K&R Explanation of echo program is confusing, when debugging it using gdb. What's wrong?

Question

As per K&R, his explanation of echo program: echo hello world "By convention, argv[0] is the name by which the program was invoked, so argc is at least 1. If the argc is 1, there are no command-line arguments after the program name. In the example above, argc is 3, and argv[0], argv[1], and argv[2] are "echo", "hello", and "world" respectively. The first optional argument is argv[1] and the last is argv[argc-1]."

But, when debugging the program, using gdb, I can see the following:

(gdb) p argv[0]
$2 = 0x7efff85a "/home/pi/new/a.out"
(gdb) p argv[1]
$3 = 0x0
(gdb) p argv[2]
$4 = 0x7efff86d "LS_COLORS=rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:mi=00:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arc"...
(gdb) p *argv[0]
    $5 = 47 '/'

Ex 1:

#include <stdio.h>
int main(int argc, char *argv[])
{
    int i;
    while (--argc>0)
        printf("%s%s",*++argv,(argc>1)?" ":"");
    printf("\n");
    return 0;
}

For the code example above, when compiling and executing as the below:

pi@readonly:~/new$ cc -g echo.c
pi@readonly:~/new$ a.out hello world
hello world

Debug using gdb:

Snippet 1:

(gdb) p &*argv
$11 = (char **) 0x7efff744
(gdb) p &**argv
$12 = 0x7efff85a "/home/pi/new/a.out"
(gdb) p argv
$13 = (char **) 0x7efff744
(gdb) p &argv
$14 = (char ***) 0x7efff5e0
(gdb) p &argv[0]
$15 = (char **) 0x7efff744
(gdb) p argv[0]
$16 = 0x7efff85a "/home/pi/new/a.out"

Snippet 2:

(gdb) p argv[3]
$22 = 0x7efffe5a "_=/usr/bin/gdb"
(gdb) p argv[4]
$23 = 0x7efffe69 "LANG=en_GB.UTF-8"
(gdb) p &argv[3]
$24 = (char **) 0x7efff754

Questions:

• Snippet 1:

Am I understand it correctly, that 0x7efff85a is the hexadecimal value of the string "/home/pi/new/a.out"? If yes, how it has been obtained? In other words, has it been obtained by using, for instance, atoi() function?

• Snippet 3:

Am I understand it correctly, that, _=/usr/bin/gdb is converted into 0x7efffe5a?
Also, there is some predefined values being set to the array argv, because, in case, nothing has been input, it is there. Where can I read about this predefined values? Is it implementation-defined? For instance, on windows, using Eclipse, or other IDEs, would it give different but, predefined values set as well for argv[] array?

No, they are addresses (pointers). Look at the type you're trying to print. — Sourav Ghosh
You need to use the set args command within GDB to set the program's command-line arguments. — Ian Abbott
That argv[0] is /home/pi/new/a.out is because you did not rename your program to echo. Although I must confess that K&R don't mention the path, or maybe that got added later in the definition of argv[0]. — Paul Ogilvie
You cannot access argv[2] and beyond if argc is 1. You cannot access argv[3] if argc is 2, etc. Any such value you obtain from the debugger is meaningless. — n. 1.8e9-where's-my-share m.

Ian Abbott Ian Abbott · Accepted Answer · 2019-02-13T10:52:35

Snippet 1) 0x7efff85a is a pointer to the string "/home/pi/new/a.out" for this instance of the program.

Snippet 2) 0x7efffe5a is a pointer to the string "_=/usr/bin/gdb" for this particular instance, but you shouldn't rely on argv[argc+1] onwards pointing to anything in particular.

K&R Explanation of echo program is confusing, when debugging it using gdb. What's wrong?

3 Answers