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Implicit declaration of functions is not allowed: every function should be explicitly declared before it can be called. In C89, if a function is called without an explicit prototype, the compiler provides an implicit declaration.

The C90 Standard Standard [ISO/IEC 9899:1990] includes this requirement:

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In the following noncompliant code example, if malloc() is not declared, either explicitly or by including stdlib.h, a compiler may implicitly declare malloc() as int malloc(). (Compilers that comply only with C90 are required to provide an implicit declaration of malloc().) If the platform's size of int is 32 bits, but the size of pointers is 64 bits, the resulting pointer could be truncated as a result of the implicit declaration of malloc() returning a 32-bit integer.

/* #include <stdlib.h> is missing */
 
int main(void) {
  size_t i;
  for (i = 0; i < 100; ++i) {
    char *ptr = (char*)malloc(0x10000000); /* int malloc() assumed */
    *ptr = 'a';
  }
  return 0;
}

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Bibliography

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