How to append text inside an existing file without overwrite?

[gilgil]

Hi, I'm trying to add some strings inside a file in specific locations.
I'm opening a file with fopen and "r+a"

and after a while i'm trying to add strings that way:

char *line = (char *) malloc(MAX_LINE);
while (fgets(line, sizeof(line), theFile) != NULL)
    if (strstr(line, "I want to append here") != NULL)
        fputs("My new string", theFile);
 

But "My new string" will overwrite the next chars from the file.

example:
origin file:
a
b
c
I want to append here
d
e
ffffffffffffffffffffffffffff

overwrited file
a
b
c
I want to append here
My new string
ffffffffffffffffff


How can I deal with this problem?
Thanks in advanced,
Gil

[horace1]

as far as I am aware there is no simple way of inserting records into the middle of a file without overwritting existing records, e.g. see example using fseek()
http://www.cplusplus.com/reference/c.../cstdio/fseek/
you could
(1) copy the file to another inserting the new record at the required place.
(2) read the file into an array - do all your insertions (shuffling the array to make room) - then when finished write it out
(3) if you are dealing with whole lines read the file into a linked list (each element is a line of text) - do the insertions into the list - when finished write the list out to a file