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In C99, undefined behavior can result when If control reaches the end of a non-void function, and using the return value of the function call is used. This is almost always a programming error, and can lead to unexpected behaviorresults in undefined behavior. See also undefined behavior 7 of Appendix J.

This rule is related to MSC01-C. Strive for logical completeness.

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This error is frequently diagnosed by compilers (see MSC00-C. Compile cleanly at high warning levels).

Implementation Details

When this noncompliant code example is compiled with /-Wall on most versions of the GCC compiler,

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the following warning is generated

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example.c: In function ‘main’:
example.c:5: warning: control reaches end of non-void function

Compliant Solution

This compliant solution ensures that control never reaches the end of the checkpass function.

int checkpass(char *password) {
  if (strcmp(password, "pass") == 0) {
    return 1;
  }
  return 0;
}
/* ... */
if (checkpass(userinput)) {
  printf("Success!\n");
}

Implementation Details

When this noncompliant code example is compiled with -Wall on most versions of the GCC compiler,

#include <stdio.h>

int main(void) {
  printf("test\n");
}

the following warning is generated

example.c: In function ‘main’:
example.c:5: warning: control reaches end of non-void function

Risk Assessment

Using the return value from a non-void function where control reaches the end of the function can lead to unexpected program behavior, and possibly abnormal program termination.

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