如何在C中最后一行之前添加一行
你好，我在 Unix 平台上使用 C 语言工作。请告诉我如何在 C 中的最后一行之前追加一行。我在追加模式下使用了 fopen，但我无法在最后一行之前添加一行。
我只想写入文件中的倒数第二行。
Hi I am working in C on Unix platform. Please tell me how to append one line before the last line in C. I have used fopen in appending mode but I cant add one line before the last line.
I just want to write to the second last line in the file.
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您不需要覆盖整个文件。您只需：
ftell/ftello）存储在文件及其中内容fseek/fseeko)You don't need to overwrite the whole file. You just have to:
ftell/ftello) in the file and its contentsfseek/fseeko)在标准 C 中无法直接执行此操作，主要是因为很少有文件系统支持此操作。最简单的方法是将文件读入内存结构（您可能已经拥有它），将行插入内存中，然后再次写出整个结构，覆盖原始文件。
There is no way of doing this directly in standard C, mostly because few file systems support this operation. The easiest way round this is to read the file into an in memory structure (where you probably have it anyway), insert the line in memory, then write the whole structure out again, overwriting the original file.
Append 只追加到末尾，而不追加到中间。
您需要读入整个文件，然后将其写入新文件。您可能会幸运地从后面开始，并找到倒数第二个换行符的字节偏移量。然后，您可以阻止编写整个“前奏曲”，添加新行，然后发出剩余的预告片。
Append only appends to the end, not in the middle.
You need to read in the entire file, and then write it out to a new file. You might have luck starting from the back, and finding the byte offset of the second-to-last linefeed. Then you can just block write the entire "prelude", add your new line, and then emit the remaining trailer.
您可以找到最后一行结束的位置，将最后一行读入内存，查找回该位置，写入新行，然后写入最后一行。
要找到该位置：查找到末尾，减去缓冲区大小。读缓冲区，寻找
换行符。如果未找到，则向后查找两个缓冲区大小，然后重试。
您需要对
fopen使用 r+ 模式。哦，您需要小心文本和二进制模式。您需要使用二进制模式，因为在文本模式下您无法计算跳转位置，只能跳转到从
ftell获取的位置。您可以通过阅读整个文件并在每行开头调用ftell来解决此问题。对于大文件，速度会很慢。You can find the place where the last line ends, read the last line into memory, seek back to the place, write the new line, and then the last line.
To find the place: Seek to the end, minus a buffer size. Read buffer, look for
newline. If not found, seek backwards two buffer sizes, and try again.
You'll need to use the r+ mode for
fopen.Oh, and you'll need to be careful about text and binary modes. You need to use binary mode, since with text mode you can't compute jump positions, you can only jump to locations you've gotten from
ftell. You can work around that by reading through the entire file, and callingftellat the beginning of each line. For large files, that is going to be slow.使用
fseek跳转到文件末尾，向后读取直到遇到换行符。然后插入你的线。您可能希望通过计算向后读取的字符数来保存正在读取的“最后一行”，然后将其 strncpy 到正确分配的缓冲区。
Use
fseekto jump to end of file, read backwards until you encounter a newline. Then insert your line.You might want to save the 'last line' you are reading by counting how many chars you are reading backwards then strncpy it to a properly allocated buffer.