clock function. Values of type clock_t are in units of clock ticks.

Function: clock_t clock (void)
This function returns the elapsed processor time. The base time is arbitrary but doesn't change within a single process. If the processor time is not available or cannot be represented, clock returns the value (clock_t)(-1).

Detailed Elapsed CPU Time Inquiry

The times function returns more detailed information about elapsed processor time in a struct tms object. You should include the header file `sys/times.h' to use this facility.

Data Type: struct tms
The tms structure is used to return information about process times. It contains at least the following members:

clock_t tms_utime
This is the CPU time used in executing the instructions of the calling process.
clock_t tms_stime
This is the CPU time used by the system on behalf of the calling process.
clock_t tms_cutime
This is the sum of the tms_utime values and the tms_cutime values of all terminated child processes of the calling process, whose status has been reported to the parent process by wait or waitpid; see section Process Completion. In other words, it represents the total CPU time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by wait or waitpid.
clock_t tms_cstime
This is similar to tms_cutime, but represents the total CPU time used by the system on behalf of all the terminated child processes of the calling process.

All of the times are given in clock ticks. These are absolute values; in a newly created process, they are all zero. See section Creating a Process.

Function: clock_t times (struct tms *buffer)
The times function stores the processor time information for the calling process in buffer.

The return value is the same as the value of clock(): the elapsed real time relative to an arbitrary base. The base is a constant within a particular process, and typically represents the time since system start-up. A value of (clock_t)(-1) is returned to indicate failure.

Portability Note: The clock function described in section Basic CPU Time Inquiry, is specified by the ANSI C standard. The times function is a feature of POSIX.1. In the GNU system, the value returned by the clock function is equivalent to the sum of the tms_utime and tms_stime fields returned by times.

Calendar Time

This section describes facilities for keeping track of dates and times according to the Gregorian calendar.

There are three representations for date and time information:

Simple Calendar Time

This section describes the time_t data type for representing calendar time, and the functions which operate on calendar time objects. These facilities are declared in the header file `time.h'.

Data Type: time_t
This is the data type used to represent calendar time. In the GNU C library and other POSIX-compliant implementations, time_t is equivalent to long int. When interpreted as an absolute time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This date is sometimes referred to as the epoch.)

In other systems, time_t might be either an integer or floating-point type.

Function: double difftime (time_t time1, time_t time0)
The difftime function returns the number of seconds elapsed between time time1 and time time0, as a value of type double.

In the GNU system, you can simply subtract time_t values. But on other systems, the time_t data type might use some other encoding where subtraction doesn't work directly.

Function: time_t time (time_t *result)
The time function returns the current time as a value of type time_t. If the argument result is not a null pointer, the time value is also stored in *result. If the calendar time is not available, the value (time_t)(-1) is returned.

High-Resolution Calendar

The time_t data type used to represent calendar times has a resolution of only one second. Some applications need more precision.

So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in `sys/time.h'.

Data Type: struct timeval
The struct timeval structure represents a calendar time. It has the following members:

long int tv_sec
This represents the number of seconds since the epoch. It is equivalent to a normal time_t value.
long int tv_usec
This is the fractional second value, represented as the number of microseconds. Some times struct timeval values are used for time intervals. Then the tv_sec member is the number of seconds in the interval, and tv_usec is the number of additional microseconds.

Data Type: struct timezone
The struct timezone structure is used to hold minimal information about the local time zone. It has the following members:

int tz_minuteswest
This is the number of minutes west of GMT.
int tz_dsttime
If nonzero, daylight savings time applies during some part of the year.

The struct timezone type is obsolete and should never be used. Instead, use the facilities described in section Functions and Variables for Time Zones.

It is often necessary to subtract two values of type struct timeval. Here is the best way to do this. It works even on some peculiar operating systems where the tv_sec member has an unsigned type.

