stgMallocBytes: Tolerate malloc(0) returning a NULL ptr

This is valid behaviour for `malloc()` according to ISO C99 and POSIX,
and there's at least one operating system (AIX) which actually does return
NULL for 0-sized allocations.

The `createAdjustor()` routine is currently the only known use-site of
`stgMallocBytes` which may call `stgMallocBytes()` requesting a 0-size
allocation.

Reviewed By: bgamari, austin

Differential Revision: https://phabricator.haskell.org/D2022

1 files changed, 13 insertions, 0 deletions

diff --git a/rts/RtsUtils.c b/rts/RtsUtils.c
index 72e19a0b44..a36532c40d 100644
--- a/rts/RtsUtils.c
+++ b/rts/RtsUtils.c
@@ -64,6 +64,19 @@ stgMallocBytes (int n, char *msg)
 
     n2 = (size_t) n;
     if ((space = (char *) malloc(n2)) == NULL) {
+      /* Quoting POSIX.1-2008 (which says more or less the same as ISO C99):
+       *
+       *   "Upon successful completion with size not equal to 0, malloc() shall
+       *   return a pointer to the allocated space. If size is 0, either a null
+       *   pointer or a unique pointer that can be successfully passed to free()
+       *   shall be returned. Otherwise, it shall return a null pointer and set
+       *   errno to indicate the error."
+       *
+       * Consequently, a NULL pointer being returned by `malloc()` for a 0-size
+       * allocation is *not* to be considered an error.
+       */
+      if (n == 0) return NULL;
+
       /* don't fflush(stdout); WORKAROUND bug in Linux glibc */
       rtsConfig.mallocFailHook((W_) n, msg); /*msg*/
       stg_exit(EXIT_INTERNAL_ERROR);