1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
|
#include <semaphore.h>
#include <sys/mman.h>
#include <limits.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdarg.h>
#include <errno.h>
#include <time.h>
#include <stdio.h>
#include <sys/stat.h>
#include <stdlib.h>
#include <pthread.h>
#include "libc.h"
static struct {
ino_t ino;
sem_t *sem;
int refcnt;
} *semtab;
static volatile int lock[1];
#define FLAGS (O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NONBLOCK)
sem_t *sem_open(const char *name, int flags, ...)
{
va_list ap;
mode_t mode;
unsigned value;
int fd, i, e, slot, first=1, cnt, cs;
sem_t newsem;
void *map;
char tmp[64];
struct timespec ts;
struct stat st;
char buf[NAME_MAX+10];
if (!(name = __shm_mapname(name, buf)))
return SEM_FAILED;
LOCK(lock);
/* Allocate table if we don't have one yet */
if (!semtab && !(semtab = calloc(sizeof *semtab, SEM_NSEMS_MAX))) {
UNLOCK(lock);
return SEM_FAILED;
}
/* Reserve a slot in case this semaphore is not mapped yet;
* this is necessary because there is no way to handle
* failures after creation of the file. */
slot = -1;
for (cnt=i=0; i<SEM_NSEMS_MAX; i++) {
cnt += semtab[i].refcnt;
if (!semtab[i].sem && slot < 0) slot = i;
}
/* Avoid possibility of overflow later */
if (cnt == INT_MAX || slot < 0) {
errno = EMFILE;
UNLOCK(lock);
return SEM_FAILED;
}
/* Dummy pointer to make a reservation */
semtab[slot].sem = (sem_t *)-1;
UNLOCK(lock);
flags &= (O_CREAT|O_EXCL);
pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
/* Early failure check for exclusive open; otherwise the case
* where the semaphore already exists is expensive. */
if (flags == (O_CREAT|O_EXCL) && access(name, F_OK) == 0) {
errno = EEXIST;
goto fail;
}
for (;;) {
/* If exclusive mode is not requested, try opening an
* existing file first and fall back to creation. */
if (flags != (O_CREAT|O_EXCL)) {
fd = open(name, FLAGS);
if (fd >= 0) {
if (fstat(fd, &st) < 0 ||
(map = mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED) {
close(fd);
goto fail;
}
close(fd);
break;
}
if (errno != ENOENT)
goto fail;
}
if (!(flags & O_CREAT))
goto fail;
if (first) {
first = 0;
va_start(ap, flags);
mode = va_arg(ap, mode_t) & 0666;
value = va_arg(ap, unsigned);
va_end(ap);
if (value > SEM_VALUE_MAX) {
errno = EINVAL;
goto fail;
}
sem_init(&newsem, 1, value);
}
/* Create a temp file with the new semaphore contents
* and attempt to atomically link it as the new name */
clock_gettime(CLOCK_REALTIME, &ts);
snprintf(tmp, sizeof(tmp), "/dev/shm/tmp-%d", (int)ts.tv_nsec);
fd = open(tmp, O_CREAT|O_EXCL|FLAGS, mode);
if (fd < 0) {
if (errno == EEXIST) continue;
goto fail;
}
if (write(fd, &newsem, sizeof newsem) != sizeof newsem || fstat(fd, &st) < 0 ||
(map = mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED) {
close(fd);
unlink(tmp);
goto fail;
}
close(fd);
e = link(tmp, name) ? errno : 0;
unlink(tmp);
if (!e) break;
munmap(map, sizeof(sem_t));
/* Failure is only fatal when doing an exclusive open;
* otherwise, next iteration will try to open the
* existing file. */
if (e != EEXIST || flags == (O_CREAT|O_EXCL))
goto fail;
}
/* See if the newly mapped semaphore is already mapped. If
* so, unmap the new mapping and use the existing one. Otherwise,
* add it to the table of mapped semaphores. */
LOCK(lock);
for (i=0; i<SEM_NSEMS_MAX && semtab[i].ino != st.st_ino; i++);
if (i<SEM_NSEMS_MAX) {
munmap(map, sizeof(sem_t));
semtab[slot].sem = 0;
slot = i;
map = semtab[i].sem;
}
semtab[slot].refcnt++;
semtab[slot].sem = map;
semtab[slot].ino = st.st_ino;
UNLOCK(lock);
pthread_setcancelstate(cs, 0);
return map;
fail:
pthread_setcancelstate(cs, 0);
LOCK(lock);
semtab[slot].sem = 0;
UNLOCK(lock);
return SEM_FAILED;
}
int sem_close(sem_t *sem)
{
int i;
LOCK(lock);
for (i=0; i<SEM_NSEMS_MAX && semtab[i].sem != sem; i++);
if (!--semtab[i].refcnt) {
semtab[i].sem = 0;
semtab[i].ino = 0;
}
UNLOCK(lock);
munmap(sem, sizeof *sem);
return 0;
}
|