/* Subtract the `struct timeval' values X and Y,
   storing the result in RESULT.
   Return 1 if the difference is negative, otherwise 0.  */

int
timeval_subtract (result, x, y)
     struct timeval *result, *x, *y;
{
  /* Perform the carry for the later subtraction by updating y. */
  if (x->tv_usec < y->tv_usec) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
    y->tv_usec -= 1000000 * nsec;
    y->tv_sec += nsec;
  }
  if (x->tv_usec - y->tv_usec > 1000000) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000;
    y->tv_usec += 1000000 * nsec;
    y->tv_sec -= nsec;
  }

  /* Compute the time remaining to wait.
     tv_usec is certainly positive. */
  result->tv_sec = x->tv_sec - y->tv_sec;
  result->tv_usec = x->tv_usec - y->tv_usec;

  /* Return 1 if result is negative. */
  return x->tv_sec < y->tv_sec;
}

Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)
The gettimeofday function returns the current date and time in the struct timeval structure indicated by tp. Information about the time zone is returned in the structure pointed at tzp. If the tzp argument is a null pointer, time zone information is ignored.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

ENOSYS
The operating system does not support getting time zone information, and tzp is not a null pointer. The GNU operating system does not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in section Functions and Variables for Time Zones.

Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)
The settimeofday function sets the current date and time according to the arguments. As for gettimeofday, time zone information is ignored if tzp is a null pointer.

You must be a privileged user in order to use settimeofday.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM
This process cannot set the time because it is not privileged.
ENOSYS
The operating system does not support setting time zone information, and tzp is not a null pointer.

Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)
This function speeds up or slows down the system clock in order to make gradual adjustments in the current time. This ensures that the time reported by the system clock is always monotonically increasing, which might not happen if you simply set the current time.

The delta argument specifies a relative adjustment to be made to the current time. If negative, the system clock is slowed down for a while until it has lost this much time. If positive, the system clock is speeded up for a while.

If the olddelta argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet completed.

This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it. The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM
You do not have privilege to set the time.

Portability Note: The gettimeofday, settimeofday, and adjtime functions are derived from BSD.

Broken-down Time

Calendar time is represented as a number of seconds. This is convenient for calculation, but has no resemblance to the way people normally represent dates and times. By contrast, broken-down time is a binary representation separated into year, month, day, and so on. Broken down time values are not useful for calculations, but they are useful for printing human readable time.

A broken-down time value is always relative to a choice of local time zone, and it also indicates which time zone was used.

The symbols in this section are declared in the header file `time.h'.

Data Type: struct tm
This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order:

int tm_sec
This is the number of seconds after the minute, normally in the range 0 to 59. (The actual upper limit is 61, to allow for "leap seconds".)
int tm_min
This is the number of minutes after the hour, in the range 0 to 59.
int tm_hour
This is the number of hours past midnight, in the range 0 to 23.
int tm_mday
This is the day of the month, in the range 1 to 31.
int tm_mon
This is the number of months since January, in the range 0 to 11.
int tm_year
This is the number of years since 1900.
int tm_wday
This is the number of days since Sunday, in the range 0 to 6.
int tm_yday
This is the number of days since January 1, in the range 0 to 365.
int tm_isdst
This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available.
long int tm_gmtoff
This field describes the time zone that was used to compute this broken-down time value; it is the amount you must add to the local time in that zone to get GMT, in units of seconds. The value is like that of the variable timezone (see section Functions and Variables for Time Zones). You can also think of this as the "number of seconds west" of GMT. The tm_gmtoff field is a GNU library extension.
const char *tm_zone
This field is the three-letter name for the time zone that was used to compute this broken-down time value. It is a GNU library extension.

Function: struct tm * localtime (const time_t *time)
The localtime function converts the calendar time pointed to by time to broken-down time representation, expressed relative to the user's specified time zone.

The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to any of the date and time functions. (But no other library function overwrites the contents of this object.)

Calling localtime has one other effect: it sets the variable tzname with information about the current time zone. See section Functions and Variables for Time Zones.

Function: struct tm * gmtime (const time_t *time)
This function is similar to localtime, except that the broken-down time is expressed as Coordinated Universal Time (UTC)---that is, as Greenwich Mean Time (GMT) rather than relative to the local time zone.

