Page 417 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
Figure 14.6: Some System Calls Relating to Processes

System call Description

pid = fork( ) Create a child process identical to the parent
pid = waitpid(pid, & statloc, opts) Wait for a child to terminate

S = execve(name, argv, envp) Replace a process’ core image
exit(status) Terminate process execution and return status

S = sigaction(sig. & act, & oldact) Define action to take on signals
S = sigreturn (&context) Return from a signal

S = sigprocmask(how, &set, &old) Examine or change the signal mask
S = sigpending (set) Get the set of blocked signals

s = sigsuspend(sigmask) Replace the signals mask and suspend the
process
s = kill(pid, sig) Send a signal to a process
residual = alarm(seconds) Set the alarm clock

s = pause( ) suspend the caller until the next signal

Waitpid has three parameters. The return code s is ~ 1 If an error has occurred, pid is a process
ID, and residual is the remaining time in the previous alarm. The parameters are what the name
suggests. First one allows the caller to wait for a specific child. If it is ~ 1, any old child (i.e.,
the first child to terminate) will do. The second parameter is the address of a variable that will
be set to the child’s exit status (normal or abnormal termination and exit value). The third one
determines whether the caller blocks or returns if no child is already terminated. In the case of
the shell, the child process must execute the command typed by the user. It does this by using
the exec system call, which causes its entire core image to be replaced by the file named in its
first parameter. A highly simplified shell illustrating the use of fork, waitpid, and exec is shown
in Figure. 14.7. In the most general case, exec has three parameters: the name of the file to be
executed, a pointer to the argument array, and a pointer to the environment array. These will
be described shortly. Various library procedures, including execl, execv, execle, and execve,
are provided to allow the parameters to be omitted or specified in various ways. All of these
procedures invoke the same underlying system call. Although the system call is exec, there is
no library procedure with this name; one of the others must be used. Let us consider the case
of a command typed to the shell such as cp file1 file2 used to copy file1 to file2. After the shell
has been forked, the child locates and executes the file cp and passes its information about the
files to be copied. The main program of cp (and many other programs) contains the function
declaration where argc is a count of the number of items on the command line, including the
program name. For the example above, argc is 3. The second parameter, argv, is a pointer to an
array. Element i of that array is a pointer to the i-th string on the command line. In this example,
argv[0] would point to the string ‘’cp’’. Similarly, argv[1] would point to the 5-character string ‘’file1’’
and argv[2] would point to the 5-character string ‘’file2’’. The third parameter of main, envp, is a
pointer to the environment, an main(argc, argv, envp) array of strings containing assignments of

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