Странный результат в проблеме синхронизации с использованием двоичных семафоров в C ⇐ Linux

[Гость]

Recently i came across this exercise that for some reason always gives me strange output, here is the code followed by an output example:
HERE'S THE CONSUMER PROCESS

Код: Выделить всё

#include 
#include 
#include 
#include 
#include 
#include "sem.h"

#define nome_file "consumator_productor.txt"
#define DIM 1048
#define SEM_KEY_WRITE (key_t)1234
#define SEM_KEY_READ (key_t)5678

int SEM_P(int);
int SEM_V(int);
int SEM_DEL(int);
int SEM_SET(int, int);

int main(void){
int running = 1;
int fd, WRITE_ID, READ_ID, c;
char buffer[DIM]; //needed variables

if((WRITE_ID = semget(SEM_KEY_WRITE, 1, IPC_CREAT | 0666)) == -1) exit(EXIT_FAILURE);
SEM_SET(WRITE_ID, 1);
if((READ_ID = semget(SEM_KEY_READ, 1, IPC_CREAT | 0666))== -1) exit(EXIT_FAILURE);
SEM_SET(READ_ID, 0); //dichiaro e inizializzo i semafori

while(running){
if(SEM_P(READ_ID) == -1) exit(EXIT_FAILURE); //aspetto il via alla lettura
fd = open(nome_file, O_RDONLY, 0440);
read(fd, buffer, sizeof(buffer));
close(fd);
if(strncmp(buffer, "end", 3) == 0){ //controllo se trovo end
running = 0;
} else {
printf("%s\n", buffer);
}
if(SEM_V(WRITE_ID) == -1) exit(EXIT_FAILURE); //do il via alla scrittura
}
if(SEM_DEL(WRITE_ID) == -1) exit(EXIT_FAILURE);
if(SEM_DEL(READ_ID) == -1) exit(EXIT_FAILURE);
exit(EXIT_SUCCESS);
}

HERE'S THE PRODUCTOR PROCESS:

Код: Выделить всё

#include 
#include 
#include 
#include 
#include 
#include "sem.h"

#define nome_file "consumator_productor.txt"
#define DIM 1048
#define SEM_KEY_WRITE (key_t)1234
#define SEM_KEY_READ (key_t)5678

int SEM_P(int);
int SEM_V(int);
int SEM_DEL(int);
int SEM_SET(int, int);

int main(void){
int running = 1;
int fd, WRITE_ID, READ_ID;
char buffer[DIM];

if((WRITE_ID = semget(SEM_KEY_WRITE, 1, IPC_CREAT | 0666)) == -1) exit(EXIT_FAILURE);
if((READ_ID = semget(SEM_KEY_READ, 1, IPC_CREAT | 0666)) == -1) exit(EXIT_FAILURE);

while(running){
if(SEM_P(WRITE_ID) == -1) exit(EXIT_FAILURE); //aspetto il via alla scrittura
fd = open(nome_file, O_CREAT | O_TRUNC | O_WRONLY, 0660);
printf("Write on your file:\n");
fgets(buffer, sizeof(buffer), stdin);
write(fd, buffer, strlen(buffer));
if(strncmp(buffer, "end", 3) == 0){
running = 0;
}
close(fd);
if(SEM_V(READ_ID) == -1) exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}

HERE'S THE CONSUMATOR OUTPUT:

Код: Выделить всё

os@debian:~/Esercizi/semafori/libreria$ ./consumator
Ciao, ciao e ciao // i wrote it using productor process
// i didn't
Mia, mio e miei //i wrote it
o //i didn't
//i didn't
Ok ora basta. //i wrote it
i //i didn't
o //i didn't

I tried executing the 2 processes multiple times and each time they gave me a different strange output.


Источник: https://stackoverflow.com/questions/781 ... hores-in-c