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Accessing object with temporary lifetime

Read or write operations on the object are undefined behavior

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Description

This defect occurs when you attempt to read from or write to an object with temporary lifetime that is returned by a function call. In a structure or union returned by a function, and containing an array, the array members are temporary objects. The lifetime of temporary objects ends:

  • When the full expression or full declarator containing the call ends, as defined in the C11 Standard.

  • After the next sequence point, as defined in the C90 and C99 Standards. A sequence point is a point in the execution of a program where all previous evaluations are complete and no subsequent evaluation has started yet.

For C++ code, Accessing object with temporary lifetime raises a defect only when you write to an object with a temporary lifetime.

If the temporary lifetime object is returned by address, no defect is raised.

Risk

Modifying objects with temporary lifetime is undefined behavior and can cause abnormal program termination and portability issues.

Fix

Assign the object returned from the function call to a local variable. The content of the temporary lifetime object is copied to the variable. You can now modify it safely.

Examples

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#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>

#define SIZE6 6

struct S_Array
{
    int t;
    int a[SIZE6];
};

struct S_Array func_temp(void);

/* func_temp() returns a struct value containing
* an array with a temporary lifetime.
*/
int func(void) {
 
/*Writing to temporary lifetime object is
 undefined behavior
 */
    return ++(func_temp().a[0]); 
}

void main(void) {
    (void)func();
}

In this example, func_temp() returns by value a structure with an array member a. This member has temporary lifetime. Incrementing it is undefined behavior.

Correction — Assign Returned Value to Local Variable Before Writing

One possible correction is to assign the return of the call to func_temp() to a local variable. The content of the temporary object a is copied to the variable, which you can safely increment.

 #include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <string.h>

#define SIZE6 6

struct S_Array
{
    int t;
    int a[SIZE6];
};

struct S_Array func_temp(void);

int func(void) {

/* Assign object returned by function call to 
 *local variable
 */
    struct S_Array s = func_temp(); 

/* Local variable can safely be
 *incremented
 */
    ++(s.a[0]);                                           
    return s.a[0];
}

void main(void) {
    (void)func();
}

Result Information

Group: Programming
Language: C | C++
Default: On for handwritten code, off for generated code
Command-Line Syntax: TEMP_OBJECT_ACCESS
Impact: Low

Version History

Introduced in R2018a

See Also

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