c - 处理输入文件时如何向前看(处理 2 行)

Question

当逐行浏览文本文件时，我希望能够向前看下一行并在处理当前行时检查它。我正在使用 C 语言工作。我相信 fseek() 或其他类似函数会帮助我解决这个问题，但我不确定也不知道如何使用它们。我想达到这样的效果:

    fp = fopen("test-seeking.txt", "r");

    while((fgets(line, BUFMAX, fp))) {
        // Peek over to next line
        nextline = ...;
        printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n",
               line, nextline);
    }

感谢任何帮助。

Answer 1

事实上，您可以使用 fseek 并尝试如下操作:

fp = fopen("test-seeking.txt", "r");

while ((fgets(line, BUFMAX, fp))) {
    // Get the next line
    fgets(nextline, BUFMAX, fp);

    // Get the length of nextline
    int nextline_len = strlen(nextline);

    // Move the file index back to the previous line
    fseek(fp, -nextline_len, SEEK_CUR); // Notice the - before nextline_len!

    printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n", line, nextline);
}

---

另一种方法是使用fgetpos 和fsetpos，如下所示:

fp = fopen("test-seeking.txt", "r");

while ((fgets(line, BUFMAX, fp))) {
    // pos contains the information needed from
    //   the stream's position indicator to restore
    //   the stream to its current position. 
    fpos_t pos;

    // Get the current position
    fgetpos(fp, &pos);

    // Get the next line
    fgets(nextline, BUFMAX, fp);

    // Restore the position
    fsetpos(fp, &pos);

    printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n", line, nextline);
}