Insert in a file without overwriting without using a 2nd file using C/C++

The below are the contents in a text file.

name1: 1234
name2: 2000
name3: 3000

This is an existing text file and I want to replace one value(say 1234) with another value (say 12345) in the text file. so I placed the cursor at start of the value (here its the 7th position) . Then i used the following statement:

fprintf(filepointer,"12345\n");

The resultant file is like

name1: 12345
ame2 : 2000
name3 : 3000

Its overwriting the 4 characters("1000") and a newline('\n') and 'n' with 5 characters("12345") and a newline('\n').

The solutions I know are:
1. Overwriting the entire file to add one extra character.
2. Copying each line in a linked list node and change the characters in the memory and write in the same file.
3. Create a temp file and copy the new characters to the desired place in the temp file and change the name of the temp file to source file name and delete the source file.

Also I tried adding carriage return '\r' and windows format of EOF ('\r\n') , still the next line characters are overwritten. Also I expanded the file size using [SetEndOfFile][1] API and still I face the same problem. I searched many forums and found answers like "It is not possible to insert characters without overwriting".

Is there any solution just to insert characters without overwriting the characters in the middle of the file. or any logic to insert characters in a line and not affect the next line.
Thanks in advance.