wrapper.c

   1 /*
   2  * Various trivial helper wrappers around standard functions
   3  */
   4 #include "git-compat-util.h"
   5 #include "abspath.h"
   6 #include "config.h"
   7 #include "gettext.h"
   8 #include "object.h"
   9 #include "repository.h"
  10 #include "strbuf.h"
  11 #include "trace2.h"
  12
  13 #ifdef HAVE_RTLGENRANDOM
  14 /* This is required to get access to RtlGenRandom. */
  15 #define SystemFunction036 NTAPI SystemFunction036
  16 #include <ntsecapi.h>
  17 #undef SystemFunction036
  18 #endif
  19
  20 static int memory_limit_check(size_t size, int gentle)
  21 {
  22         static size_t limit = 0;
  23         if (!limit) {
  24                 limit = git_env_ulong("GIT_ALLOC_LIMIT", 0);
  25                 if (!limit)
  26                         limit = SIZE_MAX;
  27         }
  28         if (size > limit) {
  29                 if (gentle) {
  30                         error("attempting to allocate %"PRIuMAX" over limit %"PRIuMAX,
  31                               (uintmax_t)size, (uintmax_t)limit);
  32                         return -1;
  33                 } else
  34                         die("attempting to allocate %"PRIuMAX" over limit %"PRIuMAX,
  35                             (uintmax_t)size, (uintmax_t)limit);
  36         }
  37         return 0;
  38 }
  39
  40 char *xstrdup(const char *str)
  41 {
  42         char *ret = strdup(str);
  43         if (!ret)
  44                 die("Out of memory, strdup failed");
  45         return ret;
  46 }
  47
  48 static void *do_xmalloc(size_t size, int gentle)
  49 {
  50         void *ret;
  51
  52         if (memory_limit_check(size, gentle))
  53                 return NULL;
  54         ret = malloc(size);
  55         if (!ret && !size)
  56                 ret = malloc(1);
  57         if (!ret) {
  58                 if (!gentle)
  59                         die("Out of memory, malloc failed (tried to allocate %lu bytes)",
  60                             (unsigned long)size);
  61                 else {
  62                         error("Out of memory, malloc failed (tried to allocate %lu bytes)",
  63                               (unsigned long)size);
  64                         return NULL;
  65                 }
  66         }
  67 #ifdef XMALLOC_POISON
  68         memset(ret, 0xA5, size);
  69 #endif
  70         return ret;
  71 }
  72
  73 void *xmalloc(size_t size)
  74 {
  75         return do_xmalloc(size, 0);
  76 }
  77
  78 static void *do_xmallocz(size_t size, int gentle)
  79 {
  80         void *ret;
  81         if (unsigned_add_overflows(size, 1)) {
  82                 if (gentle) {
  83                         error("Data too large to fit into virtual memory space.");
  84                         return NULL;
  85                 } else
  86                         die("Data too large to fit into virtual memory space.");
  87         }
  88         ret = do_xmalloc(size + 1, gentle);
  89         if (ret)
  90                 ((char*)ret)[size] = 0;
  91         return ret;
  92 }
  93
  94 void *xmallocz(size_t size)
  95 {
  96         return do_xmallocz(size, 0);
  97 }
  98
  99 void *xmallocz_gently(size_t size)
 100 {
 101         return do_xmallocz(size, 1);
 102 }
 103
 104 /*
 105  * xmemdupz() allocates (len + 1) bytes of memory, duplicates "len" bytes of
 106  * "data" to the allocated memory, zero terminates the allocated memory,
 107  * and returns a pointer to the allocated memory. If the allocation fails,
 108  * the program dies.
