Programming Style Guide

• Source code files must be readable by vi and contain only UNIX newlines (only line feeds). In other words, source code files must not contain non-UNIX newlines (line feed and carriage return pairs, e.g., ^M s).
• Assignments must be prepared exclusively using UNIX systems.
• Begin each source file with the following header filled-in appropriately.
  

  /*******************************************************************************
  /
  /      filename:  env.c
  /
  /   description:  Implements the UNIX env utility.
  /
  /        author:  Last, First
  /      login id:  cps444-n1.xx
  /
  /         class:  CPS 444
  /    instructor:  Perugini
  /    assignment:  Homework #1
  /
  /      assigned:  January 18, 2006
  /           due:  January 25, 2006
  /
  /******************************************************************************/
  

• Begin each shell script file with the following header filled-in appropriately.
  

  #*******************************************************************************
  #
  #      filename:  filter
  #
  #   description:  Implements a filter script.
  #
  #        author:  Last, First
  #      login id:  cps444-n1.xx
  #
  #         class:  CPS 444
  #    instructor:  Perugini
  #    assignment:  Homework #1
  #
  #      assigned:  January 18, 2006
  #           due:  January 25, 2006
  #
  #*******************************************************************************
  

• Do not allow any line of code to exceed 80 characters in length. Most text editors have an option to give you column position. Find an appropriate place to break long program statements to continue them on the following line. Break long character strings using string concatenation.
  
  
• Indent all code within a block.
  
  
• Do not use tabs anywhere in your code. For each level of indentation, use three spaces. Tabs cause different amounts of horizontal spacing on different systems. By using spaces (and a fixed-width font), you guarantee your code will be properly indented for every system, editor, and printout.
  
  
• Align corresponding opening and closing braces, begin or ends, or any other program unit delimiters. My preference for curly braces { } (or similar delimiters) is to always place the opening brace on the same line as the block it opens. This makes it easy to see where blocks of code, such as loops, begin and end, and does not waste a line of code. An alternate style is to place each brace on line by itself. You may use either of these styles, but do not mix them. Always be consistent.
  
  
• Use descriptive (variable, constant, procedure, function) identifiers and use appropriate naming conventions for variables (total_sold) and constants (OUNCES_PER_TON). Remember, syntax should imply semantics.

  Cryptic: int x, y, z

  Descriptive: int dollars, average, weight

• Initialize variables (to a value of the appropriate type) before you use them to avoid garbage. This can be done when you declare the variable or with an assignment statement before the variable is used.

  Incorrect: double radius = 3;

  Correct: double radius = 3.0;

• Avoid type mismatches. Following this guideline will make your programs more portable.

  Consider:

  char c;
  
  while ((c = getchar()) != EOF) {
     ...
  }
  

• Do not assign a variable or literal of one type to a variable of another, even if our compilers/interpreters permit it. Following this guideline will make your programs more portable.

  Consider: double avg_score = 76.7; int exam1 = 86;
  
  

  Incorrect: avg_score = exam1;

  Correct: avg_score = static_cast (exam1);
  
  

  Consider: double average = 0.0; int total = 967, num_students = 10;
  
  

  Incorrect: average = total/num_students;

  Correct: average = static_cast (total)/num_students;

• Do not use goto.
  
  
• Avoid the use of global variables.
  
  
• Avoid the use of static variables.
  
  
• Use comments to explain critical subsections or any ambiguous parts of your programs (e.g., a tricky expression).
  
  When writing C code, only use C /* ... */ style comments (even if your compiler supports C++ // ... style comments); this will make your programs more portable.
  
  If supported, do not use multi-line comments where they are likely to make your program less readable.
  
