Array of strings gets entirely rewrote when I try to append - C (xv6)

Question

I am declaring an array of strings by using:

char *commands[100];

I then proceed to enter a while loop that reads commands input from the user:

    while(getcmd(buf, sizeof(buf), cmd_count) >= 0){

            printf(2, "cmd_count: %d\n", cmd_count);
            buf[strlen(buf)-1] = 0;
            printf(2, "string buf: -%s-\n", buf);
            commands[cmd_count] = buf;
            printf(2, "commands: %s\n", commands[0]);
            printf(2, "commands: %s\n", commands[1]);
            printf(2, "commands: %s\n", commands[2]);
            printf(2, "commands: %s\n", commands[3]);
            cmd_count++;
    }

Here is the output from two iterations:

0 EZ$ ls
cmd_count: 0
string buf: -ls-
commands: ls
commands: (null)
commands: (null)
commands: (null)

EZ$ echo hello
cmd_count: 1
string buf: -echo hello-
commands: echo hello
commands: echo hello
commands: (null)
commands: (null)

Even though cmd_count was clearly 1 on the second iteration, it rewrote both the 0th and 1st position. What gives?

`printf(2, "cmd_count: %d\n", cmd_count);` should be `printf("cmd_count: %d\n", cmd_count);` Your attempt yields `"warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast"` — David C. Rankin, Jan 29 '18 at 03:21
@DavidC.Rankin you're probably not testing in an xv6 environment — Kenta, Jan 29 '18 at 03:44
Ah, you are probably right. Thanks Tom. Now I know it is one of those quirky things like CS50... — David C. Rankin, Jan 29 '18 at 04:29

score 1 · Answer 1 · answered Jan 29 '18 at 02:39

1

I'm assuming that getcmd() does not allocate space for the buffer 'buf', that you allocate that space outside of getcmd()? If so, then the following line:

commands[cmd_count] = buf;

Does not allocate a new buffer, it simply updates one of your char pointers to point to your one buffer. So after two iterations, both commands[0] and commands[1] point to the same buffer, 'buf'.

answered Jan 29 '18 at 02:39

Stephen Docy

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You would be correct.. how would you suggest I fix this? Is there a different data structure I could use for creating an array of strings rather than using char pointers? – Kenta Jan 29 '18 at 02:48

score 1 · Accepted Answer · answered Jan 29 '18 at 03:01

Not knowing your entire algorithm, a simple approach may be:

currently you have the variable command allocated to hold 100 char pointers, so that gives you room to handle cmd_count 0 - 99.
but you only have a single buffer for storing each command
if you do need to store each command past the while loop in which you read them, how about the following approach:

allocate a buffer for each pointer in commands that is big enough to hold a command

for (int i = 0; i < 100; i++) {
    commands[i] = malloc(MAXSTRINGSIZE + 1);
}

(you could probably optimize this by allocating ((MAXSTRINGSIZE + 1) * 100) and then setting each pointer in commands to the correct offset, but the approach above may be easier for you to understand if you are just learning about pointers)

And then try replacing buf in the getcmd() call with commands[i], so that you read the command directly into an entry in the commands array.

Array of strings gets entirely rewrote when I try to append - C (xv6)

2 Answers2