104 lines
3.6 KiB
Text
104 lines
3.6 KiB
Text
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__________________
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LAB 05 QUESTIONS
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__________________
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- Name: Michael Zhang
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- NetID: zhan4854
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Answer the questions below according to the lab specification. Write
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your answers directly in this text file and submit it to complete Lab01.
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PROBLEM 1 `dirops.c'
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====================
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A
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~
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Examine the source code of `dirops.c' closely. It makes use of a
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variety of system calls to produce a semi-interesting effect. Compile
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and run it several times. Describe the overall intent of the program
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based on its output and the code you understand.
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It lists the files in the current directory, their files sizes, determines the
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largest file in the directory, and then copies that to a new file
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"<filename>.copy". An observation I made was that the directories . and .. are
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fixed at 4096 bytes.
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B ~
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What set of system calls is used by the program to determine all the
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files in the current directory? Describe briefly how these calls work
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together.
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readdir returns the next entry in the directory listing.
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C
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~
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Identify the system call that `dirops.c' uses to find the sizes of
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files. Describe briefly how this call works.
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This line: stat(file->d_name, &sb); retrieves information from the operating
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system about how the file is stored on the disk, including inode numbers,
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ownership, file size, and so on.
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D
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~
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The following line sets up the read/write permissions that the copied
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file will have.
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,----
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| mode_t perms = S_IRUSR | S_IWUSR;
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`----
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Modify this line so that the copied file is readable by the group as
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well as the user.
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/Optional challenge:/ Set the permissions to be identical to the
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original file. `stat()' is one way to find out the permissions for the
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original file.
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I found the list of user permissions in the 'stat' manpage: S_IRGRP. The
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modified line looks like:
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mode_t perms = S_IRUSR | S_IWUSR | S_IRGRP;
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In order to find out what the original permissions were, another stat call must
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be performed on the file with the largest size. Ideally, this should be done by
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file descriptor, but since we are traversing the directory, it's impractical to
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keep all the file descriptors open until we identify the largest file. Here's
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how I would do it:
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struct stat s;
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stat(max_name, &s);
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mode_t perms = s.st_mode;
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E
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~
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`dirops.c' contains a subtle bug in the following bit of code towards
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the end of the file.
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,----
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| while( (nbytes = read(infd, buf, BUFSIZE)) > 0){
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| write(outfd, buf, BUFSIZE);
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| }
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`----
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You should observe that every time program is run, it will identify a
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copied file as the largest and make another copy due to this bug. It
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may help to examine the ends of the copied files with the `tail
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file.txt.copy' command which will show the last 10 lines.
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Explain what is wrong with the loop and paste a fixed version below.
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The problem is that the buffer is full before the final loop, which may be less
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than BUFSIZE bytes, but the write command is still writing BUFSIZE bytes. Sicne
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there's no mechanism to automatically clear out the remaining bytes, they get
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written to the file as well. The solution is to only write the number of bytes
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that have been read, since it was stored anyways but not used.
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44 while ((nbytes = read(infd, buf, BUFSIZE)) > 0) {
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45 write(outfd, buf, nbytes);
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46 }
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