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C no longer allows the absence of type specifiers in a declaration. Subclause 6.7.2 of the C Standard [ISO/IEC 9899:2011] states:

At least one type specifier shall be given in the declaration specifiers in each declaration, and in the specifier-qualifier list in each struct declaration and type name.

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However, to conform with the C Standard, you must explicitly prototype every function before invoking it. An implementation that conforms to the C Standard may or may not perform implicit function declarations. However, C does require the implementation to issue a diagnostic if it encounters an undeclared function being used.

In the following this noncompliant code example, if malloc() is not declared, either explicitly or by including stdlib.h, a compiler that only complies with C90 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 would likely 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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Noncompliant Code Example (Implicit Return Type)

Similarly, do Do not declare a function with implicit return type. If it returns For example, if a function returns a meaningful integer value, declare it int. If it returns no meaningful value, declare it void.

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Because the compiler assumes that foo() returns a value of type int for this noncompliant code example, UINT_MAX is incorrectly converted to −1.

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This compliant solution explicitly defines the return type of foo() as unsigned int:. As a result, the function correctly returns UINT_MAX.

#include <limits.h>
#include <stdio.h>

unsigned int foo(void) {
  return UINT_MAX;
}

int main(void) {
  long long c = foo();
  printf("%lld\n", c);
  return 0;
}

Risk Assessment

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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