Do not access or modify the result of a function call after a subsequent sequence point. According to C99 Section 6.5.2.2, "Function calls":

If an attempt is made to modify the result of a function call or to access it after the next sequence point, the behavior is undefined.

Non-Compliant Code Example

In C, the lifetime of a return value ends at the next sequence point.

#include <stdio.h>

struct X { char a[6]; };

struct X addressee() {
  struct X result = { "world" };
  return result;
}

int main(void) {
  printf("Hello, %s!\n", addressee().a);
  return 0;
}

Because there is a sequence point after the call to addressee() and before the call to {{printf()]], this program has
undefined behavior.

Implementation Details

This code compiles cleanly and runs without error under Microsoft Visual C++ Version 8.0. On gcc version 4.1, the program compiles with a warning when the -Wall switch is used and execution on Linux results in a segmentation fault.

Compliant Solution

This compliant solution does not have undefined behavior because the reference to the returned value is evaluated before the subsequent sequence point.

#include <stdio.h>

struct X { char a[6]; };

struct X addressee() {
  struct X result = { "world" };
  return result;
}

int main(void) {
  printf(
    "Hello, %c!\n", 
    addressee().a[0],
    addressee().a[1],
    addressee().a[2],
    addressee().a[3],
    addressee().a[4]
  );
  return 0;
}

Risk Assessment

Examples of vulnerabilities resulting from the violation of this rule can be found on the CERT website.

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]] Section 6.5.2.2, "Function calls"