Advanced Programming in the UNIX® Environment

ptg10805159

908 Solutions to Selected Exercises Appendix C

process table entry

...

fd1:

fd2:

fd3:

flagsfd pointerfile

file status flags

current file offset

v-node pointer

file status flags

current file offset

v-node pointer

file table

v-node information

v_data

i-node information

current file size

i_vnode

v-node table

Figure C.2 Result ofdupandopen

Iffdis 3, however,after the three calls todup2,four descriptors arepointing to

the same file table entry.Inthis case, we need to close descriptor 3.

3.5 Since the shells process their command line from left to right, the command

./a.out > outfile 2>&1

first sets standardoutput tooutfile and then dupsstandardoutput onto

descriptor 2 (standarderror). Theresult is that standardoutput and standard

error areset to the same file. Descriptors 1 and 2 both point to the same file table

entry.With

./a.out 2>&1 > outfile

however,the dupis executed first, causing descriptor 2 to be the terminal

(assuming that the command is run interactively). Then standardoutput is

redirected to the fileoutfile.The result is that descriptor 1 points to the file

table entry foroutfile,and descriptor 2 points to the file table entry for the

terminal.

3.6 Youcan stilllseekandreadanywhere in the file, but awriteautomatically

resets the file offset to the end of file beforethe data is written. This makes it

impossible towriteanywhereother than at the end of file.

Chapter 4

4.1 Ifstatis called, it always tries to follow a symbolic link (Figure4.17), so the

program will never print a file type of ‘‘symbolic link.’’For the example shown in

the text, where/dev/cdromis a symbolic link to/dev/sr0,statreports that

/dev/cdromis a block special file, not a symbolic link. If the symbolic link

points to a nonexistent file,statreturns an error.

4.2 All permissions areturned off: