Lecture 06
Time and Process Info
Outline
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Time
Time System Calls
Time Conversions
System and Process
Information
• Live Coding
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Time
• Within a program, two types of time:
1. Real time
2. Process time
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Real Time
Time measured from some standard point in process’s life
• Calendar time- useful for database interactions
• Start of process- useful for periodic actions
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Process Time
Amount of CPU time used by the process
• Useful for checking program performance
• Useful for optimizing algorithm performance
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Functions for Time
Functions to:
• Set time
• Get time
• Convert time from internal representation to readable format
• Convert time from readable format to internal representation
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Calendar Time
UNIX represents time internally as seconds since Epoch
• Epoch => midnight 1/1/1970
• “birth” of UNIX
• Time stored in variables of type time_t (integer)
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Outline
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Time
Time System Calls
Time Conversions
System and Process
Information
• Live Coding
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gettimeofday()
int gettimeofday( struct timeval *tv, struct timezone *tz );
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#include <sys/time.h>
Returns 0 on success, -1 on error
Fills calendar time in buffer pointed to by tv
tz is historical artifact (just set to NULL)
System call
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struct timeval
struct timeval {
time_t
suseconds_tv
};
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tv_sec;
tv_usec;
//seconds since 1/1/1970
//additional microseconds
time()
time_t time(time_t *timep);
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#include <time.h>
Returns number of seconds since Epoch
Returns -1 on error
If timep == NULL, current time placed in timep
Often use t = time(NULL); without error checking!
System call
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time()
time() gets
time_t from
Kernel
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Outline
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Time
Time System Calls
Time Conversions
System and Process
Information
• Live Coding
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ctime()
char *ctime(const time_t *timep);
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#include <time.h>
Returns NULL on error
Returns pointer to statically allocated string
String terminated by newline and ‘\0’
• 26-byte string: e.g., “Wed Jun 8 14:22:34 2011”
• Automatically accounts for timezone and DST settings
• Future calls to ctime() overwrites string
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ctime()
ctime() converts
time_t to fixed
format string
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gmtime()
struct tm *gmtime( const time_t *timep );
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#include <time.h>
Converts calendar time into broken-time time for UTC
Returns pointer to allocated broken-down time structure
Returns NULL on error
gm -> Greenwich Mean Time
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gmtime()
gmtime()
converts time_t
to broken-down
time format (struct
tm)
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struct tm
Contains date and time broken-down into individual parts
struct tm {
int tm_sec;
int tm_min;
int tm_hour;
int tm_mday
int tm_mon;
int tm_year;
…
}
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// seconds
//minutes
//hours
//day
//month
//year
localtime()
struct tm *localtime( const time_t *timep );
• #include <time.h>
• Converts calendar local time to broken-down time
• Accounts for timezone and DST settings
• Returns pointer to allocated broken-down time structure
• Returns NULL on error
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localtime()
localtime()
converts time_t
to broken-down
time format (struct
tm)
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mktime()
time_t mktime( struct tm *timeptr );
• #include <time.h>
• Returns seconds since Epoch (time_t) from corresponding to
broken-down time (struct tm)
• Returns -1 on error
• Adjusts for out of range data fields in struct tm
• E.g., -1 seconds => 59 seconds of previous minute
• Very useful for time calculations
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mktime()
mktime() converts
broken-down time
(struct tm) to
Epoch seconds
(time_t)
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asctime()
char *asctime( const struct tm *timeptr );
• #include <time.h>
• Takes pointer to broken-down time (struct tm)
=> returns pointer to string containing time
• Returns NULL on error
• Returns pointer to allocated string (terminated by ‘\n\0’)
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asctime()
asctime() converts
broken-down time
(struct tm) to string
(char *)
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strftime()
size_t strftime( char *outstr, size_t maxsize, const char *format,
const struct tm *timeptr );
• #include <time.h>
• More precise control when converting broken-down time into
printable format
• String returned in outstr formatted according to format
• Similar to printf
• Uses % for placeholders
• Table 10-1 has list of placeholders
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strftime()
size_t strftime( char *outstr, size_t maxsize, const char *format,
const struct tm *timeptr );
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Returns number of bytes placed in outstr
Returns 0 on error
maxsize specifies maximum space in outstr
strftime() does NOT include a newline in string
• Unless specified in format
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strftime()
strftime() converts
broken-down time
(struct tm) to
user-formatted
string (char *)
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Accuracy of Time
• Accuracy of time-related system calls limited to resolution of
system software clock
• jiffies- measure of time units (Hz)
• Defined within the kernel source
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Process Time
• Amount of CPU time used by process since creation
• Separated into user and system CPU time
• User CPU time: time spent executing in user mode
• System CPU time: time spent executing in kernel mode
• System calls
• Performing other tasks on behalf o program (e.g., page faults)
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times()
clock_t times( struct tms *buf );
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#include <sys/time.h>
Returns number of clocks ticks since arbitrary past time
Returns -1 on error
clock_t integer time in units of clock ticks
• Must convert to time
• tms structure holds
• user time
• system time
• (time spent waiting on) child’s user and system time
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times()
clock_t times( struct tms *buf );
• Return value of times() is unreliable!!
• Begins at arbitrary point
• More reliable method: use gettimeofday()
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clock()
clock_t clock();
• #include <time.h>
• Returns -1 on error
• Returns total CPU clocks (user & system) of calling process
• Must convert return value (clocks) to time
• Divide clock_t return value by CLOCK_PER_SEC => time
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Outline
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Time
Time System Calls
Time Conversions
System and Process
Information
• Live Coding
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System and Process Information
May want to answer following questions:
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How many processes are running on system? Who owns?
What files does a process have open?
What files are locked? Which processes hold the locks?
What sockets are being used by the system?
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/proc
Modern UNIX provides /proc virtual filesystem
• Resides under /proc
• Contains various files that expose kernel info
• Allows process to read using normal file I/O calls
• /proc is virtual => does NOT reside on disk
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/proc/PID
For each process, kernel provides directory named /proc/pid
• pid is process identifier
• Directory contains various files and subdirectories with
information about that process
• E.g., Information for PID 1 (init)
• Look at files uner /proc/1
• File named “status” => /proc/1/status
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/proc/PID
Many files in /proc/pid directory, e.g.,
• cmdline: command line arguments used
• cwd: symbolic link to current working directory
• fd: directory containing symbolic links to open file descriptors
• mem: process virtually memory
• Any process can access its own /proc/pid using /proc/self
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System Info Under /proc
• Various files and subdirectories => access to system-wide info
• E.g.,
• /proc/net - status info about networking and sockets
• /proc/sys/kernel – kernel settings
• /proc/sys/vm – memory management settings
• See Table 12-2 for more
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Accessing /proc Files
• Most are read-only (owned by root)
• Some can be written by user
• /proc/pid directories are volatile
• Each pid directory created when process created
• Directory disappears when process is terminated
• Must handle this accordingly.. (check existence first)
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Outline
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Time
Time System Calls
Time Conversions
System and Process
Information
• Live Coding
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Live Coding
• Print current time
• Print current time using custom formatted string
• Print a made up time
• Explore /proc directory structure
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