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cfwritefseek

Using fseek and fwrite to overwrite a line in a file

I have a text file where each line is terminated by a "\n". Like so

0000
0000
0000
0000

Now, a file initially starts like above but a line can be overwritten with another line of variable length. I am using fseek() in order to set the position indicator of the stream. I have no problems moving to a line and replacing for example 0000 with 1111.

Now lets say I want to replace the first line with 1111 1111. I do the following

char *str = "1111 1111\n";
fwrite(str, 1, strlen(str), file);

This deletes line 2 and 1 byte from line 3 like so

1111 1111
000
0000

What is the cause of this?

Solution

  • If you think of your file like: "0000\n0000\n0000\n0000\n"

    It will be easier to understand that when you overwrite first line you are overwriting first n-number of chars really. So that is why your second line disappears.