Message queue/shared memory method - linux

Question

I have a bit of a problem in using IPC (inter-process communication) program below.
Please let me explain:
I want to pass Linux commands such as "ls" or "wc file.txt"
from a parent to a child to execute using the message queue, and
then have the child returning the command outputs back to
the parent process using shared memory method.
But this is what I got: The parent process always got the output 1 step behind;
in the following fashion:
Step1) ls file.txt
(Nothing showed up.)
Step2) wc file.txt
(Output of earlier command "ls file.txt" showed up here instead.)
Step 3) cat file.txt
(Output of earlier command "wc file.txt" showed up instead.)
Any help is appreciated.
To compile: gcc -o program ./program.c
To run: -./program -v
Code:
#define BUFSZ 512
#define ERRBUFSZ 512
#define TIMEOUT_TIMEDIO 20
#define SHM_SIZE 5120
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <setjmp.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
static sigjmp_buf jmpbuf;
int timed_io(char* buf, int len, FILE* rfp, int sec);
static void sigalrm_handler(int signo);
void do_cmd(char *buf, int len, int linenum, char *errbuf);
int parse_cmd(char *buf, char **vbuf, char *errbuf);
int process_cmd_ipc(char *argv, int linenum, char *errbuf);
struct my_msgbuf {
long mtype;
char mtext[256];
};
static void sigalrm_handler(int signo)
{
siglongjmp(jmpbuf, 1);
}
int timed_io(char* buf, int len, FILE* rfp, int sec)
{
struct sigaction nsigaction[1];
struct sigaction osigaction[1];
int prev_alrm;
int st = 0;
if(sigsetjmp(jmpbuf, 1) == 0)
{
nsigaction->sa_handler = sigalrm_handler;
sigemptyset(&nsigaction->sa_mask);
nsigaction->sa_flags = SA_RESTART;
prev_alrm = alarm(0);
sigaction(SIGALRM, nsigaction, osigaction);
alarm(sec);
if (fgets(buf, len, rfp) == NULL)
st = -1; // EOF
buf[strlen(buf) - 1] = 0;
}
else { st = -2; } // Time-out
alarm(0); // Reset old alarm and handler
sigaction(SIGALRM, osigaction, 0);
return st;
}
int process_cmd_ipc(char *argv, int linenum, char* errbuf)
{
struct my_msgbuf buf;
int msqid, msqid_parent, st, shmid, str_len;
key_t key, key_shm;
char* shared_buf;
FILE *fd;
// create key for shared memory segment
if ((key_shm = ftok("shm_key.txt", 'R')) == -1) {
perror("ftok");
exit(1);
}
// Connect to shared memory segment
if ((shmid = shmget(key_shm, SHM_SIZE, 0644 | IPC_CREAT)) == -1)
{
perror("shmget");
exit(1);
}
// Attach to shared memory segment
shared_buf = shmat(shmid, (void *) 0, 0);
if (shared_buf == (char *) (-1)) {
perror("shmat");
exit(1);
}
// End of shared memory section` //
// Begin: message queue section
pid_t cpid=fork();
if (cpid<0) {
fprintf(stderr,"ERR: \"fork\" error! (Line=%d)\n", linenum);
exit (-1);
} else if (cpid==0) // child process
{ // Begin: message queue
if ((key = ftok("mysh.c", 'B')) == -1) {
perror("ftok");
exit(1);
}
if ((msqid = msgget(key, 0644)) == -1) {
perror("msgget from child");
exit(1);
}
memset(buf.mtext, 0, sizeof(buf.mtext)); // Clear buffer
if(msgrcv(msqid, (struct msgbuf*) &buf, sizeof(buf), 0,0) == -1)
{
perror("msgrcv");
exit(1);
}
// End: message queue
// begin: shared memory segment
memset(shared_buf, 0, SHM_SIZE); // zeroize shared_buf
fd = popen(buf.mtext, "r");
str_len = 0;
while(fgets(shared_buf + str_len, SHM_SIZE, fd) != NULL)
{ str_len = strlen(shared_buf); }
pclose(fd);
// end: shared memory segment
}
else { // parent
// Begin - message queue
if ((key = ftok("mysh.c", 'B')) == -1) {
perror("ftok");
exit(1);
}
if ((msqid_parent = msgget(key, 0644 | IPC_CREAT)) == -1) {
perror("msgget from parent");
exit(1);
}
buf.mtype = 1;
strncpy(buf.mtext, argv, strlen(argv));
if(msgsnd(msqid_parent, (struct my_msgbuf*) &buf, strlen(buf.mtext), 0) == -1)
perror("msgsnd");
// End - message queue
// Begin - shared memory
// usleep(10000);
printf("%s", shared_buf);
// End - shared memory
} // if-else fork
}
int parse_cmd(char *buf, char **vbuf, char *errbuf)
{
int i=0;
char *delim=" ,\t\n";
char *tok;
tok=strtok(buf,delim);
while (tok) {
vbuf[i]=(char *)malloc(BUFSZ*sizeof(char));
strcpy(vbuf[i],tok);
tok=strtok(NULL,delim);
i++;
}
vbuf[i]=0;
return i;
}
void do_cmd(char *buf, int len, int linenum, char *errbuf) {
int i=0; int numargs;
char *vbuf[128];
char* copy = (char *) malloc(strlen(buf) + 1);
int maxargs=sizeof(vbuf)/sizeof(char *);
strcpy(copy, buf);
numargs = parse_cmd(copy,vbuf,errbuf);
process_cmd_ipc(buf,linenum, errbuf);
for (i=0;i<numargs; i++) { free(vbuf[i]); }
free(copy);
copy = NULL;
return;
}
int main(int argc, char **argv)
{
int i; int st; int linenum=0;
char *buf=(char *)malloc(BUFSZ*sizeof(char));
char *errbuf=(char *)malloc(ERRBUFSZ*sizeof(char));
char *mysh = "";
FILE *rfp=stdin;
if (isatty(fileno(rfp))) {
mysh = "mysh (Ctrl-C to exit)>";
fprintf(stderr,"%s",mysh);
}
while(1)
{
st = timed_io(buf, BUFSZ, stdin, TIMEOUT_TIMEDIO);
if (st != 0)
{
fprintf(stderr, "ERR: No input %s (Status=%d)\n", errbuf, st);
return -1;
}
else
{
linenum++;
if (*buf)
{ do_cmd(buf, BUFSZ, linenum,errbuf); }
if (mysh)
fprintf(stderr,"%s",mysh);
}
}
}