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#1
09-18-2005
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how to round up a memory address(memory alignment problem)
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. 
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#2
09-18-2005
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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);
}
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#3
09-21-2005
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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.
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