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In particular, do not default the test for non-zerononzero. For instance, suppose a foo() function returns 0 to indicate failure , or a non-zero nonzero value to indicate success. Testing for inequality with zero:0,

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

is preferable to

if (foo()) ...

despite the convention that 0 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

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1. and

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1. nonzero if true [

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1. StackOvflw 2009].

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1. Function failures can typically be indicated by

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1. −1 or any nonzero number.

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1. Comparison functions

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1. (such as the standard library function strcmp(), which has a trinary return value

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1. ) return 0 if the arguments are equal and 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();
}

...

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 zero0, there are  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 foovalidateUser() 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 no explicit test in processRequest(), the logical error is not obvious and seems correct by certain conventions.

Compliant Solution

The following This compliant code sample is preferable for 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 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 strcmp() function) return 0 if the arguments are equal and non-zero nonzero otherwise.

Because many comparison functions 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 a function testing for equality, but the programmer did not following follow the same convention as strcmp(), they 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_user(usr);
  if (!strcmp((user->password),pw_given)) {
    grantAccess();
  }
  else {
    denyAccess("Incorrect Password");
  }
}

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

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

}

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

In an attempt to leave the previous logic intact, the developer just replaces the strcmp() with a call to their the new function. However, doing so would produce produces incorrect behavior. In the this case above, any user which 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 *usr, char *pw) {
  User user = find_user(usr);
  if (strcmp((user->password),pw_given) == 0) {
    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.

References

Automated Detection

Bibliography

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Image Added Image Added Image Added Wiki Markup[\[StackOvflw 09\]|AA. References#StackOvflw 09] "Should I return TRUE / FALSE values from a C function? \[[ISO/IEC 9899:1999|AA. References#ISO/IEC 9899-1999]\]