Programming FAQ

I think if you just typecast it to (unsigned char), then it should be okay. So, do:

printf("%x", (unsigned char)your_ch);

printf("%02x", (unsigned char)your_ch);

But what if the input is too long? When you call fgets(), you must give a buffer and its size. If the input line you are reading is actually longer than the buffer size, then it doesn't really read a complete line.

One way is to write a wrapper function to read a possibly very very long line. In this function, you call fgets() using a local buffer.

1. You set the last character in the buffer to be '\0' before you call fgets().
2. Call fgets().
3. Check the last character and see if it's still '\0'.
   1. If it's not '\0' and it's not '\n', then you have not finished reading a line yet. So, you allocate a new buffer and copy the data into this new buffer and go back to step (1) above. If there is already an allocated buffer, call realloc() to make it bigger.
   2. Otherwise, you are done. Return the data in an allocated buffer.

If you don't know if you are reading a text file or a binary file, you should always assume that you are reading a binary file.

Also, if you want to know how many bytes you have read, check the return code of fread(). Don't use strlen() because strlen() only works for null-terminated string data.

You should always check the return code of fwrite() to see if it's the same as the number of bytes that you were writing. If not, may be the filesystem is full.

    uint32_t word;

    fwrite(&word, sizeof(uint32_t), 1, fp);

For example, you are reading one byte of data at a time from a file. When you just finished reading the last byte of a file, the EOF condition will not be set, yet. The next time you read from the file, read will return a failure code and the EOF condition will be set.

If the file handle you use is of the type (FILE*), you can use feof() to check if you have reached the end-of-file condition. But you don't have to call feof() to detect the end-of-file condition. If you use fread() to read data from the file handle, you should check the return code. I think if it returns anything less than or equal to zero, then you have reached the end-of-file. (It can also be a read error. But in either case, you should not read any more.)

If the file handle you use is of the type int (i.e., it's a file descriptor), then I'm not sure if there is a function you can use to check the end-of-file condition. If you use read() to read data from the file descriptor, you should check the return code. If it returns anything less than or equal to zero, then you have reached the end-of-file. (It can also be a read error. But in either case, you should not read any more.)

To create a mask, you start with 0x01 and you left shift the right number of bits.

To create a mask, you start with 0x01 and you left shift the right number of bits.

You first start from the last character of the buffer. If it's a whitespace character, you replace it with a null ('\0') character and keep moving toward the front. When you see the first non-whitespace character, you stop. At each step, you should check if you are at the beginning of the buffer just in case there are no non-whitespace characters in the string at all.

After you have removed all the trailing whitespace characters, you start from the beginning of the buffer and look for the first non-whitespace character. If this is the first character, then you are done. Otherwise, you copy the characters, one by one, from the current position to the beginning of the buffer until you have copied a null character.

    program_name Arg1 Arg2 ... [optional stuff] ... Arg_N_minus_2 Arg_N_minus_1

    int main(int argc, char *argv[])
    {
        ...
        return 0;
    }

argv[1] maps to Arg1
argv[2] maps to Arg2
...
argv[argc-2] maps to Arg_N_minus_2
argv[argc-1] maps to Arg_N_minus_1

Let's take the following as an example:

    program {fee|fie|foe} [-o offset] [-m] hostname:port string

    void Usage()
    {
        /* print out usage informaiton, i.e. commandline syntax */
        exit(1);
    }

    int ParseCommandLine(int argc, char *argv[])
    {
        argc--, argv++; /* skip the original argv[0] */
        if (argc <= 0) {
            /* ... print specific error */
            Usage();
        }
        if (strcmp(*argv, "fee") == 0) {
            msg_type = ADR_RQST;
        } else if (strcmp(*argv, "fie") == 0) {
            msg_type = FSZ_RQST;
        } else if (strcmp(*argv, "foe") == 0) {
            msg_type = GET_RQST;
        } else {
            /* ... print specific error */
            Usage();
        }
        for (argc--, argv++; argc > 0; argc--, argv++) {
            if (*argv[0] == '-') {
                if (strcmp(*argv, "-m") == 0) {
                   /* set a global flag */
                } else if (strcmp(*argv, "-o") == 0) {
                    argc--, argv++; /* move past "-o"
                    if (argc <= 0) {
                        /* ... print specific error */
                        Usage();
                    }
                    /* read offset from *argv */
                }
            } else {
                /*
                 * must be "hostname:port" followed by "string"
                 *
                 * the idea here is that you know that you
                 *     are looking for two things
                 * so, the first time you get here, you
                 *     should copy *argv into what holds
                 *     "hostname:port"
                 * the second time you get here, you
                 *     should copy *argv into what holds
                 *     "string"
                 */
            }
        }
    }

Regarding bit/byte data manipulation, there is really not much difference between C and C++.

