将每行末尾的元素追加到文件中

Question

I want to append elements into a file line by line, not the end of the file. I wrote this code, but the result was that I could only append elements at the end of the file.

#include <stdio.h>
#include <stdio.h>

int main(void)
{
    static const char filename[] = "m.txt";
    FILE *file = fopen(filename, "r+a");
    char *a = "ok";

    if (file != NULL)
    {
        char line[128];         /* or other suitable maximum line size */

        while (fgets(line, sizeof line, file) != NULL)  /* read a line */
        {
            fputs(a, file);     /* write the line */
        }
        fclose(file);
    }
    else
    {
        perror(filename);       /* why didn't the file open? */
    }
    return 0;
}

Some answers have mentioned buffer.
I am a little confused with the buffer here. Does it mean I have to read more than one line into memory and then modify them? And is buffer the same as BufferedReader class in Java?

Answer 1

Files are not stored as an array of lines you can append to and manipulate at will, they are one big chunk of data you interpret. So if you were to do such a thing, you'd overwrite the rest of the file that follows your current line.

What you have to do is re-write at least the entire rest of the file whenever you do this. You can do that easily using a buffer.

Lines in files are just formatting and logical points marked by the newline character.

Answer 2

You need to read each line one at a time, modify it as you see fit, and then rewrite the whole line to a new file . Then when you have read the whole file you rename the new file as the old file.

Or you read the whole file into memory, for example into an array, one line per element of the array, modify the lines, and overwrite the contents of the file.

The problem with modifying a variable-length text file is that you nee4d to shift all data after the insertion point, which is not easy to do in a file. That's why it's easier to read the whole file into memory and do the modifications there, or use a temporary file that you then rename.