Undefined elements in struct array

Question

I am creating a client/server application and want to call functions dynamically. i have create the following struct:

typedef struct _cmd cmd;
struct _cmd
{
    const char *name;
    void (*func)(int s,int ac, char **av);
};

When the client send a command to the server the server will browse through an array of commands:

cmd cmds[] = {
        { "CREATE", cmd_create },
        { "EXIT" ,  cmd_exit },
        { "LIST", cmd_list },
        { "READ", cmd_read },
        { "DELETE", cmd_delete },
        { "UPDATE", cmd_update }
};


cmd *find_cmd(const char *name) {
    cmd *c;
    for (c = cmds; c->name; c++) {
        if (stricmp(name, c->name) == 0)
            return c;
    }
    return NULL;
}

Please not that

stricmp()

is not a typo, it a case-insensitive version of strcmp.

I now the following problem. When I call find_cmd() and pass an invalid commmand, my application crashes. my debugging messages showed the following:

Browsing Command: CREATE
Browsing Command: EXIT
Browsing Command: LIST
Browsing Command: READ
Browsing Command: DELETE
Browsing Command: UPDATE
Browsing Command: �p�
Browsing Command: �(�

After that i get the segfault. This looks to me as if there are some undefined elements in that struct arraym but where do they come from? What am I overlooking? Thanks in advance for any pointers.

You need a sentinel value to terminate your command list. Since this is a struct, null command and null function pointer will work. — Joe
– Joe, Commented Jun 20, 2014 at 17:13
Not the answer but just note. Don't use identifier starting with _ because it's reserved for implementation. — Jack
– Jack, Commented Jun 20, 2014 at 17:15
Thanks for the hints, this works fine, see my comment below :) — Fish-Guts
– Fish-Guts, Commented Jun 20, 2014 at 17:18
Also, I like as this thread is well-written, well-structured and is very clear what the author is looking for. :) — Jack
– Jack, Commented Jun 20, 2014 at 17:20

DoxyLover · Accepted Answer · 2014-06-20 17:11:28Z

11

You need a "null" element at the end of your list to trigger the c->name (!= NULL) test in your for loop.

Change

cmd cmds[] = {
        { "CREATE", cmd_create },
        { "EXIT" ,  cmd_exit },
        { "LIST", cmd_list },
        { "READ", cmd_read },
        { "DELETE", cmd_delete },
        { "UPDATE", cmd_update }
};

to

cmd cmds[] = {
        { "CREATE", cmd_create },
        { "EXIT" ,  cmd_exit },
        { "LIST", cmd_list },
        { "READ", cmd_read },
        { "DELETE", cmd_delete },
        { "UPDATE", cmd_update },
        { NULL, NULL }
};

answered Jun 20, 2014 at 17:11

DoxyLover

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3 Comments

Fish-Guts Over a year ago

Thanks for the quick reply, this works as a charme. How come I need to explicitly add a NULL, NULL element to the list? I'm not checking for (c->name == NULL) but return NULL when nothing was found?

DoxyLover Over a year ago

But you are testing c->name == NULL. That's the termination condition on your for loop. That's what the center expression on the for statement does.

DaV Over a year ago

@Fish-Guts, your loop termination condition is c->name i.e c->name != NULL. By adding the sentinel NULL element, it allows the for loop to exit.

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