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1 /* ========================================================================
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2 * Copyright 1988-2006 University of Washington
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3 *
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4 * Licensed under the Apache License, Version 2.0 (the "License");
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5 * you may not use this file except in compliance with the License.
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6 * You may obtain a copy of the License at
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7 *
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8 * http://www.apache.org/licenses/LICENSE-2.0
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9 *
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10 *
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11 * ========================================================================
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12 */
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13
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14 /*
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15 * Program: flock emulation via fcntl() locking
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16 *
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17 * Author: Mark Crispin
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18 * Networks and Distributed Computing
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19 * Computing & Communications
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20 * University of Washington
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21 * Administration Building, AG-44
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22 * Seattle, WA 98195
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23 * Internet: MRC@CAC.Washington.EDU
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24 *
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25 * Date: 10 April 2001
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26 * Last Edited: 30 August 2006
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27 */
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28
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29
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30 /* Cygwin does not seem to have the design flaw in fcntl() locking that
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31 * most other systems do (see flocksim.c for details). If some cretin
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32 * decides to implement that design flaw, then Cygwin will have to use
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33 * flocksim. Also, we don't test NFS either.
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34 *
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35 * However, Cygwin does have the Windows misfeature (introduced in NT 4.0)
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36 * that you can not write to any segment which has a shared lock, and you
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37 * can't lock a zero-byte segment either. This screws up the shared-write
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38 * mailbox drivers (mbx, mtx, mx, and tenex). As a workaround, we'll only
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39 * lock the first byte of the file, meaning that you can't write that byte
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40 * shared. It's been suggested to lock the maximum off_t type, but that
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41 * risks having a future version of Windows (or Cygwin) deciding that this
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42 * also means "no lock".
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43 */
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44
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45 #undef flock /* name is used as a struct for fcntl */
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46
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47 /* Emulator for flock() call
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48 * Accepts: file descriptor
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49 * operation bitmask
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50 * Returns: 0 if successful, -1 if failure under BSD conditions
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51 */
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52
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53 int flocksim (int fd,int op)
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54 {
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55 char tmp[MAILTMPLEN];
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56 int logged = 0;
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57 struct flock fl;
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58 /* lock one bytes at byte 0 */
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59 fl.l_whence = SEEK_SET; fl.l_start = 0; fl.l_len = 1;
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60 fl.l_pid = getpid (); /* shouldn't be necessary */
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61 switch (op & ~LOCK_NB) { /* translate to fcntl() operation */
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62 case LOCK_EX: /* exclusive */
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63 fl.l_type = F_WRLCK;
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64 break;
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65 case LOCK_SH: /* shared */
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66 fl.l_type = F_RDLCK;
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67 break;
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68 case LOCK_UN: /* unlock */
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69 fl.l_type = F_UNLCK;
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70 break;
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71 default: /* default */
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72 errno = EINVAL;
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73 return -1;
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74 }
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75 while (fcntl (fd,(op & LOCK_NB) ? F_SETLK : F_SETLKW,&fl))
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76 if (errno != EINTR) {
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77 /* Can't use switch here because these error codes may resolve to the
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78 * same value on some systems.
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79 */
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80 if ((errno != EWOULDBLOCK) && (errno != EAGAIN) && (errno != EACCES)) {
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81 sprintf (tmp,"Unexpected file locking failure: %s",strerror (errno));
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82 /* give the user a warning of what happened */
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83 MM_NOTIFY (NIL,tmp,WARN);
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84 if (!logged++) syslog (LOG_ERR,"%s",tmp);
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85 if (op & LOCK_NB) return -1;
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86 sleep (5); /* slow things down for loops */
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87 }
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88 /* return failure for non-blocking lock */
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89 else if (op & LOCK_NB) return -1;
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90 }
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91 return 0; /* success */
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92 }
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