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The C Language standard library facilities setjmp() and longjmp() can be used as a poor attempt to simulate the throwing and catching of exceptions, but they are very low-level facilities, and using them can bypass proper resource management and the proper calling of destructors.

Non-Compliant Code Example

Compliant Solution

. However, these facilities bypass automatic resource management and can result in undefined behavior, commonly including resource leaks and denial-of-service attacks.

The C++ Standard, [support.runtime], paragraph 4 [ISO/IEC 14882-2014], states the following:

The function signature longjmp(jmp_buf jbuf, int val) has more restricted behavior in this International Standard. A setjmp/longjmp call pair has undefined behavior if replacing the setjmp and longjmp by catch and throw would invoke any non-trivial destructors for any automatic objects.

Do not call setjmp() or longjmp(); their usage can be replaced by more standard idioms such as throw expressions and catch statements.

Noncompliant Code Example

If a throw expression would cause a nontrivial destructor to be invoked, then calling longjmp() in the same context will result in undefined behavior. In the following noncompliant code example, the call to longjmp() occurs in a context with a local Counter object. Since this object’s destructor is nontrivial, undefined behavior results.

#include <csetjmp>
#include <iostream>

static jmp_buf env;

struct Counter {
  static int instances;
  Counter() { ++instances; }
  ~Counter() { --instances; }
};

int Counter::instances = 0;

void f() {
  Counter c;
  std::cout << "f(): Instances: " << Counter::instances << std::endl;
  std::longjmp(env, 1);
}

int main() {
  std::cout << "Before setjmp(): Instances: " << Counter::instances << std::endl;
  if (setjmp(env) == 0) {
    f();
  } else {
    std::cout << "From longjmp(): Instances: " << Counter::instances << std::endl;
  }
  std::cout << "After longjmp(): Instances: " << Counter::instances << std::endl;
}

Implementation Details

The above code produces the following results when compiled with Clang 3.8 for Linux, demonstrating that the program, on this platform, fails to destroy the local Counter instance when the execution of f() is terminated. This is permissible as the behavior is undefined.

Before setjmp(): Instances: 0
f(): Instances: 1
From longjmp(): Instances: 1
After longjmp(): Instances: 1

Compliant Solution

This compliant solution replaces the calls to setjmp() and longjmp() with a throw expression and a catch statement.

#include <iostream>

struct Counter {
  static int instances;
  Counter() { ++instances; }
  ~Counter() { --instances; }
};

int Counter::instances = 0;

void f() {
  Counter c;
  std::cout << "f(): Instances: " << Counter::instances << std::endl;
  throw "Exception";
}

int main() {
  std::cout << "Before throw: Instances: " << Counter::instances << std::endl;
  try {
    f();
  } catch (const char *E) {
    std::cout << "From catch: Instances: " << Counter::instances << std::endl;
  }
  std::cout << "After catch: Instances: " << Counter::instances << std::endl;
}

This solution produces the following output.

Before throw: Instances: 0
f(): Instances: 1
From catch: Instances: 0
After catch: Instances: 0

Risk Assessment

Using setjmp() and longjmp() could lead to a denial-of-service attack due to resources not being properly destroyed.

References

\[[Henricson 97|AA. C++ References#Henricson 97]\] Rule 13.3 Do not use {{setjmp()}} and {{longjmp()}}.

Automated Detection

Related Vulnerabilities

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

Bibliography

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Image Added Image Added Image Added MEM44-CPP. Do not leak resources when throwing exceptions      08. Memory Management (MEM)      09. Input Output (FIO)