 109  */
 110 void *xmemdupz(const void *data, size_t len)
 111 {
 112         return memcpy(xmallocz(len), data, len);
 113 }
 114
 115 char *xstrndup(const char *str, size_t len)
 116 {
 117         char *p = memchr(str, '\0', len);
 118         return xmemdupz(str, p ? p - str : len);
 119 }
 120
 121 int xstrncmpz(const char *s, const char *t, size_t len)
 122 {
 123         int res = strncmp(s, t, len);
 124         if (res)
 125                 return res;
 126         return s[len] == '\0' ? 0 : 1;
 127 }
 128
 129 void *xrealloc(void *ptr, size_t size)
 130 {
 131         void *ret;
 132
 133         if (!size) {
 134                 free(ptr);
 135                 return xmalloc(0);
 136         }
 137
 138         memory_limit_check(size, 0);
 139         ret = realloc(ptr, size);
 140         if (!ret)
 141                 die("Out of memory, realloc failed");
 142         return ret;
 143 }
 144
 145 void *xcalloc(size_t nmemb, size_t size)
 146 {
 147         void *ret;
 148
 149         if (unsigned_mult_overflows(nmemb, size))
 150                 die("data too large to fit into virtual memory space");
 151
 152         memory_limit_check(size * nmemb, 0);
 153         ret = calloc(nmemb, size);
 154         if (!ret && (!nmemb || !size))
 155                 ret = calloc(1, 1);
 156         if (!ret)
 157                 die("Out of memory, calloc failed");
 158         return ret;
 159 }
 160
 161 void xsetenv(const char *name, const char *value, int overwrite)
 162 {
 163         if (setenv(name, value, overwrite))
 164                 die_errno(_("could not setenv '%s'"), name ? name : "(null)");
 165 }
 166
 167 /**
 168  * xopen() is the same as open(), but it die()s if the open() fails.
 169  */
 170 int xopen(const char *path, int oflag, ...)
 171 {
 172         mode_t mode = 0;
 173         va_list ap;
 174
 175         /*
 176          * va_arg() will have undefined behavior if the specified type is not
 177          * compatible with the argument type. Since integers are promoted to
 178          * ints, we fetch the next argument as an int, and then cast it to a
 179          * mode_t to avoid undefined behavior.
 180          */
 181         va_start(ap, oflag);
 182         if (oflag & O_CREAT)
 183                 mode = va_arg(ap, int);
 184         va_end(ap);
 185
 186         for (;;) {
 187                 int fd = open(path, oflag, mode);
 188                 if (fd >= 0)
 189                         return fd;
 190                 if (errno == EINTR)
 191                         continue;
 192
 193                 if ((oflag & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL))
 194                         die_errno(_("unable to create '%s'"), path);
 195                 else if ((oflag & O_RDWR) == O_RDWR)
 196                         die_errno(_("could not open '%s' for reading and writing"), path);
 197                 else if ((oflag & O_WRONLY) == O_WRONLY)
 198                         die_errno(_("could not open '%s' for writing"), path);
 199                 else
 200                         die_errno(_("could not open '%s' for reading"), path);
 201         }
 202 }
 203
 204 static int handle_nonblock(int fd, short poll_events, int err)
 205 {
 206         struct pollfd pfd;
 207
 208         if (err != EAGAIN && err != EWOULDBLOCK)
 209                 return 0;
 210
 211         pfd.fd = fd;
 212         pfd.events = poll_events;
 213
 214         /*
 215          * no need to check for errors, here;
 216          * a subsequent read/write will detect unrecoverable errors
 217          */
 218         poll(&pfd, 1, -1);
 219         return 1;
 220 }
 221
 222 /*
 223  * xread() is the same a read(), but it automatically restarts read()
 224  * operations with a recoverable error (EAGAIN and EINTR). xread()
 225  * DOES NOT GUARANTEE that "len" bytes is read even if the data is available.
