CITS2002 Systems Programming  
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System-calls and system-defined structures

Most system-calls accept integers and pointers to characters as parameters, and typically return integer values indicating their success. When more information must be passed to a system-call, or the call needs to return multiple values, we employ system-defined (C11) structures defined in operating system provided header files.

Accessing structures using pointers

We've seen that we can access fields of a structure using a single dot ('.' or fullstop).
What if, instead of accessing the structure directly, we only have a pointer to a structure?

We've seen "one side" of this situation, already - when we passed the address of a structure to a function: 

    struct timeval   start_time;

    gettimeofday( &start_time, NULL );

The function gettimeofday(), must have been declared to receive a pointer: 

    extern int gettimeofday( struct timeval *time, ......);

Consider the following example, in which a pointer to a structure is returned from a function.
We now use the → operator (pronounced the 'arrow', or 'points-to' operator) to access the fields via the pointer: 

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

void greeting(void)
{
    time_t      NOW     = time(NULL);
    struct tm   *tm     = localtime(&NOW);

    printf("Today's date is %i/%i/%i\n",
             tm->tm_mday, tm->tm_mon + 1, tm->tm_year + 1900);

    if(tm->tm_hour < 12) {
        printf("Good morning\n");
    }
    else if(tm->tm_hour < 17) {
        printf("Good afternoon\n");
    }
    else {
        printf("Good evening\n");
    }
}

 

CITS2002 Systems Programming, Lecture 16, p1, 19th September 2023.