learning gdb

[convenientstore]

Here's the code I am studying from "modern day c programming"

#include <string.h>

#define MAX_REMIND 50
#define MSG_LEN 60

int read_line(char str[], int n);

main()
{
    char reminders[MAX_REMIND][MSG_LEN+3];
    char day_str[3], msg_str[MSG_LEN+1];
    int day, i, j, num_remind = 0;

    for (;;) {
        if ( num_remind == MAX_REMIND) {
               printf("-- No space left --\n");
               break;
        }

        printf("Enter day and reminder: ");

        scanf("%2d", &day);
        if ( day == 0 )
             break;
        sprintf(day_str, "%2d", day);
        read_line(msg_str, MSG_LEN);

        for (i = 0; i < num_remind; i++ )
            if ( strcmp(day_str, reminders[i]) < 0 )
                   break;
        for (j = num_remind; j > i; j-- )
            strcpy(reminders[j], reminders[j-1]);

        strcpy(reminders[i], day_str);
        strcat(reminders[i], msg_str);

        num_remind++;
    }

    printf("\nDay Reminders\n");
    for ( i = 0; i < num_remind; i++ )
       printf(" %s\n", reminders[i]);

    return 0;
}

int read_line(char str[], int n )
{
    char ch;
    int i = 0;

    while (  (ch = getchar() ) != '\n' )
          if ( i < n )
               str[i++] = ch;

          str[i] = '\0';
          return i;

}
~

And here's gdb session I am doing:

Two question,
1)why can't I print out value of MAX_REMIND
2)why can't I print out the value of msg_str

GNU gdb Red Hat Linux (6.3.0.0-1.162.el4rh)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-redhat-linux-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb) b main
Breakpoint 1 at 0x80484bf: file big_to_small.c, line 13.
(gdb) r
Starting program: /root/pro/c/big_to_small 

Breakpoint 1, main () at big_to_small.c:13
13          int day, i, j, num_remind = 0;
(gdb) s
16              if ( num_remind == MAX_REMIND) {
(gdb) disp num_remind
1: num_remind = 0
(gdb) disp MAX_REMIND
No symbol "MAX_REMIND" in current context.
(gdb) s
21              printf("Enter day and reminder: ");
1: num_remind = 0
(gdb) 
23              scanf("%2d", &day);
1: num_remind = 0
(gdb) 
Enter day and reminder: 1^H1^H^H^H
24              if ( day == 0 )
1: num_remind = 0
(gdb) k
Kill the program being debugged? (y or n) y
(gdb) b main
Note: breakpoint 1 also set at pc 0x80484bf.
Breakpoint 2 at 0x80484bf: file big_to_small.c, line 13.
(gdb) r
Starting program: /root/pro/c/big_to_small 

Breakpoint 1, main () at big_to_small.c:13
13          int day, i, j, num_remind = 0;
1: num_remind = 0
(gdb) s
16              if ( num_remind == MAX_REMIND) {
1: num_remind = 0
(gdb) 
21              printf("Enter day and reminder: ");
1: num_remind = 0
(gdb) 
23              scanf("%2d", &day);
1: num_remind = 0
(gdb) 
Enter day and reminder: 16 birthday
24              if ( day == 0 )
1: num_remind = 0
(gdb) s
26              sprintf(day_str, "%2d", day);
1: num_remind = 0
(gdb) 
27              read_line(msg_str, MSG_LEN);
1: num_remind = 0
(gdb) disp msg_str
2: msg_str = '\0' <repeats 60 times>
(gdb) disp MSG_LEN
No symbol "MSG_LEN" in current context.
(gdb) print msg_str
$1 = '\0' <repeats 60 times>
(gdb)

[anduril462]

1)why can't I print out value of MAX_REMIND

MAX_REMIND is a pre-processor symbol (#define). Before compilation even starts, the preprocessor does a string replace and everywhere it sees MAX_REMIND it replaces it with 50, thus, when your code is compiled, it's as if you typed in 50 everywhere instead of MAX_REMIND. Two options:
1. Make it a const int MAX_REMIND = 50;
2. Compile with -ggdb3, which should put pre-processor symbols in your executable. Note that this can increase the size of your executable significantly.