 226  */
 227 ssize_t xread(int fd, void *buf, size_t len)
 228 {
 229         ssize_t nr;
 230         if (len > MAX_IO_SIZE)
 231                 len = MAX_IO_SIZE;
 232         while (1) {
 233                 nr = read(fd, buf, len);
 234                 if (nr < 0) {
 235                         if (errno == EINTR)
 236                                 continue;
 237                         if (handle_nonblock(fd, POLLIN, errno))
 238                                 continue;
 239                 }
 240                 return nr;
 241         }
 242 }
 243
 244 /*
 245  * xwrite() is the same a write(), but it automatically restarts write()
 246  * operations with a recoverable error (EAGAIN and EINTR). xwrite() DOES NOT
 247  * GUARANTEE that "len" bytes is written even if the operation is successful.
 248  */
 249 ssize_t xwrite(int fd, const void *buf, size_t len)
 250 {
 251         ssize_t nr;
 252         if (len > MAX_IO_SIZE)
 253                 len = MAX_IO_SIZE;
 254         while (1) {
 255                 nr = write(fd, buf, len);
 256                 if (nr < 0) {
 257                         if (errno == EINTR)
 258                                 continue;
 259                         if (handle_nonblock(fd, POLLOUT, errno))
 260                                 continue;
 261                 }
 262
 263                 return nr;
 264         }
 265 }
 266
 267 /*
 268  * xpread() is the same as pread(), but it automatically restarts pread()
 269  * operations with a recoverable error (EAGAIN and EINTR). xpread() DOES
 270  * NOT GUARANTEE that "len" bytes is read even if the data is available.
 271  */
 272 ssize_t xpread(int fd, void *buf, size_t len, off_t offset)
 273 {
 274         ssize_t nr;
 275         if (len > MAX_IO_SIZE)
 276                 len = MAX_IO_SIZE;
 277         while (1) {
 278                 nr = pread(fd, buf, len, offset);
 279                 if ((nr < 0) && (errno == EAGAIN || errno == EINTR))
 280                         continue;
 281                 return nr;
 282         }
 283 }
 284
 285 ssize_t read_in_full(int fd, void *buf, size_t count)
 286 {
 287         char *p = buf;
 288         ssize_t total = 0;
 289
 290         while (count > 0) {
 291                 ssize_t loaded = xread(fd, p, count);
 292                 if (loaded < 0)
 293                         return -1;
 294                 if (loaded == 0)
 295                         return total;
 296                 count -= loaded;
 297                 p += loaded;
 298                 total += loaded;
 299         }
 300
 301         return total;
 302 }
 303
 304 ssize_t write_in_full(int fd, const void *buf, size_t count)
 305 {
 306         const char *p = buf;
 307         ssize_t total = 0;
 308
 309         while (count > 0) {
 310                 ssize_t written = xwrite(fd, p, count);
 311                 if (written < 0)
 312                         return -1;
 313                 if (!written) {
 314                         errno = ENOSPC;
 315                         return -1;
 316                 }
 317                 count -= written;
 318                 p += written;
 319                 total += written;
 320         }
 321
 322         return total;
 323 }
 324
 325 ssize_t pread_in_full(int fd, void *buf, size_t count, off_t offset)
 326 {
 327         char *p = buf;
 328         ssize_t total = 0;
 329
 330         while (count > 0) {
 331                 ssize_t loaded = xpread(fd, p, count, offset);
 332                 if (loaded < 0)
 333                         return -1;
 334                 if (loaded == 0)
 335                         return total;
 336                 count -= loaded;
 337                 p += loaded;
 338                 total += loaded;
 339                 offset += loaded;
 340         }
 341
 342         return total;
 343 }
 344
 345 int xdup(int fd)
 346 {
 347         int ret = dup(fd);
 348         if (ret < 0)
 349                 die_errno("dup failed");
 350         return ret;
 351 }
 352
 353 /**
 354  * xfopen() is the same as fopen(), but it die()s if the fopen() fails.
