Inserting a text into a specific line - c

Question

I am trying to create a function which inserts texts between lines according to the user input. The user has to specify the line number and the index in order to insert his line.
Currently, I have managed to insert the text before the line, but I can't insert it into the line "index".
Does anyone know how to insert according to the index number?
PS. I am still starter at C programming. I know there are too many times that the file is opened and closed!
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
void inserttext(void);
main ()
{
inserttext();
}
void inserttext(void)
{
FILE *file1,*file2;
char *f = malloc(sizeof(char)), *t = malloc(sizeof(char));
int l,i,r,y,n,index,nl=0;
printf("Enter a text file name: ");
scanf("%s",f);
if (access(f,F_OK)!=-1)//if the text file exists
{
file1=fopen(f, "r+");
file2=fopen("f2.txt", "w+");
printf("\nThe file before editing:\n\n");
while((n=fgetc(file1))!=EOF)// to show the contents of the file before the edit
{
putchar(n);
}
fclose(file1);
fclose(file2);
if(access(f,W_OK)!=-1)//if the file has the write permission
{
file1=fopen(f, "r+");
file2=fopen("f2.txt", "w+");
printf("\n\nPlease enter your text: \n");
scanf(" %[^\n]s ",t);
printf("Specify the line number where you want to insert: ");
scanf("%d", &l);
printf("\nindex:\n");
scanf("%d", &index);
while((r=fgetc(file1))!=EOF)//copying file1 contents into file2 contents
{
fputc(r,file2);
if(r == '\n' && ++nl == l){
fprintf(file2, "%s ", t);//adding the inserted text
}
}
fclose(file1);
fclose(file2);
file1=fopen(f, "w+");
file2=fopen("f2.txt", "r");
while((y=fgetc(file2))!=EOF){
fputc(y,file1);
}
fclose(file2);
fclose(file1);
remove("f2.txt");
file1=fopen(f, "r");
printf("\n");
while((i=fgetc(file1))!=EOF)//showing the result after inserting
{
putchar(i);
}
fclose(file1);
free(f);
free(t);
}
else{
printf("\n%s text file does not have the Write Permission!", f);
free(f);
free(t);
return;
}
}else{
printf("file doesn't exits!\n");
}
}

Answer 1

This uses ftell() and fseek() to save the file position at the start of the selected line, read the length of the line and return to the start of the line.
The user is prompted to input an index into the line less than the line length.
I did get a segmentation fault with the original malloc's for *t and *f. I tried some longer inputs, so this allocates 100 characters to each pointer.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
void inserttext(void);
int main ()
{
inserttext();
return 0;
}
void inserttext(void)
{
FILE *file1,*file2;
char *f = malloc(100), *t = malloc(100);
int l,i,r,y,n,index,nl=0;
int linelength = 0;;
long offset = 0;
printf("Enter a text file name: ");
scanf("%99s",f);
if (access(f,F_OK)!=-1)//if the text file exists
{
file1=fopen(f, "r+");
file2=fopen("f2.txt", "w+");
printf("\nThe file before editing:\n\n");
while((n=fgetc(file1))!=EOF)// to show the contents of the file before the edit
{
putchar(n);
}
fclose(file1);
fclose(file2);
if(access(f,W_OK)!=-1)//if the file has the write permission
{
file1=fopen(f, "r+");
file2=fopen("f2.txt", "w+");
printf("\n\nPlease enter your text: \n");
scanf(" %99[^\n]",t);
printf("Specify the line number where you want to insert: ");
scanf("%d", &l);
while((r=fgetc(file1))!=EOF)//copying file1 contents into file2 contents
{
fputc(r,file2);
if(r == '\n' && ++nl == l){
offset = ftell ( file1);//save location in file
while ( ( r = fgetc ( file1)) != '\n' && r != EOF) {
linelength++;//count characters in line
}
fseek ( file1, offset, SEEK_SET);//seek back to start of line
//get index where to insert text
do {
printf("\nindex(less than %d):\n", linelength);
if ( ( scanf("%d", &index)) != 1) {
scanf ( "%*[^\n]");//input not an integer. clear buffer
index = linelength;
}
} while ( index >= linelength || index < 0);
while ( index) {
r = fgetc ( file1);
fputc(r,file2);
index--;
}
fprintf(file2, "%s ", t);//adding the inserted text
}
}
printf("\nDONE:\n");
fclose(file1);
fclose(file2);
file1=fopen(f, "w+");
file2=fopen("f2.txt", "r");
while((y=fgetc(file2))!=EOF){
fputc(y,file1);
}
fclose(file2);
fclose(file1);
remove("f2.txt");
file1=fopen(f, "r");
printf("\n");
while((i=fgetc(file1))!=EOF)//showing the result after inserting
{
putchar(i);
}
fclose(file1);
free(f);
free(t);
}
else{
printf("\n%s text file does not have the Write Permission!", f);
free(f);
free(t);
return;
}
}else{
printf("file doesn't exits!\n");
}
}