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

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;
}

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() {
  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.

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"

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