 355  */
 356 FILE *xfopen(const char *path, const char *mode)
 357 {
 358         for (;;) {
 359                 FILE *fp = fopen(path, mode);
 360                 if (fp)
 361                         return fp;
 362                 if (errno == EINTR)
 363                         continue;
 364
 365                 if (*mode && mode[1] == '+')
 366                         die_errno(_("could not open '%s' for reading and writing"), path);
 367                 else if (*mode == 'w' || *mode == 'a')
 368                         die_errno(_("could not open '%s' for writing"), path);
 369                 else
 370                         die_errno(_("could not open '%s' for reading"), path);
 371         }
 372 }
 373
 374 FILE *xfdopen(int fd, const char *mode)
 375 {
 376         FILE *stream = fdopen(fd, mode);
 377         if (!stream)
 378                 die_errno("Out of memory? fdopen failed");
 379         return stream;
 380 }
 381
 382 FILE *fopen_for_writing(const char *path)
 383 {
 384         FILE *ret = fopen(path, "w");
 385
 386         if (!ret && errno == EPERM) {
 387                 if (!unlink(path))
 388                         ret = fopen(path, "w");
 389                 else
 390                         errno = EPERM;
 391         }
 392         return ret;
 393 }
 394
 395 static void warn_on_inaccessible(const char *path)
 396 {
 397         warning_errno(_("unable to access '%s'"), path);
 398 }
 399
 400 int warn_on_fopen_errors(const char *path)
 401 {
 402         if (errno != ENOENT && errno != ENOTDIR) {
 403                 warn_on_inaccessible(path);
 404                 return -1;
 405         }
 406
 407         return 0;
 408 }
 409
 410 FILE *fopen_or_warn(const char *path, const char *mode)
 411 {
 412         FILE *fp = fopen(path, mode);
 413
 414         if (fp)
 415                 return fp;
 416
 417         warn_on_fopen_errors(path);
 418         return NULL;
 419 }
 420
 421 int xmkstemp(char *filename_template)
 422 {
 423         int fd;
 424         char origtemplate[PATH_MAX];
 425         strlcpy(origtemplate, filename_template, sizeof(origtemplate));
 426
 427         fd = mkstemp(filename_template);
 428         if (fd < 0) {
 429                 int saved_errno = errno;
 430                 const char *nonrelative_template;
 431
 432                 if (strlen(filename_template) != strlen(origtemplate))
 433                         filename_template = origtemplate;
 434
 435                 nonrelative_template = absolute_path(filename_template);
 436                 errno = saved_errno;
 437                 die_errno("Unable to create temporary file '%s'",
 438                         nonrelative_template);
 439         }
 440         return fd;
 441 }
 442
 443 /* Adapted from libiberty's mkstemp.c. */
 444
 445 #undef TMP_MAX
 446 #define TMP_MAX 16384
 447
 448 int git_mkstemps_mode(char *pattern, int suffix_len, int mode)
 449 {
 450         static const char letters[] =
 451                 "abcdefghijklmnopqrstuvwxyz"
 452                 "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 453                 "0123456789";
 454         static const int num_letters = ARRAY_SIZE(letters) - 1;
 455         static const char x_pattern[] = "XXXXXX";
 456         static const int num_x = ARRAY_SIZE(x_pattern) - 1;
 457         char *filename_template;
 458         size_t len;
 459         int fd, count;
 460
 461         len = strlen(pattern);
 462
 463         if (len < num_x + suffix_len) {
 464                 errno = EINVAL;
 465                 return -1;
 466         }
 467
 468         if (strncmp(&pattern[len - num_x - suffix_len], x_pattern, num_x)) {
 469                 errno = EINVAL;
 470                 return -1;
 471         }
 472
 473         /*
 474          * Replace pattern's XXXXXX characters with randomness.
 475          * Try TMP_MAX different filenames.
