Inserting text into a txt file - c

Question

I'm trying to accomplish the following:
I have a text file with the last printable character "]" in a separate line.
This line is not necessary to be the last line of the file. Some blank lines (line returns) can be there.
The purpose of the project is to insert new text before the line with "]".
The way I try to implement this is to search the file from the end of the file to find the line number with the character "]" (char_line).
Copy line by line from the original file rules1.txt to rules2.txt up to the line char_line. Next step is to append the new text with "]" at the end.
After that, I can delete the original file and rename the new file from rules2.txt to rules1.txt.
The problem I have is that the program finds the line with the character "]" and I can do a printf and see the correct line number.
I am assigning char_line = "%d".
When I'm using if(i < char_line) the file is copied all the way to EOF.
If I assign a numerical value, char_line = 23, the file is copied up to line 22, which is what I want.
This is the part of the code which should find line number for "]", copy line by line rules1.txt to rules2.txt up the line with "]".
#include <stdio.h>
int main(void) {
int end, loop, line;
char str[64];
FILE *file;
FILE *write;
int char_line;
int ret;
file = fopen("rules1.txt", "r");
if (file == NULL) {
printf("Failed to open file\n");
return -1;
}
int ch, line_num = 0;
do {
ch = fgetc(file);
if(ch == '\n')
line_num++;
} while (ch != EOF);
// last line doesn't end with a new line!
// but there has to be a line at least before the last line
if(ch != '\n' && line_num != 0)
line_num++;
fclose(file);
line = line_num-1;
start:
file = fopen("rules1.txt", "r");
if (file == NULL) {
printf("Failed to open file\n");
return -1;
}
for(end = loop = 0;loop<line;++loop){
if(0==fgets(str, sizeof(str), file)){//include '\n'
end = 1;//can't input (EOF)
break;
}
}
if(!end)
if (strncmp ("]", str, 1) == 0){
char_line ="%d";
goto next;
} else if(line >1){
line == line--;//WTF?
fclose(file);
goto start;
} else //What to do here?
next:
file = fopen("rules1.txt", "r");
write = fopen("rules2.txt", "w");
char linec [64]; /* line size */
int i = 0;
while (fgets(linec, sizeof(linec), file)) { /* read line from file */
i++;
if(i < char_line) {
fprintf (write , linec); /* write line to file */
}
}
fclose(file);
fclose(write);
return(0);
}

Answer 1

You have declared int char_line; and later you code
char_line ="%d";
This is nonsense. Since a literal string is some char[] array (better think of it as some constant array), decayed to a pointer, and assigning a pointer to an int does not make sense at all. On many machines (x86-64 notably), a pointer has 64 bits but an int has only 32 bits.
Please, compile your code with all warnings and debug info, so gcc -Wall -Wextra -g with GCC, improve your code to get no warnings, then use the debugger gdb.
Take several days to read some good books on C programming. Be aware and work hard to avoid undefined behavior. Use the debugger to run your program step by step and query its state to understand what is happening.
Read the documentation of every standard function you are using such as fgets.
You probably want a loop that reads every line, and copies sometimes that line to another file.