  
• Use named constants rather than magic numbers. This gives you a single point of modification which will save you time and reduce bugs.
  
  
  examples:

  #define SIZE 76
  
  const int NUMBER_OF_RECORDS = 101;
  
  const char A = 'a';
  const char E = 'e';
  const char I = 'i';
  const char O = 'o';
  const char U = 'u';
  
  switch (character) {
      case A:
      case E:
      case I:
      case O:
      case U:
  }
  

• Always use enumerated types where they make your code more readable.
  
  
  example:

  typedef enum { JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC } months;
  
     main() {
        months my_months;
  
        switch (my_months) {
           case JAN:
              ...
              break;
           case FEB:
              ...
              break;
  
           ...
  
           case NOV:
              ...
              break;
           case DEC:
              ...
              break;
        }
     }
  

• Enforce the principle of least privilege.
  
  
• Avoid using local variables with same name in different scopes (they are different variables).
  
  
• Always exit from main with a 0 exit status to indicate success and a non-zero status to indicate failure. Use exit rather than return to make your program more uniform. Use an int as a return type for main.
  
  
  example:

  int main() {
     FILE* fp = NULL;
     char* filename = "input.txt";
  
     if ((fp = fopen (filename, "r")) == NULL) {
        fprintf (stderr, "cannot open %s\n", filename);
        exit (1);
     } else {
          ...
          exit (0);
       }
  

• Always initialize pointer variables.
  
  
  examples:

  Node* node_ptr = NULL;
  
  char* filename = "input.txt";
  
  FILE* myinstream = fopen (filename, "r");
  

• Avoid allocating more memory than necessary for anything.
  
  
• When allocating memory, always verify that the memory was allocated successfully.
  
  
  example:

  if ((node_ptr = malloc (sizeof (Node))) == NULL) {
     fprintf (stderr, "out of memory!");
     exit (1);
  } else {
       ...
       exit (0);
     }
  

• Once finished, always free memory that you explicitly allocated.
  
  
  example:

  if ((node_ptr = malloc (sizeof (Node))) == NULL) {
     fprintf (stderr, "out of memory!");
     exit (1);
  } else {
       ...
       free (node_ptr);
       exit (0);
     }
  

• When opening a file, always verify that the file was opened successfully.
  
  
  example:

  if ((fp = fopen (filename, "r")) == NULL) {
     fprintf (stderr, "cannot open %s\n", filename);
     exit (1);
  } else {
       ...
       exit (0);
     }
  

• Always close files that you explicitly opened.
  
  
  example:

  if ((fp = fopen (filename, "r")) == NULL) {
     fprintf (stderr, "cannot open %s\n", filename);
     exit (1);
  } else {
       ...
       fclose (fp);
       exit (0);
    }
  

• Always print error and debugging messages to stderr (output written to stdout is line buffered).
  
  
  example:

  if ((fp = fopen (filename, "r")) == NULL) {
     fprintf (stderr, "cannot open %s\n", filename);
     exit (1);
  } else {
       ...
    }
  

• Avoid buffer overflows.

  Consider:

  char password[17];
  printf ("Please enter your password: ");
  

  Incorrect: scanf ("%s", password);

  Correct: scanf ("16%s", password);

• Use perror (errno.h) and/or strerror (string.h) to display error messages where appropriate.
  
  
• Make appropriate use of qualifiers such as const, restrict, volatile, and register on function parameters and elsewhere (in the case of const, volatile, and register).
  
  
• Functions:
  
  
  • Always use a procedure/function prototype.
  • Use parameter names in procedure/function prototypes.
  • Use different identifiers for formal parameters and actual parameters to reinforce that they are different variables.
  • Precede every procedure/function with the following header explaining its purpose, the meaning of each parameter, precondition, postcondition, and the general strategy of its implementation, if applicable.
    

    /*******************************************************************************
    /
    / purpose: To compute the factorial of a non-negative integer.
    /
    /******************************************************************************/
    int factorial (int n) {
       if (n == 0) then
          return 1;
       else
          return n*factorial (n-1);
    }
    

  • No routine/subprogram, block, procedure, function, or method (or message) should exceed 50 lines of code.
  
