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1.1 root 1: Shared Memory and How To Use It
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4:
5: MiNT has a special directory, U:\SHM, which provides a place for
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7: programs to advertise memory that they are offering to share.
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9: To create a shared memory file, a program uses the Fcreate call
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11: to create a file in U:\SHM, e.g.:
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15: fd = Fcreate("U:\\SHM\\MY.SHARE", 0);
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19: It then uses an Fcntl call to attach a block of memory (previously
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21: allocated by Malloc or Mxalloc) to the file:
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25: blk = Malloc(128L);
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27: Fcntl(fd, blk, SHMSETBLK);
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32:
33: Several things should be noted when creating a shared memory
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35: file:
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37:
38:
39: (1) The file's attributes must be 0. Read-only shared memory, or
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41: shared memory with other attributes, is not yet implemented,
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43: but may be in the future.
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47: (2) Two shared memory files cannot have the same name. An attempt
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49: to create a new shared memory file with the same name as an
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51: existing one will fail with an access denied error (EACCDN).
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55: (3) Once the block of memory has been attached to the file, it
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57: may be accessed by any application that opens the file.
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61: (4) A shared memory file (and associated block) remain allocated
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63: even after the program which created it terminates. It can be
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65: deleted (and the associated memory freed) with an Fdelete()
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67: system call.
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71: (5) The size of the shared memory file will be the actual size
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73: of the memory block. This may be somewhat larger than the
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75: size requested in the Malloc or Mxalloc request, due to memory
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77: rounding.
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83: To use a shared memory block, a client application must open
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85: the file and use the SHMGETBLK Fcntl to gain access to it.
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87: For example:
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91: fd = Fopen("U:\\SHM\\MY.SHARE", 2);
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93: blk = Fcntl(fd, 0L, SHMGETBLK);
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95: Fclose(fd); /* optional -- see below */
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97:
98:
99: Things to note:
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103: (1) The address of the shared memory block is returned by the
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105: Fcntl call. NOTE THAT THIS ADDRESS MAY BE DIFFERENT FOR
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107: DIFFERENT PROGRAMS. That is, a shared memory block that appears
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109: at address 0x01000100 in one program may appear at address
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111: 0x0007f000 in another. In particular, shared memory blocks
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113: should not contain absolute addresses (e.g. pointers).
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117: (2) The extra argument passed to Fcntl is reserved for future
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119: expansion; use 0L for now to ensure compatibility with
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121: future versions of MiNT.
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125: (3) The mode argument in the Fopen function must be an accurate
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127: reflection of how the program plans to use the memory; read and
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129: write access permissions will be enforced in future versions
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131: of MiNT.
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135: (4) If no SHMSETBLK has been made for the file, a SHMGETBLK Fcntl
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137: will return a NULL pointer to indicate an error.
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141: (5) If a program is finished with a shared memory block and no
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143: longer wishes to use it, it should call Mfree() with the address
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145: of the block (i.e. the address returned by Fcntl(fd, 0L, SHMGETBLK)).
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151: Deleting a Shared Memory File
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154:
155: The Fdelete() system call may be used to delete a shared memory
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157: file. This will *not* necessarily free the associated memory;
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159: the memory will actually be freed only after (1) the file has
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161: been deleted, and (2) all processes using the memory have
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163: freed the memory, either directly or as a result of the process
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165: terminating.
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169: Fdelete() will fail if the shared memory file is still open.
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171: Processes may omit the Fclose() call if they wish this to happen;
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173: it's a way of informing the process trying to delete the file
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175: that people are still interested in it. Note that it is *not*
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177: harmful to allow the Fdelete to occur, since (as noted above)
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179: the memory will not actually be freed until everyone is finished
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181: with it; but sometimes it may be useful for programs to know that
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183: the memory is still in use.
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