 476          */
 477         filename_template = &pattern[len - num_x - suffix_len];
 478         for (count = 0; count < TMP_MAX; ++count) {
 479                 int i;
 480                 uint64_t v;
 481                 if (csprng_bytes(&v, sizeof(v)) < 0)
 482                         return error_errno("unable to get random bytes for temporary file");
 483
 484                 /* Fill in the random bits. */
 485                 for (i = 0; i < num_x; i++) {
 486                         filename_template[i] = letters[v % num_letters];
 487                         v /= num_letters;
 488                 }
 489
 490                 fd = open(pattern, O_CREAT | O_EXCL | O_RDWR, mode);
 491                 if (fd >= 0)
 492                         return fd;
 493                 /*
 494                  * Fatal error (EPERM, ENOSPC etc).
 495                  * It doesn't make sense to loop.
 496                  */
 497                 if (errno != EEXIST)
 498                         break;
 499         }
 500         /* We return the null string if we can't find a unique file name.  */
 501         pattern[0] = '\0';
 502         return -1;
 503 }
 504
 505 int git_mkstemp_mode(char *pattern, int mode)
 506 {
 507         /* mkstemp is just mkstemps with no suffix */
 508         return git_mkstemps_mode(pattern, 0, mode);
 509 }
 510
 511 int xmkstemp_mode(char *filename_template, int mode)
 512 {
 513         int fd;
 514         char origtemplate[PATH_MAX];
 515         strlcpy(origtemplate, filename_template, sizeof(origtemplate));
 516
 517         fd = git_mkstemp_mode(filename_template, mode);
 518         if (fd < 0) {
 519                 int saved_errno = errno;
 520                 const char *nonrelative_template;
 521
 522                 if (!filename_template[0])
 523                         filename_template = origtemplate;
 524
 525                 nonrelative_template = absolute_path(filename_template);
 526                 errno = saved_errno;
 527                 die_errno("Unable to create temporary file '%s'",
 528                         nonrelative_template);
 529         }
 530         return fd;
 531 }
 532
 533 /*
 534  * Some platforms return EINTR from fsync. Since fsync is invoked in some
 535  * cases by a wrapper that dies on failure, do not expose EINTR to callers.
 536  */
 537 static int fsync_loop(int fd)
 538 {
 539         int err;
 540
 541         do {
 542                 err = fsync(fd);
 543         } while (err < 0 && errno == EINTR);
 544         return err;
 545 }
 546
 547 int git_fsync(int fd, enum fsync_action action)
 548 {
 549         switch (action) {
 550         case FSYNC_WRITEOUT_ONLY:
 551                 trace2_counter_add(TRACE2_COUNTER_ID_FSYNC_WRITEOUT_ONLY, 1);
 552
 553 #ifdef __APPLE__
 554                 /*
 555                  * On macOS, fsync just causes filesystem cache writeback but
 556                  * does not flush hardware caches.
 557                  */
 558                 return fsync_loop(fd);
 559 #endif
 560
 561 #ifdef HAVE_SYNC_FILE_RANGE
 562                 /*
 563                  * On linux 2.6.17 and above, sync_file_range is the way to
 564                  * issue a writeback without a hardware flush. An offset of
 565                  * 0 and size of 0 indicates writeout of the entire file and the
 566                  * wait flags ensure that all dirty data is written to the disk
 567                  * (potentially in a disk-side cache) before we continue.
 568                  */
 569
 570                 return sync_file_range(fd, 0, 0, SYNC_FILE_RANGE_WAIT_BEFORE |
 571                                                  SYNC_FILE_RANGE_WRITE |
 572                                                  SYNC_FILE_RANGE_WAIT_AFTER);
 573 #endif
 574
 575 #ifdef fsync_no_flush
 576                 return fsync_no_flush(fd);
 577 #endif
 578
 579                 errno = ENOSYS;
 580                 return -1;
 581
 582         case FSYNC_HARDWARE_FLUSH:
 583                 trace2_counter_add(TRACE2_COUNTER_ID_FSYNC_HARDWARE_FLUSH, 1);
 584
 585                 /*
 586                  * On macOS, a special fcntl is required to really flush the
 587                  * caches within the storage controller. As of this writing,
 588                  * this is a very expensive operation on Apple SSDs.
