9 System Functions

The C programming language is a good choice if you wish to write operating systems. For example, much of the UNIX operating system is written in C. When writing system programs, it is sometimes necessary to retrieve or modify the environment in which the program is running. This chapter describes the DEC C Run-Time Library (RTL) functions that accomplish this and other system tasks.

Table 9-1 lists and describes all the system functions found in the DEC C RTL. For a more detailed description of each function, see the Reference Section.

Table 9-1 System Functions

Example 9-1 shows how the cuserid function is used.

Example 9-1 Accessing the User Name

/*  Using cuserid, this program returns the user name.         */

#include <stdio.h>

main()

{
   static char string[L_cuserid];
   cuserid(string);
   printf("Initiating user: %s\n", string);
}

If a user named TOLLIVER runs the program, the following is displayed on stdout:

$ RUN  EXAMPLE1
Initiating user: TOLLIVER

Example 9-2 shows how the getenv function is used.

Example 9-2 Accessing Terminal Information

cfunc()
{
    printf("Terminal type: %s\n", getenv("TERM"));
}

Running Example 9-2 on a Digital VT100 terminal in 132-column mode displays the following:

$ RUN  EXAMPLE3
Terminal type: vt100-132

Example 9-3 shows how to use getenv to find the user's default login directory and how to use chdir to change to that directory.

Example 9-3 Manipulating the Default Directory

/*  This program performs the equivalent to the DCL command    *
 *  SET DEFAULT SYS$LOGIN. Once the program exits, however,    *
 *  the directory is reset to the directory from which the     *
 *  program was run.                                           */

#include <stdio.h>
#include <unistd.h>

main()
{
   char *dir;
   int i;

   dir = getenv("HOME");
   if ((i = chdir(dir)) != 0)
      {
         perror("Cannot set directory");
         exit();
      }

   printf("Current directory: %s\n", dir);
}

Running Example 9-3 displays the following:

$ RUN EXAMPLE4
Current directory: dba0:[tolliver]
$

Example 9-4 shows how to use the time, localtime, and strftime functions to print the correct date and time at the terminal.

Example 9-4 Printing the Date and Time

/*  The time function returns the time in seconds; the         *
 *  localtime function converts the time to hours, minutes,    *
 *  and so on; and the strftime function uses these values to  */
 *  obtain a string in the desired format.                     */

#include <time.h>
#define MAX_STRING 80

main()
{
   struct  tm  *time_structure;
   time_t time_val;
   char output_str[MAX_STRING];

   time(&time_val);
   time_structure = localtime(&time_val);

   /* Print the date  */

   strftime(output_str, MAX_STRING,
               "Today is %A, %B %d, %Y", time_structure);

   printf("%s\n", output_str);

   /* Print the time using a 12-hour clock format. */

   strftime(output_str, MAX_STRING,
               "The time is %I:%M %p", time_structure);

   printf("%s\n", output_str);

}

Running Example 9-4 displays the following:

$ RUN  EXAMPLE5
Today is Thursday, May 20, 1993
The time is 10:18 AM
$