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#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"
char *__shm_mapname(const char *, char *);
static struct {
ino_t ino;
sem_t *sem;
int refcnt;
} *semtab;
static volatile int lock[2];
#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;
/* 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;
}
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