how to round up a memory address(memory alignment problem)

nj302 · #1 (**permalink**) 09-19-2005

Hi, I try to marshal a unsigned int and a char * into a buffer, and then unmarshal them later to get them out. I need to put the char * in the front and unsigned int at the end of the buffer. However, my system always give me "BUS ERROR". I am using Sun Sparcs Sloris 2.10.

My code to marshal the data:

unsigned dev = 111;
unsigned inode = 222;

int s_docname = strlen(docname) + 1;
int s_key = s_docname + sizeof(unsigned) * 2;

keybuf = malloc(s_key);
memset(keybuf, 0, s_key);
memcpy(keybuf, docname, s_docname);
memcpy(keybuf + s_docname, &(dev), sizeof(unsigned));
memcpy(keybuf + s_docname + sizeof(unsigned), &(inode),
sizeof(unsigned));

My code to unmarshal the data:
char * docname = (char *)key.data;
int s_docname = strlen(docname) + 1;
dev =*((unsigned *)((char *)key.data + s_docname));
inode = *((unsigned *)((char *)key.data + s_docname +
sizeof(unsigned)));

I can print the dev and inode out in dbx debugger by typing the same
code, but running the program always have the BUS error.

I realize this is a problems of memory alignement. s_docname has byte granularity, somehow I need to "round up" my s_docname by some suitable calculation. But I really don't know how to? Help is appreciated.

blowtorch · #2 (**permalink**) 09-19-2005

Here is a possible alternate solution (I really haven't understood the problem very well - I may be completely off here)

Code:

#include<stdlib.h>
#include<string.h>

int main(argc,argv)
int argc;
char *argv[];
{
        unsigned int dev=111,inode=222;
        char docname[]="/tmp/test";
        char *keybuf;
        int s_docname,s_key;

        s_docname=strlen(docname);
        s_key=s_docname+(sizeof(unsigned)*2)+1;
        keybuf=(char*)malloc(s_key);
        if(!keybuf) {
                perror("error in malloc!");
                exit(-1);
        }
        sprintf(keybuf,"%s %u %u",docname,dev,inode);
        fprintf(stdout,"%s\n",keybuf);
        docname[0]=0;
        dev=0;
        inode=0;
        fprintf(stdout,"doc: %s, dev: %d, inode: %d\n",docname,dev,inode);
        sscanf(keybuf,"%s %u %u",docname,&dev,&inode);
        fprintf(stdout,"post sscanf\n");
        fprintf(stdout,"doc: %s, dev: %d, inode: %d\n",docname,dev,inode);
        exit(0);
}

nj302 · #3 (**permalink**) 09-21-2005

Still I got same bus error. The problem is that the dev is not at the address where keybuff + s_docname is

Some processors are not able to load an integer, unsigned etc, from an address which is not a multiple of the size of that integer. For 4-byte integers, the address it is loaded from, looking at the lowest two bit only, must be 00. It is called 4-byte alignment.

I need to "round up" my s_docname by some suitable calculation, leaving what is called 'padding' between docname and dev.

I need some advice on how to round this up. Thanks a lot.

Perderabo · #4 (**permalink**) 09-21-2005

Don't dereference the pointer in place. Since you memcpy'ed it in, memcpy it back out. Then deference it.

nj302 · #5 (**permalink**) 09-21-2005

Sorry, but I don't understand what exactly you mean?

Perderabo · #6 (**permalink**) 09-21-2005

Replace:
dev =*((unsigned *)((char *)key.data + s_docname));
with:
memcpy(dev, keybuf + s_docname, sizeof(unsigned));
and make a similar for inode.

nj302 · #7 (**permalink**) 09-21-2005

Problem solved. Thank you very much, Perderabo!

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