Interprocess message queue in c? - c

Question

I'm writing a program in C using inter-process communication, specifically I'm trying to write a program using an inter-process message queue. The program should work like this:
The command line accepts n-files (at least one). N-processes will be created as much as files.
The n processes must send the contents of the file to a process called Receiver which will have the task of printing the messages received.
The problem is: not all the contents of the file are printed, even if the processes send the messages. Why ? Could anyone tell me where I'm going wrong?
This is my code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/stat.h>
#define DIM_MSG 1024
#define TYPE_W 2
typedef struct
{
long mtype;
char eof;
char mtext[DIM_MSG];
} msg;
void child_r(int coda, const char *file)
{
FILE *r_stream;
if ((r_stream = fopen(file, "r")) == NULL)
{
perror("errore apertura file");
exit(1);
}
printf("%s:\n",file);
msg messaggio;
while (fgets(messaggio.mtext, DIM_MSG, r_stream) != NULL)
{
messaggio.mtype = TYPE_W;
messaggio.eof = 0;
if (msgsnd(coda, &messaggio, sizeof(msg) - sizeof(long) , 0) == -1)
{
perror("msgsnd");
exit(1);
}
printf("\tMessage send: %s", messaggio.mtext);
}
strcpy(messaggio.mtext, "quit");
messaggio.eof = 1;
messaggio.mtype = TYPE_W;
if (msgsnd(coda, &messaggio, sizeof(msg) - sizeof(long) , 0) == -1)
{
perror("msgsnd");
exit(1);
}
fclose(r_stream);
exit(0);
}
void child_f(int coda)
{
msg messaggio;
printf("\nReceiver\n");
do
{
if (msgrcv(coda, &messaggio, sizeof(msg) - sizeof(long), TYPE_W, 0) == -1)
{
perror("msgrcv");
exit(1);
}
if (strcmp(messaggio.mtext, "quit") != 0)
{
printf("\tMessage rcv: %s ", messaggio.mtext);
}
} while (messaggio.eof != 1);
exit(0);
}
int main(int argc, char const *argv[])
{
char *file_name = NULL;
struct stat sb;
int child=0;
int ds_coda;
if(argc<1)
{
fprintf(stderr,"Utilizzo %s <file-1> <file-2> <file-n>....",argv[0]);
exit(1);
}
if((ds_coda=msgget(IPC_PRIVATE,IPC_CREAT|IPC_EXCL|0600))==-1)
{
perror("coda");
exit(1);
}
/* analizza la command-line */
for (int i = 1; i < argc; i++) {
if ((stat(argv[i], &sb) == 0) && (S_ISREG(sb.st_mode)))
{
file_name = (char*)argv[i];
child++;
if(fork()==0)
{
child_r(ds_coda, file_name);
}
}
else {
perror(argv[i]);
exit(1);
}
}
if(child==0)
{
fprintf(stderr,"Parametri non validi!\n");
exit(1);
}
if(fork() == 0)
{
// child_w
sleep(1);
child_f(ds_coda);
}
else wait(NULL);
msgctl(ds_coda, IPC_RMID, NULL);
return 0;
}

