1 | #include <semaphore.h>
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2 | #include <sys/mman.h>
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3 | #include <limits.h>
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4 | #include <fcntl.h>
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5 | #include <unistd.h>
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6 | #include <string.h>
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7 | #include <stdarg.h>
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8 | #include <errno.h>
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9 | #include <time.h>
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10 | #include <stdio.h>
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11 | #include <sys/stat.h>
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12 | #include <stdlib.h>
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13 | #include <pthread.h>
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14 | #include "libc.h"
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15 |
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16 | char *__shm_mapname(const char *, char *);
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17 |
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18 | static struct {
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19 | ino_t ino;
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20 | sem_t *sem;
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21 | int refcnt;
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22 | } *semtab;
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23 | static volatile int lock[2];
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24 |
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25 | #define FLAGS (O_RDWR|O_NOFOLLOW|O_CLOEXEC|O_NONBLOCK)
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26 |
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27 | sem_t *sem_open(const char *name, int flags, ...)
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28 | {
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29 | va_list ap;
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30 | mode_t mode;
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31 | unsigned value;
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32 | int fd, i, e, slot, first=1, cnt, cs;
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33 | sem_t newsem;
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34 | void *map;
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35 | char tmp[64];
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36 | struct timespec ts;
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37 | struct stat st;
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38 | char buf[NAME_MAX+10];
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39 |
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40 | if (!(name = __shm_mapname(name, buf)))
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41 | return SEM_FAILED;
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42 |
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43 | LOCK(lock);
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44 | /* Allocate table if we don't have one yet */
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45 | if (!semtab && !(semtab = calloc(sizeof *semtab, SEM_NSEMS_MAX))) {
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46 | UNLOCK(lock);
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47 | return SEM_FAILED;
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48 | }
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49 |
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50 | /* Reserve a slot in case this semaphore is not mapped yet;
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51 | * this is necessary because there is no way to handle
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52 | * failures after creation of the file. */
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53 | slot = -1;
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54 | for (cnt=i=0; i<SEM_NSEMS_MAX; i++) {
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55 | cnt += semtab[i].refcnt;
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56 | if (!semtab[i].sem && slot < 0) slot = i;
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57 | }
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58 | /* Avoid possibility of overflow later */
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59 | if (cnt == INT_MAX || slot < 0) {
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60 | errno = EMFILE;
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61 | UNLOCK(lock);
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62 | return SEM_FAILED;
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63 | }
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64 | /* Dummy pointer to make a reservation */
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65 | semtab[slot].sem = (sem_t *)-1;
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66 | UNLOCK(lock);
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67 |
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68 | flags &= (O_CREAT|O_EXCL);
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69 |
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70 | pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
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71 |
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72 | /* Early failure check for exclusive open; otherwise the case
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73 | * where the semaphore already exists is expensive. */
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74 | if (flags == (O_CREAT|O_EXCL) && access(name, F_OK) == 0) {
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75 | errno = EEXIST;
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76 | goto fail;
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77 | }
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78 |
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79 | for (;;) {
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80 | /* If exclusive mode is not requested, try opening an
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81 | * existing file first and fall back to creation. */
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82 | if (flags != (O_CREAT|O_EXCL)) {
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83 | fd = open(name, FLAGS);
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84 | if (fd >= 0) {
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85 | if (fstat(fd, &st) < 0 ||
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86 | (map = mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED) {
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87 | close(fd);
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88 | goto fail;
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89 | }
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90 | close(fd);
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91 | break;
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92 | }
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93 | if (errno != ENOENT)
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94 | goto fail;
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95 | }
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96 | if (!(flags & O_CREAT))
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97 | goto fail;
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98 | if (first) {
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99 | first = 0;
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100 | va_start(ap, flags);
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101 | mode = va_arg(ap, mode_t) & 0666;
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102 | value = va_arg(ap, unsigned);
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103 | va_end(ap);
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104 | if (value > SEM_VALUE_MAX) {
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105 | errno = EINVAL;
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106 | goto fail;
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107 | }
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108 | sem_init(&newsem, 1, value);
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109 | }
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110 | /* Create a temp file with the new semaphore contents
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111 | * and attempt to atomically link it as the new name */
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112 | clock_gettime(CLOCK_REALTIME, &ts);
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113 | snprintf(tmp, sizeof(tmp), "/dev/shm/tmp-%d", (int)ts.tv_nsec);
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114 | fd = open(tmp, O_CREAT|O_EXCL|FLAGS, mode);
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115 | if (fd < 0) {
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116 | if (errno == EEXIST) continue;
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117 | goto fail;
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118 | }
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119 | if (write(fd, &newsem, sizeof newsem) != sizeof newsem || fstat(fd, &st) < 0 ||
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120 | (map = mmap(0, sizeof(sem_t), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED) {
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121 | close(fd);
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122 | unlink(tmp);
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123 | goto fail;
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124 | }
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125 | close(fd);
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126 | e = link(tmp, name) ? errno : 0;
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127 | unlink(tmp);
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128 | if (!e) break;
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129 | munmap(map, sizeof(sem_t));
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130 | /* Failure is only fatal when doing an exclusive open;
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131 | * otherwise, next iteration will try to open the
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132 | * existing file. */
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133 | if (e != EEXIST || flags == (O_CREAT|O_EXCL))
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134 | goto fail;
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135 | }
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136 |
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137 | /* See if the newly mapped semaphore is already mapped. If
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138 | * so, unmap the new mapping and use the existing one. Otherwise,
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139 | * add it to the table of mapped semaphores. */
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140 | LOCK(lock);
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141 | for (i=0; i<SEM_NSEMS_MAX && semtab[i].ino != st.st_ino; i++);
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142 | if (i<SEM_NSEMS_MAX) {
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143 | munmap(map, sizeof(sem_t));
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144 | semtab[slot].sem = 0;
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145 | slot = i;
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146 | map = semtab[i].sem;
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147 | }
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148 | semtab[slot].refcnt++;
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149 | semtab[slot].sem = map;
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150 | semtab[slot].ino = st.st_ino;
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151 | UNLOCK(lock);
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152 | pthread_setcancelstate(cs, 0);
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153 | return map;
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154 |
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155 | fail:
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156 | pthread_setcancelstate(cs, 0);
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157 | LOCK(lock);
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158 | semtab[slot].sem = 0;
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159 | UNLOCK(lock);
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160 | return SEM_FAILED;
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161 | }
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162 |
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163 | int sem_close(sem_t *sem)
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164 | {
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165 | int i;
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166 | LOCK(lock);
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167 | for (i=0; i<SEM_NSEMS_MAX && semtab[i].sem != sem; i++);
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168 | if (!--semtab[i].refcnt) {
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169 | semtab[i].sem = 0;
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170 | semtab[i].ino = 0;
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171 | }
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172 | UNLOCK(lock);
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173 | munmap(sem, sizeof *sem);
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174 | return 0;
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175 | }
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