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Attempting to compile a program with a function declarator that does not include the appropriate type information typically generates a warning but does not prevent program compilation. These warnings should be resolved. (See MSC00-C. Compile cleanly at high warning levels.)

Noncompliant Code Example (Non-Prototype-Format Declarators)

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Section 6.11.7 of the C Standard [ISO/IEC 9899:2011] states that "the use of function definitions with separate parameter identifier and declaration lists (not prototype-format parameter type and identifier declarators) is an obsolescent feature."

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Declaring a function without any prototype forces the compiler to assume that the correct number and type of parameters have been supplied to a function. This can result in unintended and undefined behavior.

In this noncompliant code example, the definition of func() in file_a.c expects three parameters but is supplied only two.

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C99 eliminated implicit function declarations from the C language. However, many compilers still allow the compilation of programs containing implicitly declared functions, although they may issue a warning message. These warnings should be resolved. (See MSC00-C. Compile cleanly at high warning levels.)

Compliant Solution (Function Prototypes)

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Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

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