compat/mkstemps.c

   1 #include "../git-compat-util.h"
   2
   3 /* Adapted from libiberty's mkstemp.c. */
   4
   5 #undef TMP_MAX
   6 #define TMP_MAX 16384
   7
   8 int gitmkstemps(char *pattern, int suffix_len)
   9 {
  10         static const char letters[] =
  11                 "abcdefghijklmnopqrstuvwxyz"
  12                 "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
  13                 "0123456789";
  14         static const int num_letters = 62;
  15         uint64_t value;
  16         struct timeval tv;
  17         char *template;
  18         size_t len;
  19         int fd, count;
  20
  21         len = strlen(pattern);
  22
  23         if (len < 6 + suffix_len) {
  24                 errno = EINVAL;
  25                 return -1;
  26         }
  27
  28         if (strncmp(&pattern[len - 6 - suffix_len], "XXXXXX", 6)) {
  29                 errno = EINVAL;
  30                 return -1;
  31         }
  32
  33         /*
  34          * Replace pattern's XXXXXX characters with randomness.
  35          * Try TMP_MAX different filenames.
  36          */
  37         gettimeofday(&tv, NULL);
  38         value = ((size_t)(tv.tv_usec << 16)) ^ tv.tv_sec ^ getpid();
  39         template = &pattern[len - 6 - suffix_len];
  40         for (count = 0; count < TMP_MAX; ++count) {
  41                 uint64_t v = value;
  42                 /* Fill in the random bits. */
  43                 template[0] = letters[v % num_letters]; v /= num_letters;
  44                 template[1] = letters[v % num_letters]; v /= num_letters;
  45                 template[2] = letters[v % num_letters]; v /= num_letters;
  46                 template[3] = letters[v % num_letters]; v /= num_letters;
  47                 template[4] = letters[v % num_letters]; v /= num_letters;
  48                 template[5] = letters[v % num_letters]; v /= num_letters;
  49
  50                 fd = open(pattern, O_CREAT | O_EXCL | O_RDWR, 0600);
  51                 if (fd > 0)
  52                         return fd;
  53                 /*
  54                  * Fatal error (EPERM, ENOSPC etc).
  55                  * It doesn't make sense to loop.
  56                  */
  57                 if (errno != EEXIST)
  58                         break;
  59                 /*
  60                  * This is a random value.  It is only necessary that
  61                  * the next TMP_MAX values generated by adding 7777 to
  62                  * VALUE are different with (module 2^32).
  63                  */
  64                 value += 7777;
  65         }
  66         /* We return the null string if we can't find a unique file name.  */
  67         pattern[0] = '\0';
  68         errno = EINVAL;
  69         return -1;
  70 }