Miscellaneous System Calls

A variety of other system calls exist as well. We will look at just four of them here. The chdir call changes the current working directory. After the call

chdir("/usr/ast/test");

an open on the file xyz will open /usr/ast/test/xyz. The concept of a working directory eliminates the need for typing (long) absolute path names all the time.

In UNIX every file has a mode used for protection. The mode includes the read-write-execute bits for the owner, group, and others. The chmod system call makes it possible to change the mode of a file. For example, to make a file read-only by everyone except the owner, one could execute

chmod("file", 0644);

The kill system call is the way users and user processes send signals. If a process is prepared to catch a particular signal, then when it arrives, a signal handler is run. If the process is not prepared to handle a signal, then its arrival kills the process (hence the name of the call).

POSIX defines several procedures for dealing with time. For example, time just returns the current time in seconds, with 0 corresponding to Jan. 1, 1970 at midnight (just as the day was starting, not ending). On computers with 32-bit words, the maximum value time can return is 2³² −1 seconds (assuming an unsigned integer is used). This value corresponds to a little over 136 years. Thus in the year 2106, 32-bit UNIX systems will go berserk, imitating the famous Y2K problem. If you currently have a 32-bit UNIX system, you are advised to trade it in for a 64-bit one sometime before the year 2106.