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The C11 Standard [ISO/IEC 9899:2011] introduced a new term: temporary lifetime. This term still remains in the C23 Standard. Modifying an object with temporary lifetime is undefined behavior. According to subclause 6.2.4, paragraph 8 [ISO/IEC 9899:2024]

A non-lvalue expression with structure or union type, where the structure or union contains a member with array type (including, recursively, members of all contained structures and unions) refers to an object with automatic storage duration and temporary lifetime. Its lifetime begins when the expression is evaluated and its initial value is the value of the expression. Its lifetime ends when the evaluation of the containing full expression ends. Any attempt to modify an object with temporary lifetime results in undefined behavior.

This definition differs from the C99 Standard (which defines modifying

C99 Section 6.5.2.2 says \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\],

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the result of a function call or

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accessing it after the next sequence point as undefined behavior) because a temporary object's lifetime ends when the evaluation containing the full expression or full declarator ends,

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so the result of a function call can be accessed. This extension to the lifetime of a temporary also removes a quiet change to C90 and improves compatibility with C++. 

C functions may not return arrays; however, functions can return a pointer to an array or a  struct or union that contains arrays. Consequently, in any version of C, if a function call returns by value a struct or union containing an array, do not modify those arrays within the expression containing the function call. In C99 and older, do not access an array returned by a function after the next sequence point or after the evaluation of the containing full expression or full declarator ends.

Noncompliant Code Example

This noncompliant code example conforms to the C11 Standard; however, it fails to conform to C99. If compiled with a C99-conforming implementation, this code has undefined behavior 34 because the sequence point preceding the call to printf() comes between the call and the access by printf() of the string in the returned object

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[68]; };

struct X salutation(void) {
  struct X result = { "Hello" };
  return result;
}

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

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

In C, the lifetime of a return value ends at the next sequence point. This program has undefined behavior because despite a sequence point before printf() is called, there is an attempt to to access the return value of addressee().

Implementation Details

This code compiles cleanly and runs without error under Microsoft Visual C++ Version 8.0. On GCC Compiler 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

Compliant Solution (C11 and newer)

This compliant solution checks __STDC_VERSION__ to ensure that a pre-C11 compiler will fail to compile the code, rather than invoking undefined behavior.

#include <stdio.h>

#if __STDC_VERSION__ < 201112L
#error This code requires a compiler supporting the C11 standard or newer
#endif

struct X { char a[8]; };

struct X salutation(void) {
  struct X result = { "Hello" };
  return result;
}

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

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

Compliant Solution

This compliant solution stores the structures returned by the call to addressee() before calling the printf() function. Consequently, this program conforms to both C99 and C11.

#include <stdio.h>

struct X { char a[8]; };
 
struct X salutation(void) {
  struct X result = { "Hello" };
  return result;
}

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

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

Noncompliant Code Example

This noncompliant code example attempts to retrieve an array and increment the array's first value. The array is part of a struct that is returned by a function call. Consequently, the array has temporary lifetime, and modifying the array is undefined behavior in both C99 and C11.

#include <stdio.h>

struct X { int a[6]; };

struct X addressee(void) {
  struct X result = { { 1, 2, 3, 4, 5, 6 } };
  return result;
}

int main(void) {
  printf("%x", ++(addressee().a[0]));
  return 0;
}

Compliant Solution

This compliant solution stores the structure returned by the call to addressee() as my_x before calling the printf() function. When the array is modified, its lifetime is no longer temporary but matches the lifetime of the block in main().

#include <stdio.h>

struct X { int a[6]; };

struct X addressee(void) {
  struct X result = { { 1, 2, 3, 4, 5, 6 } };
  return result;
}

int main(void) {
  struct X my_x = addressee();
  printf("%x", ++(my_x.a[0]));
  return 0;
}

Noncompliant Code Example

This noncompliant code example attempts to save a pointer to an array that is part of a struct that is returned by a function call. Consequently, the array has temporary lifetime, and using the pointer to it outside of the full expression is undefined behavior in both C99 and C11.

#include <stdio.h>

struct X { int a[6]; };

struct X addressee(void) {
  struct X result = { { 1, 2, 3, 4, 5, 6 } };
  return result;
}

int main(void) {
  int *my_a = addressee().a;
  printf("%x", my_a[0]);
  return 0;
}

Compliant Solution

This compliant solution stores 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() functionsaving a pointer to its array member. When the pointer is used, its lifetime is no longer temporary but matches the lifetime of the block in main().

#include <stdio.h>

struct X { charint a[6]; };

struct X addressee(void) {
  struct X result = { { "world" 1, 2, 3, 4, 5, 6 } };
  return result;
}

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

Risk Assessment

Attempting to access or modify the result of a function call after a subsequent sequence point modify an array or access it after its lifetime expires may result in unexpected and perhaps unintended erroneous program behavior.

Automated Detection

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Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

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Bibliography

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Image Added Image Added Image Added "Function calls" \[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "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