直接讀取檔案
說明：使用 O_DIRECT 旗標來繞過緩衝區，直接讀寫設備或檔案。使用memalign()函數分配一塊記憶體，該記憶體區塊與其第一個參數以倍數對齊。
此範例使用O_DIRECT旗標開啟與讀寫文件，這個程式最多接受四個參數，依次指定要讀取的文件、要從文件中讀取的位元數、程式在從文件讀取之前應該查詢的偏移量，以及傳遞給 read() 的資料緩衝區。最後兩個參數是非必要的選項，預設是 0 偏移量以及 4096 個位元組。
Listing 13-1
direct_read.c
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/*************************************************************************\
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* Copyright (C) Michael Kerrisk, 2022. *
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* *
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* This program is free software. You may use, modify, and redistribute it *
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* under the terms of the GNU General Public License as published by the *
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* Free Software Foundation, either version 3 or (at your option) any *
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* later version. This program is distributed without any warranty. See *
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* the file COPYING.gpl-v3 for details. *
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\*************************************************************************/
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/* Listing 13-1 */
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/* direct_read.c
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Demonstrate the use of O_DIRECT to perform I/O bypassing the buffer cache
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("direct I/O").
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Usage: direct_read file length [offset [alignment]]
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This program is Linux-specific.
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*/
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#define _GNU_SOURCE /* Obtain O_DIRECT definition from <fcntl.h> */
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#include <fcntl.h>
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#include <malloc.h>
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#include "tlpi_hdr.h"
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int
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main(int argc, char *argv[])
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{
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int fd;
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ssize_t numRead;
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size_t length, alignment;
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off_t offset;
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char *buf;
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if (argc < 3 || strcmp(argv[1], "--help") == 0)
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usageErr("%s file length [offset [alignment]]\n", argv[0]);
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length = getLong(argv[2], GN_ANY_BASE, "length");
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offset = (argc > 3) ? getLong(argv[3], GN_ANY_BASE, "offset") : 0;
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alignment = (argc > 4) ? getLong(argv[4], GN_ANY_BASE, "alignment") : 4096;
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fd = open(argv[1], O_RDONLY | O_DIRECT);
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if (fd == -1)
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errExit("open");
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/* memalign() allocates a block of memory aligned on an address that
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is a multiple of its first argument. By specifying this argument as
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2 * 'alignment' and then adding 'alignment' to the returned pointer,
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we ensure that 'buf' is aligned on a non-power-of-two multiple of
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'alignment'. We do this to ensure that if, for example, we ask
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for a 256-byte aligned buffer, we don't accidentally get
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a buffer that is also aligned on a 512-byte boundary. */
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buf = memalign(alignment * 2, length + alignment);
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if (buf == NULL)
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errExit("memalign");
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buf += alignment;
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if (lseek(fd, offset, SEEK_SET) == -1)
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errExit("lseek");
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numRead = read(fd, buf, length);
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if (numRead == -1)
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errExit("read");
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printf("Read %ld bytes\n", (long) numRead);
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exit(EXIT_SUCCESS);
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}
Last modified 7mo ago