  
• The following guidelines are from UNIX Systems Programming: Concurrency, Communication, and Threads by K.A. Robbins and S. Robbins. Prentice Hall, 2003 (pp. 29-30):

  ``Error handling is a key issue in writing reliable systems programs. When you are writing a function, think in terms of that function being called millions of times by the same application. How do you want the function to behave? In general, functions should never exit on their own, but rather should always indicate an error to the calling program. This strategy gives the caller an opportunity to recover or shut down gracefuly.

  Functions should also not make unexpected changes to the process state that persist beyond the return from the function. For example, if a function blocks signals, it should restore the signal mask to its previous value before returning.

  Finally, the function should release all the hidden resources that it uses during its execution. Suppose a function allocates a temporary buffer by calling malloc and does not free it before returning. One call to this function may not cause a problem, but hundreds or thousands of successive calls may cause the process memory usage to exceed its limits. Usually, a function that allocates memory should either free the memory or make a pointer available to the calling program. Otherwise, a long-running program may have a memory leak; that is, memory "leaks" out of the system and is not available until the process terminates.

  You should also be aware that the failure of a library function usually does not cause your program to stop executing. Instead, the program continues, possibly using inconsistent or invalid data. You must examine the return value of every library function that can return an error that affects the running of your program, even if you think the chance of such an error occurring is remote.

  Your own functions should also engage in careful error handling and communication. Standard approaches to handling errors in UNIX programs include the following.

  • Print out an error message and exit the program (only in main).
  • Return -1 or NULL and set an error indicator such as errno.
  • Return an error code.
  In general, functions should never exit on their own but should always report an error to the calling program. Error messages within a function may be useful during the debugging phase but generaly should not appear in the final version. A good way to handle debugging is to enclose debugging print statements in a conditional compilation block so you can reactivate them if necessary.''
  
  
• The following guidelines are from UNIX Systems Programming: Concurrency, Communication, and Threads by K.A. Robbins and S. Robbins. Prentice Hall, 2003 (pp. 30-31):
  
  ``Most library functions provide good models for implementing functions. Here are some guidelines to follow.
  1. Make use of return values to communicate information and to make error trapping easy for the calling program.
  2. Do not exit from functions. Instead, return an error value to allow the calling program flexibility in handling the error [Explicitly set errno for all errors, and do not rely on the fact that a function which fails may set errno automatically for you. Common errors include exceeding available memory or file I/O open/close, read/write errors. See the GNU webpage for libc for a list error codes which are #defined in error.h (e.g., use ENOMEM for the former and EIO for the latter errors above)].
  3. Make functions general but usable. (Sometimes there are conflicting goals.)
  4. Do not make unnecessary assumptions about sizes of buffers. (This is often hard to implement.)
  5. When it is necessary to use limits, use standard system-defined limits, [e.g., MAX_CANON, #defined in limits.h] rather than arbitrary constants.
  6. Do not reinvent the wheel -- use standard library functions when possible.
  7. Do not modify input parameter values unless it makes sense to do so.
  8. Do not use static variable or dynamic memory allocation if automatic allocation will do just as well.
  9. Analyze all the calls to the malloc family to make sure the program frees the memory that was allocated.
  10. Consider whether a function is ever called recursively or from a signal handler or from a thread. Functions with variables of static storage class may not behave in the desired way. (The error number can cause a big problem here.)
  11. Analyze the consequences of interruptions by signals.
  12. Carefully consider how the entire program terminates.''
  
  
• Shell scripts:
  
  
  • Always terminate with a proper exit statement (0 for success and non-zero for failure).
  • Always start with a proper interpreter directive.
    

    #!/bin/sh
    #!/bin/ksh
    #!/bin/csh
    

• Be consistent in your application of the above guidelines.
  
  
• Overall, write your programs such that they are self-documenting. In other words, structure your code such that the program itself provides its own documentation. Self-documentation means using descriptive identifiers and a consistent, aligned format.