Recall that calendar times are always expressed in coordinated universal time.

Function: time_t mktime (struct tm *brokentime)
The mktime function is used to convert a broken-down time structure to a calendar time representation. It also "normalizes" the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

The mktime function ignores the specified contents of the tm_wday and tm_yday members of the broken-down time structure. It uses the values of the other components to compute the calendar time; it's permissible for these components to have unnormalized values outside of their normal ranges. The last thing that mktime does is adjust the components of the brokentime structure (including the tm_wday and tm_yday).

If the specified broken-down time cannot be represented as a calendar time, mktime returns a value of (time_t)(-1) and does not modify the contents of brokentime.

Calling mktime also sets the variable tzname with information about the current time zone. See section Functions and Variables for Time Zones.

Formatting Date and Time

The functions described in this section format time values as strings. These functions are declared in the header file `time.h'.

Function: char * asctime (const struct tm *brokentime)
The asctime function converts the broken-down time value that brokentime points to into a string in a standard format:

"Tue May 21 13:46:22 1991\n"

The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.

The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.

The return value points to a statically allocated string, which might be overwritten by subsequent calls to any of the date and time functions. (But no other library function overwrites the contents of this string.)

Function: char * ctime (const time_t *time)
The ctime function is similar to asctime, except that the time value is specified as a time_t calendar time value rather than in broken-down local time format. It is equivalent to

asctime (localtime (time))

ctime sets the variable tzname, because localtime does so. See section Functions and Variables for Time Zones.

Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)
This function is similar to the sprintf function (see section Formatted Input), but the conversion specifications that can appear in the format template template are specialized for printing components of the date and time brokentime according to the locale currently specified for time conversion (see section Locales and Internationalization).

Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, and are replaced in the output string as follows:

%a
The abbreviated weekday name according to the current locale.
%A
The full weekday name according to the current locale.
%b
The abbreviated month name according to the current locale.
%B
The full month name according to the current locale.
%c
The preferred date and time representation for the current locale.
%d
The day of the month as a decimal number (range 01 to 31).
%H
The hour as a decimal number, using a 24-hour clock (range 00 to 23).
%I
The hour as a decimal number, using a 12-hour clock (range 01 to 12).
%j
The day of the year as a decimal number (range 001 to 366).
%m
The month as a decimal number (range 01 to 12).
%M
The minute as a decimal number.
%p
Either `am' or `pm', according to the given time value; or the corresponding strings for the current locale.
%S
The second as a decimal number.
%U
The week number of the current year as a decimal number, starting with the first Sunday as the first day of the first week.
%W
The week number of the current year as a decimal number, starting with the first Monday as the first day of the first week.
%w
The day of the week as a decimal number, Sunday being 0.
%x
The preferred date representation for the current locale, but without the time.
%X
The preferred time representation for the current locale, but with no date.
%y
The year as a decimal number, but without a century (range 00 to 99).
%Y
The year as a decimal number, including the century.
%Z
The time zone or name or abbreviation (empty if the time zone can't be determined).
%%
A literal `%' character.

The size parameter can be used to specify the maximum number of characters to be stored in the array s, including the terminating null character. If the formatted time requires more than size characters, the excess characters are discarded. The return value from strftime is the number of characters placed in the array s, not including the terminating null character. If the value equals size, it means that the array s was too small; you should repeat the call, providing a bigger array.

If s is a null pointer, strftime does not actually write anything, but instead returns the number of characters it would have written.

For an example of strftime, see section Time Functions Example.

Specifying the Time Zone with TZ

In POSIX systems, a user can specify the time zone by means of the TZ environment variable. For information about how to set environment variables, see section Environment Variables. The functions for accessing the time zone are declared in `time.h'.

You should not normally need to set TZ. If the system is configured properly, the default timezone will be correct. You might set TZ if you are using a computer over the network from a different timezone, and would like times reported to you in the timezone that local for you, rather than what is local for the computer.

In POSIX.1 systems the value of the TZ variable can be of one of three formats. With the GNU C library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world. The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise. But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database.

