===== Yet another reason that makes concurrent programming harder. =====
Below, there is a piece of code that can be compiled by issuing:
gcc -g -O0 sb.c -o sb
The program forks and uses shared memory, so concurrency is involved.
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__Analyze the code and tell which results can be output.__
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Remember that newly allocated anonymous shared memory is always zeroed.
Run the program multiple times with the following command, and observe the results:
( for((i=0;i<10000;++i)); do ./sb; done ) | sort | uniq -c
Finally, explain the results.
#include
#include
#include
#include
struct data {
atomic_char vA, vB, control[3];
volatile char data[64*3];
};
int main(){
struct data * const s = mmap(NULL, sizeof(struct data),
PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
atomic_char * const control = s->control;
volatile char * const a = s->data + 64;
volatile char * const b = s->data + 64*2;
volatile char dummy = s->vA + s->vB + *a + *b;
if(!fork()){
control[0]++;
while(control[0]!=2);
*b = 1;
//while(--dummy);
s->vA = *a;
control[1]=1;
return 0;
} else {
control[0]++;
while(control[0]!=2);
*a = 1;
s->vB = *b;
while(control[1]!=1);
printf("%d %d\n",s->vA, s->vB);
return 0;
}
}
~~META:
language = en
~~