Advanced Programming in the UNIX Environment: Second Edition [Electronic resources]

When reading or writing a disk file, are the functions described in this chapter really unbuffered? Explain.

Assume that a process executes the following three function calls:

fd1 = open(pathname, oflags); fd2 = dup(fd1); fd3 = open(pathname, oflags);

Draw the resulting picture, similar to Figure 3.8. Which descriptors are affected by an fcntl on fd1 with a command of F_SETFD? Which descriptors are affected by an fcntl on fd1 with a command of F_SETFL?

The following sequence of code has been observed in various programs:

dup2(fd, 0); dup2(fd, 1); dup2(fd, 2); if (fd > 2) close(fd);

To see why the if test is needed, assume that fd is 1 and draw a picture of what happens to the three descriptor entries and the corresponding file table entry with each call to dup2. Then assume that fd is 3 and draw the same picture.

The Bourne shell, Bourne-again shell, and Korn shell notation

digit1 >&

digit2

says to redirect descriptor

digit1 to the same file as descriptor

digit2 . What is the difference between the two commands

./a.out > outfile 2>&1 ./a.out 2>&1 > outfile

(Hint: the shells process their command lines from left to right.)

If you open a file for readwrite with the append flag, can you still read from anywhere in the file using lseek? Can you use lseek to replace existing data in the file? Write a program to verify this.