The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone:

std offset

The std string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon or embedded digits, commas, or plus or minus signs. There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.

The offset specifies the time value one must add to the local time to get a Coordinated Universal Time value. It has syntax like [+|-]hh[:mm[:ss]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 24, and the minute and seconds between 0 and 59.

For example, here is how we would specify Eastern Standard Time, but without any daylight savings time alternative:

EST+5

The second format is used when there is Daylight Saving Time:

std offset dst [offset],start[/time],end[/time]

The initial std and offset specify the standard time zone, as described above. The dst string and offset specify the name and offset for the corresponding daylight savings time time zone; if the offset is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when daylight savings time is in effect. The start field is when daylight savings time goes into effect and the end field is when the change is made back to standard time. The following formats are recognized for these fields:

Jn
This specifies the Julian day, with n between 1 and 365. February 29 is never counted, even in leap years.
n
This specifies the Julian day, with n between 0 and 365. February 29 is counted in leap years.
Mm.w.d
This specifies day d of week w of month m. The day d must be between 0 (Sunday) and 6. The week w must be between 1 and 5; week 1 is the first week in which day d occurs, and week 5 specifies the last d day in the month. The month m should be between 1 and 12.

The time fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is 02:00:00.

For example, here is how one would specify the Eastern time zone in the United States, including the appropriate daylight saving time and its dates of applicability. The normal offset from GMT is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am.

EST+5EDT,M4.1.0/M10.5.0

The schedule of daylight savings time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below).

The third format looks like this:

:characters

Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.

If the TZ environment variable does not have a value, the operation chooses a time zone by default. In the GNU C library, the default time zone is like the specification `TZ=:/etc/localtime' (or `TZ=:/usr/local/etc/localtime', depending on how GNU C library was configured; see section How to Install the GNU C Library). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.

If characters begins with a slash, it is an absolute file name; otherwise the library looks for the file `/share/lib/zoneinfo/characters'. The `zoneinfo' directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, `America/New_York', `Europe/London', `Asia/Hong_Kong'. These data files are installed by the system administrator, who also sets `/etc/localtime' to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain.

Functions and Variables for Time Zones

Variable: char * tzname [2]
The array tzname contains two strings, which are the standard three-letter names of the pair of time zones (standard and daylight savings) that the user has selected. tzname[0] is the name of the standard time zone (for example, "EST"), and tzname[1] is the name for the time zone when daylight savings time is in use (for example, "EDT"). These correspond to the std and dst strings (respectively) from the TZ environment variable.

The tzname array is initialized from the TZ environment variable whenever tzset, ctime, strftime, mktime, or localtime is called.

Function: void tzset (void)
The tzset function initializes the tzname variable from the value of the TZ environment variable. It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone.

The following variables are defined for compatibility with System V Unix. These variables are set by calling localtime.

Variable: long int timezone
This contains the difference between GMT and local standard time, in seconds. For example, in the U.S. Eastern time zone, the value is 5*60*60.

Variable: int daylight
This variable has a nonzero value if the standard U.S. daylight savings time rules apply.

Time Functions Example

Here is an example program showing the use of some of the local time and calendar time functions.

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

#define SIZE 256

int
main (void)
{
  char buffer[SIZE];
  time_t curtime;
  struct tm *loctime;

  /* Get the current time. */
  curtime = time (NULL);

  /* Convert it to local time representation. */
  loctime = localtime (&curtime);

  /* Print out the date and time in the standard format. */
  fputs (asctime (loctime), stdout);

  /* Print it out in a nice format. */
  strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
  fputs (buffer, stdout);
  strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
  fputs (buffer, stdout);

  return 0;
}

It produces output like this:

Wed Jul 31 13:02:36 1991
Today is Wednesday, July 31.
The time is 01:02 PM.

Setting an Alarm

The alarm and setitimer functions provide a mechanism for a process to interrupt itself at some future time. They do this by setting a timer; when the timer expires, the process receives a signal.

Each process has three independent interval timers available:

You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value.

