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if (foo() != 0) ...

is preferable to

if (foo()) ...

despite the convention that 0 indicates failure. Explicitly testing for inequality with 0 benefits maintainability if foo() is later modified to return −1 rather than 0 on failure.

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1. Functions return 0 if false and nonzero if true [StackOvflw 2009 AA. Bibliography#StackOvflw 09].
2. Function failures can typically be indicated by −1 or any nonzero number.
3. Comparison functions (such as the standard library function strcmp(), which has a trinary return value) return 0 if the arguments are equal and nonzero otherwise (see strcmp function).

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LinkedList bannedUsers;

int is_banned(User usr) {
  int x = 0;

  Node cur_node = (bannedUsers->head);

  while (cur_node != NULL) {
    if(!strcmp((char *)cur_node->data, usr->name)) {
      x++;
    }
    cur_node = cur_node->next;
  }

  return x;
}

void processRequest(User usr) {
  if(is_banned(usr) == 1) {
    return;
  }
  serveResults();
}

If a banned user is listed twice, the user is granted access. Although is_banned() follows the common convention of returning nonzero for true, processRequest checks for equality only with 1.

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Noncompliant Code Example

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int validateUser(User usr) {
  if(listContains(validUsers, usr)) {
    return 1;
  }

  return 0;
}

void processRequest(User usr, Request request) {
  if(!validateUser(usr)) {
    return "invalid user";
  }
  else {
    serveResults();
  }
}

Although the code will work as intended, it is possible that a future modification will result in the following:

errno_t validateUser(User usr) {
  if(list_contains(allUsers, usr) == 0) {
    return 303; /* User not found error code */
  }
  if(list_contains(validUsers, usr) == 0) {
    return 304; /* Invalid user error code */
  }

  return 0;
}

void processRequest(User usr, Request request) {
  if(!validateUser(usr)) {
    return "invalid user";
  }
  else {
    serveResults();
  }
}

In this code, the programmer intended to add error code functionality to indicate the cause of a validation BB. Definitions#validation failure. The new code, however, validates any invalid or nonexisting user. Because there is no explicit test in processRequest(), the logical error is not obvious and seems correct by certain conventions.

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int validateUser(User usr) {
  if(list_contains(validUsers, usr)) {
    return 1;
  }

  return 0;
}

void processRequest(User usr, Request request) {
  if(validateUser(usr) == 0) {
    return "invalid user";
  }
  else {
    serveResults();
  }
}

Noncompliant Code Example

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void login(char *usr, char *pw) {
  User user = find_user(usr);
  if (!strcmp((user->password),pw)) {
    grantAccess();
  }
  else {
    denyAccess("Incorrect Password");
  }
}

The preceding code works correctly. However, to simplify the login code or to facilitate checking a user's password more than once, a programmer can separate the password-checking code from the login function in the following way:

In an attempt to leave the previous logic intact, the developer just replaces strcmp() with a call to the new function. However, doing so produces incorrect behavior. In this case, any user who inputs an incorrect password is granted access. Again, two conventions conflict and produce code that is easily corrupted when modified. To make code maintainable and to avoid these conflicts, such a result should never be defaulted.

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Risk Assessment

Code that does not conform to the common practices presented is difficult to maintain. Bugs can easily arise when modifying helper functions that evaluate true/false or success/failure. Bugs can also easily arise when modifying code that tests for equality using a comparison function that obeys the same conventions as standard library functions such as strcmp.

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Bibliography

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