Problem with shared memory in C under Linux

d1s4st3r · 06-27-2005, 04:26 AM

Hi guys,
I'm writing a program using shared memory segments and I have the following problem:
first of all, I create a key, get a memory segment and then attach it (I will post only the necessary parts of code):

Code:

#define		MAX	5
#define		LEN	40

/* here the main() begins */

const key_t GENKEY = ftok(".", 'x');
char *smp1;
int shmid;

shmid = shmget(GENKEY, sizeof(char)*(MAX*LEN), IPC_CREAT|0655);

if (shmid==(-1))
{
    fprintf(stderr, "Error message.\n");
    exit(1);
}
else
{
    smp1 = (char *)shmat(shmid, 0, 0);
}

the program proceeds then by reading messages from a message queue, and for each message received it forks.
This is the following snippet:

Code:

pid = fork();

if (pid==(-1))
{
    fprintf(stderr, "Error message.\n");
    exit(1);
}
else if (pid==0) /* child process */
{
    char *answer = malloc(sizeof(char)*LEN);
    char *p;
    int objs = 0;
    int i;
    p = smp1; /* this points to the first character of the shared memory area (a string MAX*LEN chars long) ---> that's equal to "p = &smp1[0]" */

    for (i=0; i<MAX; i++)
    {
        if (p[i*LEN]=='v')
            objs++;
    }
    printf("Objects: %d\n", objs); /* the result is always right here */
				
    if (objs<MAX)
    {
        p = &smp1[objs*LEN]; /* here p points to a certain char of smp1 depending on objs that change at each cycle (it's like an offset) */

        /* here the string "answer" is filled with some data, then sent on
           the message queue. For now, everything works */

        strncpy(p, answer, LEN);
        p[0] = 'v';

     /* p = smp1(objs*LEN);            <--- THIS CODE DOESN'T WORK ALSO
        strncpy(p, answer, LEN);
        *(p) = 'v'; */

        printf("Objects (later): %d\n", objs); /* this is OK */
        printf("Entry: \"%s\" (%d; %d)\n", p, strlen(p), LEN); /* this is OK also */
        printf("SHMEM: \"%s\" (%d)\n", smp1, strlen(smp1)); /* HERE IS THE PROBLEM!!! smp1 seems to be *empty* and strlen returns 0 */
    }
    else
    {
        /* do something else */
    }

    shmdt(smp1);
    free(answer);
    exit(0);
}
else /* parent process */
{
    int w;
    w = waitpid(pid, 0, 0|WNOHANG);
}

I truly dont know why the shared memory segment (pointed by smp1 and p) results as empty when trying to print its contents. The two pointers point to the same area (expecially when objs=0 at the first iteration).
Could somebody please gimme a hand? I'm realy getting mad...

Many thank to everyone!

Mara · 06-28-2005, 04:17 PM

Quote:

Originally posted by d1s4st3r

Code:

p = smp1(objs*LEN); <--- THIS CODE DOESN'T WORK ALSO

Are you sure it's correct? It makes sense as p = smp1[objs*LEN], but I'm not sure what do you want to archieve with p = smp1(objs*LEN);

aluser · 06-28-2005, 06:22 PM

I get this output:

Code:

Objects: 0
Objects (later): 0
Entry: "v" (1; 40)
SHMEM: "v" (1)

from the following:

Code:

#include <sys/shm.h>
#include <stdio.h>
#include <sys/wait.h>
#include <sys/types.h>

#define		MAX	5
#define		LEN	40

/* here the main() begins */

int main() {
	const key_t GENKEY = ftok(".", 'x');
	char *smp1;
	int shmid;
	pid_t pid;

	shmid = shmget(GENKEY, sizeof(char)*(MAX*LEN), IPC_CREAT|0655);

	if (shmid==(-1))
	{
	    fprintf(stderr, "Error message.\n");
	    exit(1);
	}
	else
	{
	    smp1 = (char *)shmat(shmid, 0, 0);
	}

	pid = fork();

	if (pid==(-1))
	{
	    fprintf(stderr, "Error message.\n");
	    exit(1);
	}
	else if (pid==0) /* child process */
	{
	    char *answer = malloc(sizeof(char)*LEN);
	    char *p;
	    int objs = 0;
	    int i;
	    p = smp1; /* this points to the first character of the shared memory area (a string MAX*LEN chars long) ---> that's equal to "p = &smp1[0]" */

	    for (i=0; i<MAX; i++)
	    {
		if (p[i*LEN]=='v')
		    objs++;
	    }
	    printf("Objects: %d\n", objs); /* the result is always right here */
					
	    if (objs<MAX)
	    {
		p = &smp1[objs*LEN]; /* here p points to a certain char of smp1 depending on objs that change at each cycle (it's like an offset) */

		/* here the string "answer" is filled with some data, then sent on
		   the message queue. For now, everything works */

		strncpy(p, answer, LEN);
		p[0] = 'v';

	     /* p = smp1(objs*LEN);            <--- THIS CODE DOESN'T WORK ALSO
		strncpy(p, answer, LEN);
		*(p) = 'v'; */

		printf("Objects (later): %d\n", objs); /* this is OK */
		printf("Entry: \"%s\" (%d; %d)\n", p, strlen(p), LEN); /* this is OK also */
		printf("SHMEM: \"%s\" (%d)\n", smp1, strlen(smp1)); /* HERE IS THE PROBLEM!!! smp1 seems to be *empty* and strlen returns 0 */
	    }
	    else
	    {
		/* do something else */
	    }

	    shmdt(smp1);
	    free(answer);
	    exit(0);
	}
	else /* parent process */
	{
	    int w;
	    w = waitpid(pid, 0, 0|WNOHANG);
	}
}

This is about as I would expect, and smp1 is not empty. You should fix the part of the code that you haven't posted

aluser · 06-28-2005, 06:36 PM

In particular, is it possible for some other process to set the first byte of smp1 to 0 between the first iteration of the "for (i=0; i<MAX; i++)" loop and the SHMEM printf?

that race will cause your symptom sometimes, but it seems unlikely to happen consistently given the small number of iterations in that loop.

I suppose it's also possible for 'answer' to contain some control characters which erase the 'v' on your terminal. Evidently my malloc() gave me zeroed memory in this case.

d1s4st3r · 06-29-2005, 03:34 AM

It's solved!

You're all right, he posted code was right!
After a heavy bug hunting

, I found the *REAL* problem:
I got a header file that contains many #defines (where each one defines the lenght of a part of a string).
Well now, there was something like this:

Code:

#define		N		5

#define		A		1
#define		B    		16
#define		C	    	5
#define		D		4
#define 	E    		8
#define 	F		8

#define		MAX_LEN		A+B+C+D+E+F

Ok, I realized only now that the product "N*MAX_LEN" was totally wrong. This because I didn't know the sum "A+B+C+D+E+F" needs to be in round brackets ("(A+B+C+D+E+F)"), otherwise it would be "N*A+B+C+...".
This made a mess with a lot of counters and sizes (but, strangely, everything seemed to work except the shmem stuff).

Now it works fine with my code and shmem behaves it the right way

Many thanks guys!