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ye
Joined: 11 May 2005
Posts: 57
Location: london
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| Pointer problem with structure |
 Posted: Fri Jan 06, 2006 10:02 am |
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Dear all,
happy new year!~
I am using 18f8722 and was puzzled by a problem with pointer and structure. The problem is:
I've defined a structure like :
| Code: | struct node
{
int tag;
char UserID[9];
char Password[9];
int admin_index;
}; // Note this is not a node in linked list |
and define an array of pointers pointing to this structure:
| Code: | | struct node* UserRecord[10]; |
You can see I have 10 records of this type to process in the code. Initially UserRecord[i]->UserID and UserRecord[i]->Password are all blank. They then are respectively given a new content:
| Code: | j = 97;
for(i=0;i<10;i++)
{
for(k=0;k<8;k++)
UserRecord[i]->Password[k] = j;
UserRecord[i]->Password[8] = '\0';
j++;
} |
The UserIDs and Passwords in the 10 records are assigned a different string and each assignment shouldn't have affected other records. However, it seems to me the UserIDs and Passwords are cross affected by each assignment. E.G. after the first FOR loop, if UserRecord[0]->UserID and UserRecord[0]->Password are respectively given "spsp" and "mpmpmp", the UserIDs and Passwords of all the 10 records are:
| Quote: | Record 0, UserID = spsp, Password = mpmpmp
Record 1, UserID = sp, Password = mpmp
Record 2, UserID = , Password = mp
Record 3, UserID = , Password =
Record 4, UserID = , Password =
Record 5, UserID = pmpmp, Password =
Record 6, UserID = pmp, Password =
Record 7, UserID = p, Password =
Record 8, UserID = , Password =
Record 9, UserID = , Password =
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Clearly, record 2,5,6,7 have been cross affected.
However, if I do not define the array as pointers,
| Quote: | | struct node UserRecord[10]; |
this wouldn't happen.
I am suspecting this is due to overflow or memory collision but I can't see how it happened. Help? |
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treitmey
Joined: 23 Jan 2004
Posts: 1094
Location: Appleton,WI USA
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| Posted: Fri Jan 06, 2006 10:32 am |
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Looks to me like you defined the struct and pointers but didn't instatiate the struct.
| Code: | #include <18F452.H>
#case
#use delay(clock=40000000)
#fuses h4,nowdt,noprotect,nolvp
#use rs232(baud=19200,xmit=PIN_C0,invert,stream=DEBUG,disable_ints) // stderr(same as debug)
#zero_ram
//======================= MAIN ============================//
void main(void)
{
char j,i,k;
struct node
{
int tag;
char UserID[9];
char Password[9];
int admin_index;
}; // Note this is not a node in linked list
struct node UserRecord[10],*ptrUserRecord[10];//instance and pointers
//================================================
setup_adc_ports(NO_ANALOGS);
set_tris_a(0);set_tris_b(0);set_tris_c(0);
set_tris_d(0);set_tris_e(0);
//================================================
fprintf(DEBUG,"STARTING.\n\r");
ptrUserRecord[0]=&UserRecord[0];
ptrUserRecord[1]=&UserRecord[1];
ptrUserRecord[2]=&UserRecord[2];
ptrUserRecord[3]=&UserRecord[3];
ptrUserRecord[4]=&UserRecord[4];
ptrUserRecord[5]=&UserRecord[5];
ptrUserRecord[6]=&UserRecord[6];
ptrUserRecord[7]=&UserRecord[7];
ptrUserRecord[8]=&UserRecord[8];
ptrUserRecord[9]=&UserRecord[9];
j = 97;
for(i=0;i<10;i++)
{
for(k=0;k<8;k++){
ptrUserRecord[i]->Password[k] = j;//pass=97,97,97,97,97,97,97,97,/0
}
ptrUserRecord[i]->Password[8] = '\0';//string terminator
j++;
fprintf(DEBUG,"--%s---\n\r",ptrUserRecord[i]->Password);
}
fprintf(DEBUG,"DONE !\n\r");
while(1)
{
}
}
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Ttelmah
Guest
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| Posted: Fri Jan 06, 2006 10:40 am |
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Yes.
I was just about to post, and you beat me to it. It actually looks as if the pointers (since they are never allocated), end up pointing to successive words in memory, making all the structures overlap...
Best Wishes |
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ye
Joined: 11 May 2005
Posts: 57
Location: london
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| Posted: Fri Jan 06, 2006 10:50 am |
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I got you guys.
Many thanks! |
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