    ./a.out [file]

    #include <iostream>
    #include <fstream>
    #include <string>
 
    using namespace std;
 
    static
    void Process(istream& in)
    {
        string buf;
        getline(in, buf);
        while (!in.eof()) {
            cout << buf << endl;
            getline(in, buf);
        }
    }
 
    int main(int argc, char *argv[])
    {
        bool reading_from_file=false;
 
        if (argc > 1) {
            reading_from_file = true;
        }
        ifstream in;
 
        if (reading_from_file) {
            in.open(argv[1]);
            if (in.fail()) {
                cerr << "Cannot open " << argv[1] << " for reading." << endl;
                return 0;
            }
            Process(in);
            in.close();
        } else {
            Process(cin);
        }
        return 0;
    }

    #include "stdio.h"
 
    static
    void Process(FILE *fp)
    {
        char buf[1024];

        while(fgets(buf, sizeof(buf), fp) != NULL) {
            /* buf may contain '\n' */
            printf("%s", buf);
        }
    }
 
    int main(int argc, char *argv[])
    {
        int reading_from_file=0;
 
        if (argc > 1) {
            reading_from_file = 1;
        }
        FILE *fp=NULL;
 
        if (reading_from_file) {
            fp = fopen(argv[1], "r");
            if (fp == NULL) {
                fprintf(stderr, "Cannot open %s for reading.\n", argv[1]);
                return 0;
            }
            Process(fp);
            fclose(fp);
        } else {
            Process(stdin);
        }
        return 0;
    }

    prog < /tmp/xyz.txt

    cat /tmp/xyz.txt | prog

Please note that prog will read exactly the same data in both commands if prog is programmed correctly! Basically, if you are using a Unix pipe, functions like rewind(), fseek(), and lseek() do not guarantee to work.

A file pointer, which is of the type (FILE*), is a "wrapper" around a file descriptor. Lots of functions are built around file pointers to make it easier to deal with files.

When a user process is started, file descriptors 0, 1, and 2 are opened by default. File descriptor 0 is associated with the keyboard, and file descriptors 1 and 2 are associated with the display. The corresponding file pointers are also setup by default. The file pointers that's associated with file descriptor 0, 1, 2 are known as stdin, stdout, and stderr, respectively.

Similarly, in C++, cin, cout, and cerr are wrappers around file descriptors 0, 1, and 2, respectively.

It is important to understand that <Cntrl+D> signifies the end-of-input. It does not get turned into a character in the input stream for your program to detect and process. Therefore, your program needs to "detect" end-of-file condition and not "look for the <Cntrl+D> character".

    cat ~csci570b/xyz/file | prog

    prog < ~csci570b/xyz/file

    run < ~csci570b/xyz/file

As a last resort, please check out this solution.

    break foo.c:123

    break foo.c:123 if (x == 98765)

    break foo.c:123 if (strcmp(s,"abc")==0)

        set srcdir=~csci531/public/cs531/hw1
        /bin/rm -f f?.hex f??.hex
        foreach f (0 1 2 3 4 5 6 7 8)
            echo "===> $srcdir/f$f"
            ./hw1 hexdump $srcdir/f$f > f$f.hex
            diff $srcdir/f$f.hex f$f.hex
        end

In each iteration of the "foreach" loop, $f gets set to one of the items in the specified list. In this case, the first iteration throught the "foreach" loop, $f will evaluate to "0". In the 2nd iteration, $f will evaluate to "1", and so on.

The "echo" command just prints the string given to the "echo" command followed by a '\n' (i.e., linefeed) character.

1. Goto http://software.usc.edu/, select your OS and download and install FileZilla and PuTTy.
2. Transfer files from your PC to nunki.usc.edu.
   1. Read the online tutorial for FileZilla.
   2. Start FileZilla: enter nunki.usc.edu as host, your USC user name and password and 22 as port and click on Quickconnect. If this is the first time you connect to nunki using FileZilla, you will get a popup box asking if you want to trust this host, you should click Yes and check the "always trust this host, add this key to cache" checkbox.
   3. For additional features of FileZilla, please read the FileZilla Quick Guide.
   4. Transfer files by dragging and dropping. Please note that FileZilla has its quirks, you need to know what they are to have the files between your PC and nunki stay in sync.
3. Ssh to nunki.usc.edu.
   1. Start putty: enter nunki.usc.edu as hostname, 22 as port, and click on Open. If this is the first time you use putty to connect to nunki, you will get a popup box asking if you want to trust this host, you should click on yes (and the host key will be cached).
   2. Enter your user ID and press the <Enter> key on your keyword. Enter your password and press the <Enter> key on your keyword. Read the output on the screen. You are logged into nunki now. The program you are interacting with on nunki is called a UNIX Shell (or a Login Shell). Your default login shell is usually tcsh.
   3. The Login Shell has a commandline interface. It shows you a command prompt to indicate that it's ready to accept a command from you. Your "current working directory" is your "home directory" by default. To see the content of your current working directory, you can use the "ls" command. To change your current working directory, you can use the "cd" command. If the leading characters of a commandline argument is the "~/", it is interpreted as a reference to your home directory.
   4. If your programming assignment is in ~/cs###/hw1, you can use the following commands to change your current working directory to it, run "make", and run your "hw1" executable:

          cd ~/cs###/hw1
          make hw1
          ./hw1

   5. If you created your Makefile on your PC, "make" will probably fail because of the extra <CR> character added at the end of every line. To remove them, you can do:

          mv Makefile Makefile.dos
          dos2unix Makefile.dos > Makefile

   6. If the programming you are running is an X11-based program, you need to run an X11 server on your PC. You need to download and install X-Win32 from ITS Software.

First, make a backup of your current login scripts. I would use a date to remember when I performed these operations. Assume that today's date is "012914". You should use the actual date when you run these commands.

    cd ~
    cp .login .login.012914
    cp .cshrc .cshrc.012914

    cd ~
    rm .login
    rm .cshrc
    cp /usr/usc/skel/.login .login
    cp /usr/usc/skel/.cshrc .cshrc

    cd ~
    diff -C 2 .login .login.012914
    diff -C 2 .cshrc .cshrc.012914

If all else failed, run the following commands (please copy and paste from this web page and DON'T enter the command manually):

    echo 'source /usr/usc/gnu/gcc/default/setup.csh' >> ~/.login

    echo 'set path=(/usr/usc/bin $path)' >> ~/.login