The Linux Programming Interface

Solutions to Selected Exercises 1429

since it is interpreted relative to the location of the link file, and thus refers to a

nonexistent file in the parent directory. Consequently, chmod() fails with the error

ENOENT (“No such file or directory”). (A complete program containing the code of

this exercise is available in the file dirs_links/bad_symlink.c in the source code

distribution for this book.)

18-4. A solution is provided in the file dirs_links/list_files_readdir_r.c, which can be

found in the source code distribution for this book.

18-7. A solution is provided in the file dirs_links/file_type_stats.c, which can be found

in the source code distribution for this book.

18-9. Using fchdir() is more efficient. If we are performing the operation repeatedly

within a loop, then with fchdir() we can perform one call to open() before executing

the loop, and with chdir() we can place the getcwd() call outside the loop. Then we

are measuring the difference between repeated calls to fchdir(fd) and chdir(buf).

Calls to chdir() are more expensive for two reasons: passing the buf argument to the

kernel requires a larger data transfer between user space and kernel space, and the

pathname in buf must be resolved to the corresponding directory i-node on each

call. (Kernel caching of directory entry information reduces the work required for

the second point, but some work must still be done.)

Chapter 20

20-2. A solution is provided in the file signals/ignore_pending_sig.c in the source code

distribution for this book.

20-4. A solution is provided in the file signals/siginterrupt.c in the source code

distribution for this book.

Chapter 22

22-2. As with most UNIX implementations, Linux delivers standard signals before

realtime signals (SUSv3 doesn’t require this). This makes sense, because some

standard signals indicate critical conditions (e.g., hardware exceptions) that a

program should deal with as soon as possible.

22-3. Replacing sigsuspend() plus a signal handler with sigwaitinfo() in this program

provides a 25 to 40 percent speed improvement. (The exact figure varies somewhat

across kernel versions.)

Chapter 23

23-2. A modified program using clock_nanosleep() is provided in the file timers/

t_clock_nanosleep.c in the source code distribution for this book.

23-3. A solution is provided in the file timers/ptmr_null_evp.c in the source code

distribution for this book.

Chapter 24

24-1. The first fork() call creates one new child. Both parent and child carry on to execute

the second fork(), and thus each creates a further process, making a total of four