DCL21-C. Understand the storage of compound literals

Subclause 6.5.2.5 of the C Standard [ISO/IEC 9899:2011] defines a compound literal as

A postfix expression that consists of a parenthesized type name followed by a brace-enclosed list of initializers. . . . The value of the compound literal is that of an unnamed object initiated by the initializer list.

The storage for this object is either static (if the compound literal occurs at file scope) or automatic (if the compound literal occurs at block scope), and the storage duration is associated with its immediate enclosing block. For example, in the function

void func(void) {
  int *ip = (int[4]){1,2,3,4};
  /* ... */
}

following initialization, the int pointer ip contains the address of an unnamed object of type int[4], allocated on the stack. Once func returns, any attempts to access this object will produce undefined behavior.

Note that only one object is created per compound literal—even if the compound literal appears in a loop and has dynamic initializers.

This recommendation is a specific instance of DCL30-C. Declare objects with appropriate storage durations.

Noncompliant Code Example

In this noncompliant code example, the programmer mistakenly assumes that the elements of the ints array of the pointer to int_struct are assigned the addresses of distinct int_struct objects, one for each integer in the range [0, MAX_INTS - 1]:

#include <stdio.h>

typedef struct int_struct {
  int x;
} int_struct;

#define MAX_INTS 10

int main(void){
  size_t i;
  int_struct *ints[MAX_INTS];

  for (i = 0; i < MAX_INTS; i++) {
    ints[i] = &(int_struct){i};
  }

  for (i = 0; i < MAX_INTS; i++) {
    printf("%d\n", ints[i]->x);
  }
 
  return 0;
}

However, only one int_struct object is created. At each iteration of the first loop, the x member of this object is set equal to the current value of the loop counter i. Therefore, just before the first loop terminates, the value of the x member is MAX_INTS - 1.

Because the storage duration of the compound literal is associated with the for loop that contains it, dereferencing ints in the second loop results in undefined behavior 9 (Annex J of the C Standard).

Even if the region of memory that contained the compound literal is not written to between loops, the print loop will display the value MAX_INTS - 1 for MAX_INTS lines. This is contrary to the intuitive expected result, which is that the integers 0 through MAX_INTS - 1 would be printed in order.

Compliant Solution

This compliant solution uses an array of structures rather than an array of pointers. That way, an actual copy of each int_struct (rather than a pointer to the object) is stored.

#include <stdio.h>

typedef struct int_struct {
  int x;
} int_struct;

#define MAX_INTS 10

int main(void){
  size_t i;
  int_struct ints[MAX_INTS];

  for (i = 0; i < MAX_INTS; i++) {
    ints[i] = (int_struct){i};
  }

  for (i = 0; i < MAX_INTS; i++) {
    printf("%d\n", ints[i].x);
  }
 
  return 0;
}

Risk Assessment

Automated Detection

Bibliography