count.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    if (argc != 2) {
        printf("Usage: %s <file>\n", argv[0]);
        return 1;
    }

    FILE *file = fopen(argv[1], "r");
    if (file == NULL) {
        printf("Error: Failed to open file %s\n", argv[1]);
        return 1;
    }

    fseek(file, 0L, SEEK_END);
    size_t file_size = ftell(file);
    fseek(file, 0L, SEEK_SET);

    char *data = malloc(sizeof(char) * file_size);
    fread(data, 1, file_size, file);

    size_t count = 0;
    for (char *end = data + file_size; data != end; data++)
        if (*data == '\n') count++;

    fclose(file);

    printf("%zu\n", count);

    return 0;
}

Makefile

# write is for writing N newlines to a file.
# count is from counting the newlines in a file.
CC := gcc

CFLAGS := -Wall -Wextra -Werror \
		  -std=c11 -pedantic

TARGETS := count \
		   write

.PHONY: all

all: $(TARGETS)

%: %.c
	$(CC) $< -o $@ $(CFLAGS)

write.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{
    if (argc != 3) {
        printf("Usage: %s <file> <line count>\n", argv[0]);
        return 1;
    }

    FILE *file = fopen(argv[1], "w");
    if (file == NULL) {
        printf("Error: Failed to open file %s\n", argv[1]);
        return 1;
    }

    long long line_count = atoi(argv[2]);
    char *data = malloc(sizeof(char) * line_count);
    memset(data, '\n', line_count);
    fwrite(data, 1, line_count, file);

    fclose(file);

    return 0;
}