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Some C implementations do not issue a diagnostic for the violation of this constraint. These noncompliant C translators continue to treat such declarations as implying the type int.

Compliant

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Solution (Implicit int)

This compliant solution explicitly includes a type specifier:

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If a function declaration is not visible at the point at which a call to the function is made, C90-compliant platforms assume an implicit declaration of

extern int funcidentifier();

This implies that the function may take any number and type of arguments and returns a single int.

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

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#include <stdlib.h>
 
int main(void) {
  for (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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Do not declare a function with implicit an implicit return type. 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.

Compliant Solution (Implicit Return Type)

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

Related Guidelines

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