Answer 1

A few issues ...
The main process needs to loop on wait before doing IPC_RMID. Otherwise, the sender processes will fail on msgsnd because the ds_coda is no longer valid. The main process has "raced" with the sender/receiver processes and removed the id with IPC_RMID before the other processes have completed.
All senders will set messsaggio.eof but the receiver stops after receiving the first one. It must know how many senders there are and wait until all have sent EOF. (i.e.) It must maintain a count.
Before I could debug this, I had to enhance the logging. So, I created tscgetf, logopen, and logprt to create separate logs for each process with timestamps.
When I was getting close, I added the from field to the message because the receiver was getting data but didn't know which process sent it. This helped diagnose the EOF issue.
Here is the refactored code. It is annotated with the bugs and fixes.
By default, it will show the EOF issue (i.e. it will hang). To apply the fix for the EOF issue, compile with -DFIXEOF
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <time.h>
#define DIM_MSG 1024
#define TYPE_W 2
typedef struct {
long mtype;
int from;
char eof;
char mtext[DIM_MSG];
} msg;
int pididx; // linear process ID (0=main)
int child = 0; // number of spawned children
int ds_coda;
FILE *xflog; // debug log stream
#define logprt(_fmt...) \
do { \
fprintf(xflog,"[%.9f/%5.5d] ",tscgetf(),pididx); \
fprintf(xflog,_fmt); \
fflush(xflog); \
} while (0)
double tsczero;
// tscget -- get timestamp
// RETURNS: elapsed seconds
double
tscgetf(void)
{
struct timespec ts;
double sec;
clock_gettime(CLOCK_MONOTONIC,&ts);
sec = ts.tv_nsec;
sec /= 1e9;
sec += ts.tv_sec;
sec -= tsczero;
return sec;
}
void
logopen(void)
{
char logf[100];
if (xflog != NULL)
fclose(xflog);
sprintf(logf,"log%3.3d",pididx);
xflog = fopen(logf,"w");
if (xflog == NULL) {
perror(logf);
exit(1);
}
}
void
child_r(int coda, const char *file)
{
FILE *r_stream;
logopen();
if ((r_stream = fopen(file, "r")) == NULL) {
perror("errore apertura file");
exit(1);
}
logprt("child_r: reading %s\n", file);
msg messaggio;
messaggio.from = pididx;
while (fgets(messaggio.mtext, DIM_MSG, r_stream) != NULL) {
messaggio.mtype = TYPE_W;
messaggio.eof = 0;
if (msgsnd(coda, &messaggio, sizeof(msg) - sizeof(long), 0) == -1) {
perror("msgsnd");
exit(1);
}
logprt("Message send: %s", messaggio.mtext);
}
strcpy(messaggio.mtext, "quit");
messaggio.eof = 1;
messaggio.mtype = TYPE_W;
#if 1
strcpy(messaggio.mtext,"I_AM_EOF\n");
#endif
if (msgsnd(coda, &messaggio, sizeof(msg) - sizeof(long), 0) == -1) {
perror("msgsnd");
exit(1);
}
logprt("Message EOF: %s", messaggio.mtext);
fclose(r_stream);
logprt("child_r: finished %s\n", file);
fclose(xflog);
exit(0);
}
void
child_f(int coda)
{
msg messaggio;
logopen();
// NOTE: we are started last so the count we need to wait for is one less
int waitcnt = pididx - 1;
logprt("Receiver starting -- waitcnt=%d\n",waitcnt);
while (1) {
if (msgrcv(coda, &messaggio, sizeof(msg) - sizeof(long), TYPE_W, 0) == -1) {
perror("msgrcv");
exit(1);
}
if (strcmp(messaggio.mtext, "quit") != 0) {
logprt("Message rcv (from %d): %s",
messaggio.from, messaggio.mtext);
}
if (messaggio.eof == 1) {
// NOTE/BUG: we can't stop after the first EOF message -- we must wait for all
// of them
#if ! FIXEOF
logprt("got EOF\n");
break;
#else
logprt("got EOF -- waitcnt=%d\n",waitcnt);
if (--waitcnt <= 0)
break;
#endif
}
}
logprt("child_f: complete\n");
fclose(xflog);
exit(0);
}
// start_rcv -- start receiver process
void
start_rcv(void)
{
logprt("start_rcv:\n");
child++;
pid_t pid = fork();
if (pid == 0) {
pididx = child;
// child_w
// NOTE/BUG: not necessary with other fixes
#if 0
sleep(1);
#endif
child_f(ds_coda);
}
logprt("start_rcv: pid=%d child=%d\n",pid,child);
}
int
main(int argc, char const *argv[])
{
char *file_name = NULL;
struct stat sb;
tsczero = tscgetf();
#if 1
pid_t pid;
setlinebuf(stdout);
setlinebuf(stderr);
#endif
logopen();
if (argc < 1) {
fprintf(stderr, "Utilizzo %s <file-1> <file-2> <file-n>....", argv[0]);
exit(1);
}
if ((ds_coda = msgget(IPC_PRIVATE, IPC_CREAT | IPC_EXCL | 0600)) == -1) {
perror("coda");
exit(1);
}
// NOTE: early attempt to fix (receiver should start first) but didn't fix it
// and won't work because receiver needs to know the number of EOF messages to
// wait for
#if RCVEARLY
start_rcv();
#endif
/* analizza la command-line */
for (int i = 1; i < argc; i++) {
if ((stat(argv[i], &sb) == 0) && (S_ISREG(sb.st_mode))) {
file_name = (char *) argv[i];
child++;
pid = fork();
if (pid == 0) {
pididx = child;
child_r(ds_coda, file_name);
}
#if 1
else {
logprt("forked: pid=%d child=%d\n",pid,child);
}
#endif
}
else {
perror(argv[i]);
exit(1);
}
}
if (child == 0) {
fprintf(stderr, "Parametri non validi!\n");
exit(1);
}
// NOTE/FIX: main process must wait for _all_ children to complete before
// doing IPC_RMID
#if 1
#if ! RCVEARLY
start_rcv();
#endif
while (1) {
pid_t pid = wait(NULL);
logprt("waitfor: %d child=%d\n",pid,child);
if (pid <= 0)
break;
--child;
}
#endif
msgctl(ds_coda, IPC_RMID, NULL);
fclose(xflog);
return 0;
}
In the code above, I've used cpp conditionals to denote old vs. new code:
#if 0
// old code
#else
// new code
#endif
#if 1
// new code
#endif
Note: this can be cleaned up by running the file through unifdef -k