Accessing system information using structures
Operating systems (naturally) maintain a lot of (related) information,
and keep that information in structures.
So that the information about the structures
(the datatypes and names of the structure's fields)
can be known by both the operating system and users' programs,
these structures are defined in system-wide header files -
typically in
/usr/include
and
/usr/include/sys.
For example,
consider how an operating system may represent time on a computer:
#include <stdio.h>
#include <sys/time.h>
// A value accurate to the nearest microsecond but also has a range of years
struct timeval {
int tv_sec; // Seconds
int tv_usec; // Microseconds
};
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Note that the structure has now been given a name,
and we can now define multiple variables having this named datatype
(in our previous example,
the structure would be described as anonymous).
We can now request information from the operating system,
with the information returned to us in structures:
#include <stdio.h>
#include <sys/time.h>
struct timeval start_time;
struct timeval stop_time;
gettimeofday( &start_time, NULL );
printf("program started at %i.06%i\n",
(int)start_time.tv_sec, (int)start_time.tv_usec);
....
perform_work();
....
gettimeofday( &stop_time, NULL );
printf("program stopped at %i.06%i\n",
(int)stop_time.tv_sec, (int)stop_time.tv_usec);
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Here we are passing the structure by address,
with the & operator,
so that the gettimeofday() function
can modify the fields of our structure.
(we're not passing a meaningful pointer as the second parameter to
gettimeofday(), as we're not interested in timezone information)
CITS2002 Systems Programming, Lecture 6, p14, 8th August 2023.
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CITS2002