The Linux Programming Interface〈檔案 I/O〉

系統呼叫

大多的 Process 可以使用以下 3 種標準file descriptor

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0 標準輸入 stdin
1 標準輸出 stdout
2 標準錯誤 stderr

shell 一開始會將這三個一直開啟

在程式中你可以直接使用 0、1、2 表示

或是include <unistd.h> 並使用

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STDIN_FILENO
STDOUT_FILENO
STDERR_FILENO

範例檔案如下 

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int
main(int argc, char *argv[])
{
    int     inputFd, outputFd, openFlags;
    mode_t  filePerms;
    ssize_t numRead;
    char    buf[BUF_SIZE];

    if (argc != 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s old-file new-file\n", argv[0]);

    /* Open input and output files */

    inputFd = open(argv[1], O_RDONLY);
    if (inputFd == -1)
        errExit("opening file %s", argv[1]);

    openFlags = O_CREAT | O_WRONLY | O_TRUNC;
    filePerms = S_IRUSR | S_IWUSR  | S_IRGRP | S_IWGRP |
                S_IROTH | S_IWOTH;      /* rw-rw-rw- */
    outputFd = open(argv[2], openFlags, filePerms);
    if (outputFd == -1)
        errExit("opening file %s", argv[2]);

    /* Transfer data until we encounter end of input or an error */

    while ((numRead = read(inputFd, buf, BUF_SIZE)) > 0)
        if (write(outputFd, buf, numRead) != numRead)
            fatal("couldn't write whole buffer");
    if (numRead == -1)
        errExit("read");

    if (close(inputFd) == -1)
        errExit("close input");
    if (close(outputFd) == -1)
        errExit("close output");

    exit(EXIT_SUCCESS);
}

open syscall flag 參數

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O_RDONLY  # 唯讀
O_WRONLY  # 唯寫
O_RDWE    # 讀寫