Replacing strings in C - c

Question

Okay, guys, I have to write a program that forms a new string based on the given template and the given strings. The template is set as a string where it is necessary to replace all occurrences of the character '%' with a concrete string. If the template contains more characters '%' than the entered strings, characters '%' replaces strings cyclically. If no string is entered, print "ERROR". The number of strings is not known in advance. The end of the entry is indicated by a blank line ("\n").
Also, there are some conditions I need to fulfill, which are:
1) Implement char * readLine () function; which reads one row from the standard input and returns the pointer to that loaded row.
2) Implement char ** readLines (int * n) function; which reads strings that change all occurrences of the character '%' in the template. The function returns an array of pointers to the strings entered as a return value. Also, the function returns the number of values entered via argument n.
3) Implement char * format function (char * format, char ** values, int n); which formats the string format by changing each occurrence of the '%' character to a corresponding string from a string of values of length n.
4) Write a master program that, using previously implemented functions, reads the template and strings from the standard input, forms a new string as described, and prints the result to standard output.
I barely understand what I'm supposed to do so I came here to ask for help. What I did for now is the first task, but on the output I get 1 more blank line than I'm supposed to and I can't find any way to fix it. Any kind of help is welcomed since I'm really stuck here, even explaining what I'm supposed to do in some tasks, or making current code simpler is awesome. Thanks in advance. Here is my code.
EDIT: Added examples.
Input:
% be or not % be.
To
Output:
To be or not To be.
Input:
% and % make purple.
Blue
red
Output:
Blue and red make purple.
#include<stdio.h>
#include<stdlib.h>
char* readLine() {
char* line = malloc(100), * linep = line;
size_t lenmax = 100, len = lenmax;
int c;
if (line == NULL)
return NULL;
for (;;) {
c = fgetc(stdin);
if (c == EOF)
break;
if (--len == 0) {
len = lenmax;
char* linen = realloc(linep, lenmax *= 2);
if (linen == NULL) {
free(linep);
return NULL;
}
line = linen + (line - linep);
linep = linen;
}
if ((*line++ = c) == '\n')
break;
}
*line = '\0';
return linep;
}
int main() {
printf("%s", readLine());
return 0;
}
EDIT: For the 2nd task, I tried to do something similar like in the first, but I couldn't really figure out how to make it so that it works. Here is what I did:
char** readLines(int* n) {
char* line = malloc(100), * linep = line;
size_t lenmax = 100, len = lenmax;
int c, flag = 0;
if (line == NULL && flag > 1) {
char* linen = realloc(linep, lenmax *= 2);
}
else if (line == NULL && flag == 0)
return NULL;
for (;;) {
c = fgetc(stdin);
if (c == EOF || c == '\n')
break;
if (--len == 0) {
len = lenmax;
char* linen = realloc(linep, lenmax *= 2);
size_t diff = line - linep;
if (linen == NULL) {
free(linep);
return NULL;
}
line = linen + diff;
linep = linen;
flag++;
}
*line++ = c;
}
*line = '\0';
char* temp = linep;
return linep;
}

Answer 1

At least this problem
line = linen + (line - linep); is UB, as code cannot use linep after it has been free'd in realloc().
Instead calculate and save the line - linep difference before realloc()
size_t diff = line - linep; // add
char* linen = realloc(linep, lenmax *= 2);
if (linen == NULL) {
...;
}
// line = linen + (line - linep);
line = linen + diff;
OP apparently does not want to retain the '\n'. Change
// if ((*line++ = c) == '\n') break;
if (c == '\n') break;
*line++ = c;
I'd also recommend a right size realloc() step in the end.

Answer 2

If you want to read a line until EOF or '\n', but without the newline in your string, you need to do this:
for (;;) {
c = fgetc(stdin);
if (c == EOF)
break;
// realloc logic comes here
if (c == '\n')
break;
*line++ = c;
}