EXP35-C. Do not access or modify the result of a function call after a subsequent sequence point

C99 Section 6.5.2.2 says [[ISO/IEC 9899:1999]],

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.

As a result, the result of a function call after a subsequent access point must never be accessed or modified.

Non-Compliant Code Example

The following non-compliant code attempts to retrieve a field from a struct that is returned by a function call.

#include <stdio.h>

struct X { char a[6]; };

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

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

In C, the lifetime of a return value ends at the next sequence point. This program has undefined behavior because there is a sequence point before printf() is called, and printf() accesses the result of the call to addressee().

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 structure returned by the call to addressee() is stored as the variable my_x before calling the printf() function.

#include <stdio.h>

struct X { char a[6]; };

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

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

Risk Assessment

Attempting to access or modify the result of a function call after a subsequent sequence point may result in unexpected and perhaps unintended program behavior.

Automated Detection

Splint Version 3.1.1 can detect violations of this rule.

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 6.5.2.2, "Function calls"
[[ISO/IEC PDTR 24772]] "DCM Dangling references to stack frames" and "SAM Side-effects and order of evaluation"

EXP34-C. Ensure a null pointer is not dereferenced      03. Expressions (EXP)       EXP36-C. Do not convert pointers into more strictly aligned pointer types