You should establish a handler for the appropriate alarm signal using signal or sigaction before issuing a call to setitimer or alarm. Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler, and in that case it would be terminated, since that is the default action for the alarm signals. See section Signal Handling.

The setitimer function is the primary means for setting an alarm. This facility is declared in the header file `sys/time.h'. The alarm function, declared in `unistd.h', provides a somewhat simpler interface for setting the real-time timer.

Data Type: struct itimerval
This structure is used to specify when a timer should expire. It contains the following members:
struct timeval it_interval
This is the interval between successive timer interrupts. If zero, the alarm will only be sent once.
struct timeval it_value
This is the interval to the first timer interrupt. If zero, the alarm is disabled.

The struct timeval data type is described in section High-Resolution Calendar.

Function: int setitimer (int which, struct itimerval *new, struct itimerval *old)
The setitimer function sets the timer specified by which according to new. The which argument can have a value of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.

If old is not a null pointer, setitimer returns information about any previous unexpired timer of the same kind in the structure it points to.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EINVAL
The timer interval was too large.

Function: int getitimer (int which, struct itimerval *old)
The getitimer function stores information about the timer specified by which in the structure pointed at by old.

The return value and error conditions are the same as for setitimer.

ITIMER_REAL
This constant can be used as the which argument to the setitimer and getitimer functions to specify the real-time timer.
ITIMER_VIRTUAL
This constant can be used as the which argument to the setitimer and getitimer functions to specify the virtual timer.
ITIMER_PROF
This constant can be used as the which argument to the setitimer and getitimer functions to specify the profiling timer.

Function: unsigned int alarm (unsigned int seconds)
The alarm function sets the real-time timer to expire in seconds seconds. If you want to cancel any existing alarm, you can do this by calling alarm with a seconds argument of zero.

The return value indicates how many seconds remain before the previous alarm would have been sent. If there is no previous alarm, alarm returns zero.

The alarm function could be defined in terms of setitimer like this:

unsigned int
alarm (unsigned int seconds)
{
  struct itimerval old, new;
  new.it_interval.tv_usec = 0;
  new.it_interval.tv_sec = 0;
  new.it_value.tv_usec = 0;
  new.it_value.tv_sec = (long int) seconds;
  if (setitimer (ITIMER_REAL, &new, &old) < 0)
    return 0;
  else
    return old.it_value.tv_sec;
}

There is an example showing the use of the alarm function in section Signal Handlers that Return.

If you simply want your process to wait for a given number of seconds, you should use the sleep function. See section Sleeping.

You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved.

Portability Note: The setitimer and getitimer functions are derived from BSD Unix, while the alarm function is specified by the POSIX.1 standard. setitimer is more powerful than alarm, but alarm is more widely used.

Sleeping

The function sleep gives a simple way to make the program wait for short periods of time. If your program doesn't use signals (except to terminate), then you can expect sleep to wait reliably for the specified amount of time. Otherwise, sleep can return sooner if a signal arrives; if you want to wait for a given period regardless of signals, use select (see section Waiting for Input or Output) and don't specify any descriptors to wait for.

Function: unsigned int sleep (unsigned int seconds)
The sleep function waits for seconds or until a signal is delivered, whichever happens first.

If sleep function returns because the requested time has elapsed, it returns a value of zero. If it returns because of delivery of a signal, its return value is the remaining time in the sleep period.

The sleep function is declared in `unistd.h'.

Resist the temptation to implement a sleep for a fixed amount of time by using the return value of sleep, when nonzero, to call sleep again. This will work with a certain amount of accuracy as long as signals arrive infrequently. But each signal can cause the eventual wakeup time to be off by an additional second or so. Suppose a few signals happen to arrive in rapid succession by bad luck--there is no limit on how much this could shorten or lengthen the wait.

Instead, compute the time at which the program should stop waiting, and keep trying to wait until that time. This won't be off by more than a second. With just a little more work, you can use select and make the waiting period quite accurate. (Of course, heavy system load can cause unavoidable additional delays--unless the machine is dedicated to one application, there is no way you can avoid this.)

