test$ cat testfail.c

#include <stdlib.h>
#include <stdio.h>
#include <libi86/malloc.h>

void main(void) {
  char __far * pp = _fmalloc(64UL * 1024UL);
  long ii;
  for (ii = 0; ii < (64UL * 1024UL); ++ ii) {
    pp[ii] = 0;
  }
  printf("%u\n", !!pp);
}
test$ ia16-elf-gcc testfail.c -li86 -o testfail.exe
testfail.c: In function ‘main’:
testfail.c:7:30: warning: large integer implicitly truncated to unsigned type [-Woverflow]
   char __far * pp = _fmalloc(64UL * 1024UL);
                              ^~~~
test$ dos testfail
About to Execute : testfail
1
dos mem corrupt, first_mcb=02d2
prev 149a:0000|4d 9a 04 00 00 48 49 47 00 00 53 57 49 54 43 48 MÜ...HIG..SWITCH
notMZ149b:0000|00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

PANIC: MCB chain corrupted
System haltedTerminated with signal 126
test$ cat test.c

#include <stdlib.h>
#include <stdio.h>
#include <libi86/malloc.h>

void main(void) {
  char __far * pp = _fmalloc(64UL * 1024UL - 1);
  long ii;
  for (ii = 0; ii < (64UL * 1024UL); ++ ii) {
    pp[ii] = 0;
  }
  printf("%u\n", !!pp);
}
test$ ia16-elf-gcc test.c -li86 -o test.exe
test$ dos test
About to Execute : test
1
test$