如何在C中使用stdio在文件中正确移动查找指针

Question

I'm messing around with seek pointers in C, and my exercise is to write a database (format: ID NAME SURNAME AGE) into a textfile. So everything works fine (saving new data etc.) but I can't find a solution on how to delete a line in my textfile.

void cut_from_file(char* filename, int start, int stop) {
    FILE *fp = fopen(filename, "w");
    fpos_t position;
    fseek(fp, 34, SEEK_SET);
    fputs("Hello world", fp);
    fclose (fp);
}

Note: I want to make it somehow a random access file.

So my question would be how can I move the filepointer let's say 34 characters forward and overwrite the characters on that position but leave the characters from position 0 - 33 as they are?

If I try to do it like on my test function above I overwrite everything from offset 0 - 33 with '/00' and on position 34 is Hello World.

Answer 1

fopen(filename, "r+") was the answer, gosh I messed that up pretty hard.

Thanks everyone :)

Answer 2

Use SEEK_CUR instead of SEEK_SET to move forward from the currect index. SEEK_SET takes you 34 chars forward from the beginning of the file. Also "w" in opening the file overwrites all its contents.

Answer 3

There's no way of simply making a chunk of the file disappear.

The two possible solutions that I can see are:

1. Read the whole file in, write it out and skip the line(s) you want to delete.
2. Use an extra byte (and fixed-width records!) to indicate if an entry is "deleted" or "not deleted". That means deleting a record is a single write and you then know you can re-use the space for another record.

Answer 4

要从文件中删除一行，您可以打开一个临时文件，并将内容逐行写入其中，然后跳过要删除的行，稍后可以用该临时文件替换原始文件。

Answer 5

(1) Move the last entry to the entry you want to delete. (2) Call truncate to shorten the file.