android - what is purpose of 'or' a value with -1? -

i came across following code in android framework , wondering purpose of android_atomic_or(-1, &mfd) ? -1 0xffffffff 32 bit int, or -1 value should -1. if understanding correctly, code below doesn't make sense me.

https://android.googlesource.com/platform/frameworks/native/+/6eabaa3/libs/utils/memoryheapbase.cpp

line:150

void memoryheapbase::dispose() {     int fd = android_atomic_or(-1, &mfd); //mfd file descriptor     if (fd >= 0) {         if (mneedunmap) {             //logd("munmap(fd=%d, base=%p, size=%lu)", fd, mbase, msize);             munmap(mbase, msize);         }         mbase = 0;         msize = 0;         close(fd);     } } 

and, here (one of) implementation of android_atomic_or

http://androidxref.com/5.0.0_r2/xref/system/core/include/cutils/atomic-x86.h#135

134extern android_atomic_inline int32_t 135android_atomic_or(int32_t value, volatile int32_t *ptr) 136{ 137    int32_t prev, status; 138    { 139        prev = *ptr; 140        status = android_atomic_cas(prev, prev | value, ptr); 141    } while (__builtin_expect(status != 0, 0)); 142    return prev; 143}  73 android_atomic_cas(int32_t old_value, int32_t new_value, volatile int32_t *ptr) 74{ 75    int32_t prev; 76    __asm__ __volatile__ ("lock; cmpxchgl %1, %2" 77                          : "=a" (prev) 78                          : "q" (new_value), "m" (*ptr), "0" (old_value) 79                          : "memory"); 80    return prev != old_value; 81} 82 

i assume android_atomic_or atomic version of (value|*ptr) i'm not sure since can't read assembly. must misunderstand here. help.

that similar standard atomic_fetch_or() (i suspect exists c11 not available when android started).

that function returns original value before modified. previous value , set bits in value.

see lines 139 , 142, assume loop executes once. compare and set 1 of basic operations required atomics (the other compare , exchange bit more universal).