Trouble with shared memory program in C - c

Question

I have an assignment that consists of making a program with shared memory that has a parent process, which reads from a .txt and adds its contents to a buffer (this is the shared memory), and two child processes that switch the case of the contents of the buffer, and output them in two files. One child outputs them in uppercase and the other one in lowercase. I've implemented semaphores and shared memory, but my output files turn out empty. By the way, the program is run on Linux; I use a virtual machine for the deed. I'd like to get some guidance as to what I'm not doing correctly.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
int main() {
FILE *archivo, *salida1, *salida2;
int i = 0;
int j, k;
char actual;
key_t claveMC = ftok("UCAB.txt", 5);
int shmid = shmget(claveMC, 512, 0777 | IPC_CREAT);
char *buffer = (char*)shmat(shmid, NULL, 0);
key_t claveSem = ftok("UCAB.txt", 7);
int idsem = semget(claveSem, 1, IPC_CREAT | 0100);
struct sembuf valores;
valores.sem_num = 0;
valores.sem_flg = 0;
semctl(idsem, 0, SETVAL, 1);
archivo = fopen("UCAB.txt", "r");
int id = fork();
int id2;
if (id > 0) {
printf("P: %d", id);
id2 = fork();
if (id2 > 0) {
while (((actual = getc(archivo)) != EOF) && (i < 512)) {
valores.sem_op = -1;
semop(idsem, &valores, 1);
buffer[i] = actual;
valores.sem_op = 1;
semop(idsem, &valores, 1);
i++;
}
valores.sem_op = -1;
semop(idsem, &valores, 1);
printf("%s", buffer);
valores.sem_op = 1;
semop(idsem, &valores, 1);
fclose(archivo);
printf("\nPadre con id: %d", id2);
}
else {
printf("H2: %d", id);
salida2 = fopen("minusculas.txt", "w");
for (j = 0; j < i; j++) {
valores.sem_op = -1;
semop(idsem, &valores, 1);
putc(tolower(buffer[j]), salida2);
valores.sem_op = 1;
semop(idsem, &valores, 1);
}
printf("\nHijo 2 con id: %d", id2);
fclose(salida2);
}
}
else {
//hijo1
printf("H1: %d", id);
salida1 = fopen("MAYUSCULAS.txt", "w");
for (k = 0; k < i; k++) {
valores.sem_op = -1;
semop(idsem, &valores, 1);
putc(toupper(buffer[k]), salida1);
valores.sem_op = 1;
semop(idsem, &valores, 1);
}
printf("\nHijo 1 con id: %d", id);
fclose(salida1);
}
shmctl(shmid, IPC_RMID, NULL);
return 0;
}

Answer 1

The children write i characters to their file, but i is only incremented in the parent after the children have been forked, so their i remains zero.

Answer 2

After fork, the child is a clone of the parent (with differences, like pid). But the address spaces of the parent and child are different. So, the variable "i" of the parent is not the same as the variable "i" of either of the children.