Is increment an integer atomic in x86? [duplicate]

Answer 1

Here's one proof it is not atomic in a particular implementation (gcc), As you can see (?), gcc generates code that

1. loads the value from memory to a register
2. increments the content of the register
3. saves the register back to memory.

That's very far from being atomic.

$ cat t.c
volatile int a;

void func(void)
{
    a++;
}
[19:51:52 0 ~] $ gcc -O2 -c t.c
[19:51:55 0 ~] $ objdump -d t.o

t.o:     file format elf32-i386


Disassembly of section .text:

00000000 :
   0:   a1 00 00 00 00          mov    0x0,%eax
   5:   83 c0 01                add    $0x1,%eax
   8:   a3 00 00 00 00          mov    %eax,0x0
   d:   c3                      ret

Don't be fooled by the 0x0 in the mov instruction, there's room for 4 bytes there, and the linker will fill in the resulting memory address for a there when this object file is linked.