 589                  */
 590 #ifdef __APPLE__
 591                 return fcntl(fd, F_FULLFSYNC);
 592 #else
 593                 return fsync_loop(fd);
 594 #endif
 595         default:
 596                 BUG("unexpected git_fsync(%d) call", action);
 597         }
 598 }
 599
 600 static int warn_if_unremovable(const char *op, const char *file, int rc)
 601 {
 602         int err;
 603         if (!rc || errno == ENOENT)
 604                 return 0;
 605         err = errno;
 606         warning_errno("unable to %s '%s'", op, file);
 607         errno = err;
 608         return rc;
 609 }
 610
 611 int unlink_or_msg(const char *file, struct strbuf *err)
 612 {
 613         int rc = unlink(file);
 614
 615         assert(err);
 616
 617         if (!rc || errno == ENOENT)
 618                 return 0;
 619
 620         strbuf_addf(err, "unable to unlink '%s': %s",
 621                     file, strerror(errno));
 622         return -1;
 623 }
 624
 625 int unlink_or_warn(const char *file)
 626 {
 627         return warn_if_unremovable("unlink", file, unlink(file));
 628 }
 629
 630 int rmdir_or_warn(const char *file)
 631 {
 632         return warn_if_unremovable("rmdir", file, rmdir(file));
 633 }
 634
 635 int remove_or_warn(unsigned int mode, const char *file)
 636 {
 637         return S_ISGITLINK(mode) ? rmdir_or_warn(file) : unlink_or_warn(file);
 638 }
 639
 640 static int access_error_is_ok(int err, unsigned flag)
 641 {
 642         return (is_missing_file_error(err) ||
 643                 ((flag & ACCESS_EACCES_OK) && err == EACCES));
 644 }
 645
 646 int access_or_warn(const char *path, int mode, unsigned flag)
 647 {
 648         int ret = access(path, mode);
 649         if (ret && !access_error_is_ok(errno, flag))
 650                 warn_on_inaccessible(path);
 651         return ret;
 652 }
 653
 654 int access_or_die(const char *path, int mode, unsigned flag)
 655 {
 656         int ret = access(path, mode);
 657         if (ret && !access_error_is_ok(errno, flag))
 658                 die_errno(_("unable to access '%s'"), path);
 659         return ret;
 660 }
 661
 662 char *xgetcwd(void)
 663 {
 664         struct strbuf sb = STRBUF_INIT;
 665         if (strbuf_getcwd(&sb))
 666                 die_errno(_("unable to get current working directory"));
 667         return strbuf_detach(&sb, NULL);
 668 }
 669
 670 int xsnprintf(char *dst, size_t max, const char *fmt, ...)
 671 {
 672         va_list ap;
 673         int len;
 674
 675         va_start(ap, fmt);
 676         len = vsnprintf(dst, max, fmt, ap);
 677         va_end(ap);
 678
 679         if (len < 0)
 680                 BUG("your snprintf is broken");
 681         if (len >= max)
 682                 BUG("attempt to snprintf into too-small buffer");
 683         return len;
 684 }
 685
 686 void write_file_buf(const char *path, const char *buf, size_t len)
 687 {
 688         int fd = xopen(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
 689         if (write_in_full(fd, buf, len) < 0)
 690                 die_errno(_("could not write to '%s'"), path);
 691         if (close(fd))
 692                 die_errno(_("could not close '%s'"), path);
 693 }
 694
 695 void write_file(const char *path, const char *fmt, ...)
