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Perform explicit tests to determine success, true/false, and equality to improve the readability and maintainability of code and for compatibility with common conventions.

In the case that the return value will never change, an explicit test is still preferable to clearly communicate the numeric, rather than boolean, value of the test.In particular, do not default the test for non-zerononzero. For example, testing instance, suppose a foo() function returns 0 to indicate failure or a nonzero value to indicate success. Testing for inequality with zero:0,

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

is preferable to

if (foo()) ...

despite the fact convention that C considers 0 to be false0 indicates failure. Explicitly testing for inequality with zero 0 benefits maintainability if foo() is later modified to return -1 −1 rather than 0 on failure.

This recommendation is derived from and considers the implications of the following common conventions:(1)

1. Functions return 0 if false

...

1. and

...

1. nonzero if true [

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1. StackOvflw 2009].

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1. Function failures can typically be indicated by

...

1. −1 or any nonzero number.

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1. Comparison functions (such as the standard library function strcmp()

...

1. , which has a trinary return value) return 0 if the arguments are equal and

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1. nonzero otherwise (see strcmp function).

Noncompliant Code Example

In this noncompliant code example, is_banned() return returns 0 if false , and non-zero nonzero if true.:

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();
}

The call function processRequest() ignores the convention that most functions in C only guarantee a non-zero return value to indicate "true".

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

Compliant Solution

Because most functions only guarantee a return value of non-zero nonzero only for " true, " the the preceding code above is better written by checking for inequality with 0 ("false"), as follows.:

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)==0) {
      x++;
    }
    cur_node = cur_node->next;
  }

  return x;
}

void processRequest(User usr) {
  if (is_banned(usr) != 0) {
    return;
  }
  serveResults();
}

Noncompliant Code Example

In this noncompliant code example, function failures status can typically be indicated by one of the following return values: -1, a non-zero numberreturning −1 on failure or any nonnegative number on success. This is a common convention in the standard C library, but it is discouraged in ERR02-C. Avoid in-band error indicators.

Although failures are frequently indicated by a return value of zero (which C considers to be false), there are 0, some common conventions that may conflict in the future with code where in which the test for non-zero nonzero is not explicit. In this case, defaulting the test for non-zero nonzero welcomes bugs if and when a developer modifies foo validateUser() to return an error code or -1 −1 rather than 0 to indicate a failure (all of which are also common conventions). 

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();
  }
}

The Although the code above will work as intended. However, it is very feasible possible that a future modification will result in the following.:

errno_t validateUser(User usr) {
  if(list_contains(allUsers, usr) == 0) {
    return 303;  //* userUser not found error code */
  }
  if(list_contains(validUsers, usr) == 0) {
    return 304; //* invalidInvalid 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 failure. The new code, however, validates any invalid or non-existing nonexisting user. Because there is not an no explicit test in processRequest(), the logical error is not obvious (the implied "if not validated" seems correct) and seems correct by certain conventions.

Compliant Solution

The following This compliant code is preferable for code improved maintenance. By defining what constitutes a failure and explicitly testing for it, the behavior is clearly implied, and future modifications are more likely to preserve it. If there is a future modification like the one aboveis made, such as in the previous example, it is immediately obvious that the if statement in processRequest() does not correctly utilize the specification of validateUser correctly().

int
errno_t 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

Comparison functions (such as the standard library function strcmp() function) return 0 if the arguments are equal and non-zero nonzero otherwise.

Because many comparison functions (like strcmp) return 0 for equality and non-zero nonzero for inequality, they can cause confusion when used to test for equality. If someone were to switch the following strcmp() call with an equals a function , they testing for equality, but the programmer did not follow the same convention as strcmp(), the programmer might instinctively just replace the function name. Also, when quickly reviewed, the code could easily appear to test for inequality.

void login(char *usr, char *pw)
 {
  User user = find_Useruser(usr);
  if (!strcmp((user->password),pw_given)) {
    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:

int check_password(charUser *usruser, char *pw_given)
{
  User user = find_User(usr); {
  if (!strcmp((user->password),pw_given)) {
    return 1;
  }
  return 0;

}

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

In an attempt to leave the previous logic intact, the developer just replaces strcmp() with a call to the new function. However, doing so would produce incorrect behavior. As a resultproduces 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.

Compliant Solution

The following approach to This compliant solution, using a comparison function for this purpose, is the preferred approach. By performing an explicit test, any programmer who wishes to modify the equality test can clearly see the implied behavior and convention that is being followed.

void login(
char *str1;usr, 
char *str2;

pw) {
  User user = find_user(usr);
  if (strcmp(str1, str2(user->password),pw) == 0) {
    return "strings are equal";
}

References

[1] "Should I return TRUE / FALSE values from a C function?"

grantAccess();
  }
  else {
    denyAccess("Incorrect Password");
  }
}

Risk Assessment

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

Automated Detection

Bibliography

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