why GDB is not printing arrays? - gdb

Question

When running some of my binaries I found it strange that GDB does not dump the arrays and I don't know why this only happens in some of the executables.
The declaration is very simple:
tbl_account_t accounts[MAX_ACCOUNTS];
When I print arrays (just any array), I get this:
(gdb) print accounts
$16 = 0x618d20 <accounts>
Confirming the variable type, everything is ok:
(gdb) ptype accounts
type = struct tbl_account {
unsigned short email_len;
unsigned short password_len;
char auto_log_in;
char reserved_char[3];
int reserved_int;
char email[64];
char password[25];
} []
(gdb)
I do have the data in it:
(gdb) print accounts[1]
$18 = {email_len = 16, password_len = 3, auto_log_in = 0 '\000', reserved_char = "\000\000", reserved_int = 0, email = "abra#cadabra.com", '\000' <repeats 47 times>,
password = "123", '\000' <repeats 21 times>}
(gdb)
With "print accounts" command I was expecting GDB to dump the entire contents of the array, like it does in other executables. Why does this happen ?

Answer 1

I don't quite have the full answer but I suspect that if you share more of your code we can figure it out. It appears that gdb does not know the length of your array.
Consider this program
struct tbl_account {
unsigned short email_len;
unsigned short password_len;
char auto_log_in;
char reserved_char[3];
int reserved_int;
char email[64];
char password[25];
};
int main(int argc, char* agv[])
{
tbl_account table[10];
tbl_account* table_ptr = table;
return 0;
}
Now the gdb session
(gdb) b main
Breakpoint 1 at 0x4006e5: file junk.cpp, line 22.
(gdb) r
Starting program: /tmp/a.out
Breakpoint 1, main (argc=1, agv=0x7fffffffdfe8) at junk.cpp:22
22 tbl_account* table_ptr = table;
(gdb) n
23 return 0;
(gdb) ptype table_ptr
type = struct tbl_account {
unsigned short email_len;
unsigned short password_len;
char auto_log_in;
char reserved_char[3];
int reserved_int;
char email[64];
char password[25];
} *
(gdb) ptype table
type = struct tbl_account {
unsigned short email_len;
unsigned short password_len;
char auto_log_in;
char reserved_char[3];
int reserved_int;
char email[64];
char password[25];
} [10]
Note how the ptype of the pointer is indicated by the '*'. We can also see that the array type is indicated by the [10]. In my example, the array size is included and gdb will print each element. For some reason, in your example the size of the array is not included. Perhaps it is an extern variable?
Since gdb isn't certain of the size of the array, it just prints it as a pointer value. As a worked around try casting
(gdb) p (tbl_account[10])accounts

Answer 2

You can also manually tell GDB how many entries to print using '#'.
This works better if your array is not statically allocated or you just want to examine specific set of entries in the array.
struct tbl_account {
int id;
char email[64];
char password[25];
};
int main(void)
{
struct tbl_account table[] = {
{1, "abc", "1234"},
{2, "def", "5678"},
{3, "ghi", "1011"},
{4, "jkl", "1213"}};
return 0;
}
And then tell GDB to print first 3 entries in the array
(gdb) p table[0]#3
$3 = {{
id = 1,
email = "abc", '\000' <repeats 60 times>,
password = "1234", '\000' <repeats 20 times>
}, {
id = 2,
email = "def", '\000' <repeats 60 times>,
password = "5678", '\000' <repeats 20 times>
}, {
id = 3,
email = "ghi", '\000' <repeats 60 times>,
password = "1011", '\000' <repeats 20 times>
}}
If you are interested in only index 2-3 for e.g, You could do -
(gdb) p table[2]#2
$5 = {{
id = 3,
email = "ghi", '\000' <repeats 60 times>,
password = "1011", '\000' <repeats 20 times>
}, {
id = 4,
email = "jkl", '\000' <repeats 60 times>,
password = "1213", '\000' <repeats 20 times>
}}