c - 3.1.1 Noncompliant Code Example (Differing Storage Durations)

[nixawk]

3.1.1 Noncompliant Code Example (Differing Storage Durations)

In this noncompliant code example, the address of the variable c_str with automatic storage du- ration is assigned to the variable p, which has static storage duration. The assignment itself is valid, but it is invalid for c_str to go out of scope while p holds its address, as happens at the end of dont_do_this().

#include <stdio.h>

const char *p;
void dont_do_this(void) {
    const char c_str[] = "This will change";
    p = c_str; /* Dangerous */
} 

void innocuous(void) {
     printf("%s\n", p);
}

int main(void) {
    dont_do_this();
    innocuous();
    return 0;
}

[nixawk]

3.1.2 Compliant Solution (Same Storage Durations)

In this compliant solution, p is declared with the same storage duration as c_str, preventing p from taking on an indeterminate value outside of this_is_OK():

void this_is_OK(void) {
    const char c_str[] = "Everything OK";
    const char *p = c_str;
    /* ... */
}
/* p is inaccessible outside the scope of string c_str */

[nixawk]

3.1.3 Compliant Solution (Differing Storage Durations)

If it is necessary for p to be defined with static storage duration but c_str with a more limited duration, then p can be set to NULL before c_str is destroyed. This practice prevents p from tak- ing on an indeterminate value, although any references to p must check for NULL.

const char *p;
void is_this_OK(void) {
    const char c_str[] = "Everything OK?";
    p = c_str;
    /* ... */
    p = NULL;
}