On some systems, sleep can do strange things if your program uses SIGALRM explicitly. Even if SIGALRM signals are being ignored or blocked when sleep is called, sleep might return prematurely on delivery of a SIGALRM signal. If you have established a handler for SIGALRM signals and a SIGALRM signal is delivered while the process is sleeping, the action taken might be just to cause sleep to return instead of invoking your handler. And, if sleep is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of SIGALRM, this handler and sleep will interfere.

On the GNU system, it is safe to use sleep and SIGALRM in the same program, because sleep does not work by means of SIGALRM.

Resource Usage

The function getrusage and the data type struct rusage are used for examining the usage figures of a process. They are declared in `sys/resource.h'.

Function: int getrusage (int processes, struct rusage *rusage)
This function reports the usage totals for processes specified by processes, storing the information in *rusage.

In most systems, processes has only two valid values:

RUSAGE_SELF
Just the current process.
RUSAGE_CHILDREN
All child processes (direct and indirect) that have terminated already.

In the GNU system, you can also inquire about a particular child process by specifying its process ID.

The return value of getrusage is zero for success, and -1 for failure.

EINVAL
The argument processes is not valid.

One way of getting usage figures for a particular child process is with the function wait4, which returns totals for a child when it terminates. See section BSD Process Wait Functions.

Data Type: struct rusage
This data type records a collection usage amounts for various sorts of resources. It has the following members, and possibly others:

struct timeval ru_utime
Time spent executing user instructions.
struct timeval ru_stime
Time spent in operating system code on behalf of processes.
long int ru_maxrss
The maximum resident set size used, in kilobytes. That is, the maximum number of kilobytes that processes used in real memory simultaneously.
long int ru_ixrss
An integral value expressed in kilobytes times ticks of execution, which indicates the amount of memory used by text that was shared with other processes.
long int ru_idrss
An integral value expressed the same way, which is the amount of unshared memory used in data.
long int ru_isrss
An integral value expressed the same way, which is the amount of unshared memory used in stack space.
long int ru_minflt
The number of page faults which were serviced without requiring any I/O.
long int ru_majflt
The number of page faults which were serviced by doing I/O.
long int ru_nswap
The number of times processes was swapped entirely out of main memory.
long int ru_inblock
The number of times the file system had to read from the disk on behalf of processes.
long int ru_oublock
The number of times the file system had to write to the disk on behalf of processes.
long int ru_msgsnd
Number of IPC messages sent.
long ru_msgrcv
Number of IPC messages received.
long int ru_nsignals
Number of signals received.
long int ru_nvcsw
The number of times processes voluntarily invoked a context switch (usually to wait for some service).
long int ru_nivcsw
The number of times an involuntary context switch took place (because the time slice expired, or another process of higher priority became runnable).

An additional historical function for examining usage figures, vtimes, is supported but not documented here. It is declared in `sys/vtimes.h'.

Limiting Resource Usage

You can specify limits for the resource usage of a process. When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the limit. Each process initially inherits its limit values from its parent, but it can subsequently change them.

The symbols in this section are defined in `sys/resource.h'.

Function: int getrlimit (int resource, struct rlimit *rlp)
Read the current value and the maximum value of resource resource and store them in *rlp.

The return value is 0 on success and -1 on failure. The only possible errno error condition is EFAULT.

Function: int setrlimit (int resource, struct rlimit *rlp)
Store the current value and the maximum value of resource resource in *rlp.

The return value is 0 on success and -1 on failure. The following errno error condition is possible:

EPERM
You tried to change the maximum permissible limit value, but you don't have privileges to do so.

Data Type: struct rlimit
This structure is used with getrlimit to receive limit values, and with setrlimit to specify limit values. It has two fields:

rlim_cur
The current value of the limit in question. This is also called the "soft limit".
rlim_max
The maximum permissible value of the limit in question. You cannot set the current value of the limit to a larger number than this maximum. Only the super user can change the maximum permissible value. This is also called the "hard limit".

In getrlimit, the structure is an output; it receives the current values. In setrlimit, it specifies the new values.

Here is a list of resources that you can specify a limit for. Those that are sizes are measured in bytes.

