The C11 Standard requires type specifiers and forbids implicit function declarations. The C90 Standard allows implicit typing of variables and functions. Consequently, some existing legacy code uses implicit typing. Some C compilers still support legacy code by allowing implicit typing, but it should not be used for new code. Such an implementation may choose to assume an implicit declaration and continue translation to support existing programs that used this feature.
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Because implicit declarations lead to less stringent type checking, they can introduce unexpected and erroneous behavior. Occurrences of an omitted type specifier in existing code are rare, and the consequences are generally minor, perhaps resulting in abnormal program termination.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
| CERT C Secure Coding Standard | DCL07-C. Include the appropriate type information in function declarators |
| ISO/IEC TR 24772:2013 | Subprogram Signature Mismatch [OTR] |
| MISRA C:2012 | Rule 8.1 (required) |
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