 696 {
 697         va_list params;
 698         struct strbuf sb = STRBUF_INIT;
 699
 700         va_start(params, fmt);
 701         strbuf_vaddf(&sb, fmt, params);
 702         va_end(params);
 703
 704         strbuf_complete_line(&sb);
 705
 706         write_file_buf(path, sb.buf, sb.len);
 707         strbuf_release(&sb);
 708 }
 709
 710 void sleep_millisec(int millisec)
 711 {
 712         poll(NULL, 0, millisec);
 713 }
 714
 715 int xgethostname(char *buf, size_t len)
 716 {
 717         /*
 718          * If the full hostname doesn't fit in buf, POSIX does not
 719          * specify whether the buffer will be null-terminated, so to
 720          * be safe, do it ourselves.
 721          */
 722         int ret = gethostname(buf, len);
 723         if (!ret)
 724                 buf[len - 1] = 0;
 725         return ret;
 726 }
 727
 728 int is_empty_or_missing_file(const char *filename)
 729 {
 730         struct stat st;
 731
 732         if (stat(filename, &st) < 0) {
 733                 if (errno == ENOENT)
 734                         return 1;
 735                 die_errno(_("could not stat %s"), filename);
 736         }
 737
 738         return !st.st_size;
 739 }
 740
 741 int open_nofollow(const char *path, int flags)
 742 {
 743 #ifdef O_NOFOLLOW
 744         return open(path, flags | O_NOFOLLOW);
 745 #else
 746         struct stat st;
 747         if (lstat(path, &st) < 0)
 748                 return -1;
 749         if (S_ISLNK(st.st_mode)) {
 750                 errno = ELOOP;
 751                 return -1;
 752         }
 753         return open(path, flags);
 754 #endif
 755 }
 756
 757 int csprng_bytes(void *buf, size_t len)
 758 {
 759 #if defined(HAVE_ARC4RANDOM) || defined(HAVE_ARC4RANDOM_LIBBSD)
 760         /* This function never returns an error. */
 761         arc4random_buf(buf, len);
 762         return 0;
 763 #elif defined(HAVE_GETRANDOM)
 764         ssize_t res;
 765         char *p = buf;
 766         while (len) {
 767                 res = getrandom(p, len, 0);
 768                 if (res < 0)
 769                         return -1;
 770                 len -= res;
 771                 p += res;
 772         }
 773         return 0;
 774 #elif defined(HAVE_GETENTROPY)
 775         int res;
 776         char *p = buf;
 777         while (len) {
 778                 /* getentropy has a maximum size of 256 bytes. */
 779                 size_t chunk = len < 256 ? len : 256;
 780                 res = getentropy(p, chunk);
 781                 if (res < 0)
 782                         return -1;
 783                 len -= chunk;
 784                 p += chunk;
 785         }
 786         return 0;
 787 #elif defined(HAVE_RTLGENRANDOM)
 788         if (!RtlGenRandom(buf, len))
 789                 return -1;
 790         return 0;
 791 #elif defined(HAVE_OPENSSL_CSPRNG)
 792         int res = RAND_bytes(buf, len);
 793         if (res == 1)
 794                 return 0;
 795         if (res == -1)
 796                 errno = ENOTSUP;
 797         else
 798                 errno = EIO;
 799         return -1;
 800 #else
 801         ssize_t res;
 802         char *p = buf;
 803         int fd, err;
 804         fd = open("/dev/urandom", O_RDONLY);
 805         if (fd < 0)
 806                 return -1;
 807         while (len) {
 808                 res = xread(fd, p, len);
 809                 if (res < 0) {
 810                         err = errno;
 811                         close(fd);
 812                         errno = err;
 813                         return -1;
 814                 }
 815                 len -= res;
 816                 p += res;
 817         }
 818         close(fd);
 819         return 0;
 820 #endif
 821 }
 822
 823 uint32_t git_rand(void)
 824 {
 825         uint32_t result;
 826
 827         if (csprng_bytes(&result, sizeof(result)) < 0)
 828                 die(_("unable to get random bytes"));
 829
 830         return result;
 831 }