RLIMIT_CPU
The maximum amount of cpu time the process can use. If it runs for longer than this, it gets a signal: SIGXCPU. The value is measured in seconds. See section Operation Error Signals.
RLIMIT_FSIZE
The maximum size of file the process can create. Trying to write a larger file causes a signal: SIGXFSZ. See section Operation Error Signals.
RLIMIT_DATA
The maximum size of data memory for the process. If the process tries to allocate data memory beyond this amount, the allocation function fails.
RLIMIT_STACK
The maximum stack size for the process. If the process tries to extend its stack past this size, it gets a SIGSEGV signal. See section Program Error Signals.
RLIMIT_CORE
The maximum size core file that this process can create. If the process terminates and would dump a core file larger than this maximum size, then no core file is created. So setting this limit to zero prevents core files from ever being created.
RLIMIT_RSS
The maximum amount of physical memory that this process should get. This parameter is a guide for the system's scheduler and memory allocator; the system may give the process more memory when there is a surplus.
RLIMIT_MEMLOCK
The maximum amount of memory that can be locked into physical memory (so it will never be paged out).
RLIMIT_NPROC
The maximum number of processes that can be created with the same user ID. If you have reached the limit for your user ID, fork will fail with EAGAIN. See section Creating a Process.
RLIMIT_NOFILE
RLIMIT_OFILE
The maximum number of files that the process can open. If it tries to open more files than this, it gets error code EMFILE. See section Error Codes. Not all systems support this limit; GNU does, and 4.4 BSD does.
RLIM_NLIMITS
The number of different resource limits. Any valid resource operand must be less than RLIM_NLIMITS.

Constant: int RLIM_INFINITY
This constant stands for a value of "infinity" when supplied as the limit value in setrlimit.

Two historical functions for setting resource limits, ulimit and vlimit, are not documented here. The latter is declared in `sys/vlimit.h' and comes from BSD.

Process Priority

When several processes try to run, their respective priorities determine what share of the CPU each process gets. This section describes how you can read and set the priority of a process. All these functions and macros are declared in `sys/resource.h'.

The range of valid priority values depends on the operating system, but typically it runs from -20 to 20. A lower priority value means the process runs more often. These constants describe the range of priority values:

PRIO_MIN
The smallest valid priority value.
PRIO_MAX
The smallest valid priority value.

Function: int getpriority (int class, int id)
Read the priority of a class of processes; class and id specify which ones (see below). If the processes specified do not all have the same priority, this returns the smallest value that any of them has.

The return value is the priority value on success, and -1 on failure. The following errno error condition are possible for this function:

ESRCH
The combination of class and id does not match any existing process.
EINVAL
The value of class is not valid.

When the return value is -1, it could indicate failure, or it could be the priority value. The only way to make certain is to set errno = 0 before calling getpriority, then use errno != 0 afterward as the criterion for failure.

Function: int setpriority (int class, int id, int priority)
Set the priority of a class of processes to priority; class and id specify which ones (see below).

The return value is 0 on success and -1 on failure. The following errno error condition are defined for this function:

ESRCH
The combination of class and id does not match any existing process.
EINVAL
The value of class is not valid.
EPERM
You tried to set the priority of some other user's process, and you don't have privileges for that.
EACCES
You tried to lower the priority of a process, and you don't have privileges for that.

The arguments class and id together specify a set of processes you are interested in. These are the possible values for class:

PRIO_PROCESS
Read or set the priority of one process. The argument id is a process ID.
PRIO_PGRP
Read or set the priority of one process group. The argument id is a process group ID.
PRIO_USER
Read or set the priority of one user's processes. The argument id is a user ID.

If the argument id is 0, it stands for the current process, current process group, or the current user, according to class.

Function: int nice (int increment)
Increment the priority of the current process by increment. The return value is the same as for setpriority.

Here is an equivalent definition for nice:

int
nice (int increment)
{
  int old = getpriority (PRIO_PROCESS, 0);
  return setpriority (PRIO_PROCESS, 0, old + increment);
}


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