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Q&A Which platforms return a non-null pointer on malloc(0)

It is trivial enough to test: #include <stdlib.h> #include <stdio.h> #include <errno.h> #define KNOWN_GARBAGE ((int*)~0u) int main (void) { int* ptr = KNOWN_GARBAGE; ...

posted 3mo ago by Lundin‭ · edited 2mo ago by Andreas witnessed the end of the world today‭

Answer

#2: Post edited by

Andreas witnessed the end of the world today‭ · 2024-10-05T17:54:17Z (2 months ago)
Remove meta commentary. Such belongs in a comment or close flag instead.

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~~I'm not sure if "give me a list" type of questions are ideal for this site. That being said, it is trivial enough to test:~~
```c
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#define KNOWN_GARBAGE ((int*)~0u)
int main (void)
{
int* ptr = KNOWN_GARBAGE;
ptr = malloc(0);
int errno_changed = errno;
if(ptr == NULL)
puts("Returned NULL.");
else if(ptr == KNOWN_GARBAGE)
puts("Didn't modify the pointer, non-conforming?");
else
puts("Returned non-zero, modified the pointer.");
if(errno_changed)
printf("Weird use of errno detected, error code 0x%X\n", errno_changed);
}
```
Then try it with whatever compiler, version, standard lib and system you want. The vast majority of gcc-like compilers + libc flavours appear to return a new non-zero address.
The setting `errno` part appears to be some old Unix gunk from the 90s according to `man`(?), so you may have to find some sufficiently obscure computer for that, I guess.
---
Related to this question: C no longer has standard support for `realloc(ptr, 0)` since C23 likely comes with major defects here. See [realloc(ptr, 0) in C23 - now what?](https://stackoverflow.com/questions/78691087/reallocptr-0-in-c23-now-what)

It is trivial enough to test:
```c
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#define KNOWN_GARBAGE ((int*)~0u)
int main (void)
{
int* ptr = KNOWN_GARBAGE;
ptr = malloc(0);
int errno_changed = errno;
if(ptr == NULL)
puts("Returned NULL.");
else if(ptr == KNOWN_GARBAGE)
puts("Didn't modify the pointer, non-conforming?");
else
puts("Returned non-zero, modified the pointer.");
if(errno_changed)
printf("Weird use of errno detected, error code 0x%X\n", errno_changed);
}
```
Then try it with whatever compiler, version, standard lib and system you want. The vast majority of gcc-like compilers + libc flavours appear to return a new non-zero address.
The setting `errno` part appears to be some old Unix gunk from the 90s according to `man`(?), so you may have to find some sufficiently obscure computer for that, I guess.
---
Related to this question: C no longer has standard support for `realloc(ptr, 0)` since C23 likely comes with major defects here. See [realloc(ptr, 0) in C23 - now what?](https://stackoverflow.com/questions/78691087/reallocptr-0-in-c23-now-what)

#1: Initial revision by

Lundin‭ · 2024-10-01T12:02:24Z (3 months ago)

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I'm not sure if "give me a list" type of questions are ideal for this site. That being said, it is trivial enough to test:

```c
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>

#define KNOWN_GARBAGE ((int*)~0u)

int main (void)
{
  int* ptr = KNOWN_GARBAGE;
  ptr = malloc(0);
  int errno_changed = errno;

  if(ptr == NULL)
    puts("Returned NULL.");
  else if(ptr == KNOWN_GARBAGE)
    puts("Didn't modify the pointer, non-conforming?");
  else
    puts("Returned non-zero, modified the pointer.");
  if(errno_changed)
    printf("Weird use of errno detected, error code 0x%X\n", errno_changed);
}
```

Then try it with whatever compiler, version, standard lib and system you want. The vast majority of gcc-like compilers + libc flavours appear to return a new non-zero address.

The setting `errno` part appears to be some old Unix gunk from the 90s according to `man`(?), so you may have to find some sufficiently obscure computer for that, I guess.

---

Related to this question: C no longer has standard support for `realloc(ptr, 0)` since C23 likely comes with major defects here. See [realloc(ptr, 0) in C23 - now what?](https://stackoverflow.com/questions/78691087/reallocptr-0-in-c23-now-what)

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