2)why can't I print out the value of msg_str

You can and you did:

(gdb) print msg_str
$1 = '\0' <repeats 60 times>

It just happens that message string is full of 60 null characters ('\0'), so GDB saves space by saying you have a '\0' byte, and it repeats 60 times, instead of showing you the whole array/string.

EDIT: You can adjust this with 'set print repeats' in GDB. Type 'help set print repeats' for more info.

[convenientstore]

@anduril462,

Thank you for your explanation. Very detailed. I understood perfectly.

[convenientstore]

Just one more question, I am failing to see where the value 'i' (for which is being return) is being used for.. ?

int read_line(char str[], int n )
{
    char ch;
    int i = 0;

    while (  (ch = getchar() ) != '\n' )
          if ( i < n )
               str[i++] = ch;

          str[i] = '\0';
          return i;

[CommonTater]

Just one more question, I am failing to see where the value 'i' (for which is being return) is being used for.. ?

int read_line(char str[], int n )
{
    char ch;
    int i = 0;

    while (  (ch = getchar() ) != '\n' )
          if ( i < n )
               str[i++] = ch;

          str[i] = '\0';
          return i;

It looks like it's returning a count of the characters entered... i.e. the length of the string.

[anduril462]

str[i++] = ch;

I gets incremented every time this line is executed.

[tabstop]

And as a note about that function: you have most of the indentation right, but that function threw you, it looks like; the last two statements (str[i] = '\0' and return i) are not part of the loop.

[convenientstore]

Thank you again for all kind response.
My question is, yes I see that i is being incremented and points to last point position when it reads all input, so at the end, it's pointing to last position of the character.
Question is, or rather my assumption of this point is to when

 strcat(reminders[i], msg_str)

gets copied to reminders, it knows how much it needs to copied to.
Is this right assumption?

        read_line(msg_str, MSG_LEN);

        for (i = 0; i < num_remind; i++ )
            if ( strcmp(day_str, reminders[i]) < 0 )
                   break;
        for (j = num_remind; j > i; j-- )
            strcpy(reminders[j], reminders[j-1]);

        strcpy(reminders[i], day_str);
        strcat(reminders[i], msg_str);

        num_remind++;

[Bayint Naung]

Prefer enum over const especially if it is for array size. If you use const, you are using VLA. C99 feature..

[CommonTater]

And as a note about that function: you have most of the indentation right, but that function threw you, it looks like; the last two statements (str[i] = '\0' and return i) are not part of the loop.

In this case, I don't think they're supposed to be... that's null terminating the string and returning it's length.

[anduril462]

Prefer enum over const especially if it is for array size. If you use const, you are using VLA. C99 feature..

Good call with the enum, slipped my mind. I didn't think that a using a const would be considered VLA's, since the value is known at compile time...good to know though.

[Bayint Naung]

The code

const int N = 10;
int a[N];

won't compile in C89. C++ will compile.

$ cat foo.c
#include <stdio.h>

const int N = 10;

int main(void)
{
  int a[N];
  printf("%p\n",a);
return 0;
}

$ gcc -Wall -pedantic-errors -std=c89 foo.c
foo.c: In function 'main':
foo.c:7: error: ISO C90 forbids variable-size array 'a'
foo.c:8: warning: format '%p' expects type 'void *', but argument 2 has type 'in
t *'

[convenientstore]

Another weird thing. Why is J's value 1 instead of 0 here??

(gdb) b main
Breakpoint 1 at 0x100000ad4: file month_reminder.c, line 11.
(gdb) r
Starting program: /Users/javaxtra/Desktop/MY/c programs/pra/month_reminder_minus_return 
Reading symbols for shared libraries +. done

Breakpoint 1, main () at month_reminder.c:11
warning: Source file is more recent than executable.
11      {
(gdb) s
16          int day, i, j, num_remind = 0;
(gdb) disp i
1: i = 0
(gdb) disp j
2: j = 1
(gdb) disp num_remind
3: num_remind = 0
(gdb) disp day
4: day = 1
(gdb)

[stahta01]

Uninitialized variables can be any value.

Tim S.