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Implicit declaration of functions is not allowed: ; every function must be explicitly declared before it can be called. In C90, if a function is called without an explicit prototype, the compiler provides an implicit declaration.
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In this noncompliant code example, if malloc() is not declared, either explicitly or by including stdlib.h, a compiler that conforms only to C90 may implicitly declare malloc() as int malloc(). If the platform's size of int is 32 bits, but the size of pointers is 64 bits, the resulting pointer would likely be truncated as a result of the implicit declaration of malloc(), returning a 32-bit integer.
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