check string lifetime duration

I am currently wondering if it is possible to check if a C-string has a static lifetime duration or not when only the pointer to the string is known.

Basically, I want to implement such a function:

bool isStaticString(const char *str)
{
//do magic stuff
}


and the results should be like this:

isStaticString("Hello World!"); //returns true

char str[5] = {''};
isStaticString(str); //returns false


Is there any portable way to implement such a function?

PS: I hope such question was not already asked,I did not really know what to search for. My title did not give me any results.

const

const

1 Answer
1

It is impossible to do in a portable or reliable way.

However, pointers to static and read-only memory will typically be in a different memory segments than stack and dynamically-allocated memory, so you can check the difference between your pointer and some pre-allocated sample pointer, and kind of guess the memory layout.

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

enum { CHAR_ARRAY_SIZE = 32 }; // Say no to magic constants

const char readonlyFossilizedBirdBones[CHAR_ARRAY_SIZE] = "Crow skeleton from taxidermist";
char globalVariableBirdBones[CHAR_ARRAY_SIZE] = "This pigeon was hit by a car";

bool askBirdBonesIfStringIsStatic(const char *str)
{
char freshStackBirdBones[CHAR_ARRAY_SIZE] = "Chicken wings from trash bin";
char * bloodyAllocBirdBones = (char *) malloc(CHAR_ARRAY_SIZE);
// strcpy() is unsafe, and strncpy() does not always zero-terminate the result
// but snprintf() is safe and will not trigger any static code analysis tools
snprintf(bloodyAllocBirdBones, CHAR_ARRAY_SIZE, "%s", "Sparrow caught by your cat");
bool divinationSaysStatic =
abs(readonlyFossilizedBirdBones - str) < abs(freshStackBirdBones - str) &&
abs(readonlyFossilizedBirdBones - str) < abs(bloodyAllocBirdBones - str) &&
abs(globalVariableBirdBones - str) < abs(freshStackBirdBones - str) &&
abs(globalVariableBirdBones - str) < abs(bloodyAllocBirdBones - str);
printf("Asking bird bones about '%s' at %p:n", str, str);
printf("Comparing to readonly %p global %p stack %p alloc %pn",
readonlyFossilizedBirdBones, globalVariableBirdBones,
freshStackBirdBones, bloodyAllocBirdBones);
printf("Result is %snn", divinationSaysStatic ? "favorable" : "unfavorable");
free(bloodyAllocBirdBones);
return divinationSaysStatic;
}

int main()
{
static char s1[CHAR_ARRAY_SIZE] = "Static string";
char s2[CHAR_ARRAY_SIZE] = "Short-lived stack string";
char * s3 = (char *) malloc(CHAR_ARRAY_SIZE);
snprintf(s3, CHAR_ARRAY_SIZE, "%s", "String allocated by malloc()");
askBirdBonesIfStringIsStatic(s1);
askBirdBonesIfStringIsStatic(s2);
askBirdBonesIfStringIsStatic(s3);
free(s3);
return 0;
}


And the results on my 64-bit Linux PC:

Asking bird bones about 'Static string' at 0x560815f27080:
Comparing to readonly 0x560815d26b00 global 0x560815f27060 stack 0x7ffe741d0b90 alloc 0x560816365c50
Result is favorable

Asking bird bones about 'Short-lived stack string' at 0x7ffe741d0be0:
Comparing to readonly 0x560815d26b00 global 0x560815f27060 stack 0x7ffe741d0b90 alloc 0x560816365c50
Result is unfavorable

Asking bird bones about 'String allocated by malloc()' at 0x560816365c20:
Comparing to readonly 0x560815d26b00 global 0x560815f27060 stack 0x7ffe741d0b90 alloc 0x560816365c50
Result is unfavorable


Please never ever use this code in production.

readonlyFossilizedBirdBones - str

int

abs()

imaxabs()

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