How do I pass this array of c strings correctly? [closed]

Question

I have my array of C-Strings declared as:

char *argv[MAXARGS];

MAXARGS basically just tells us how many c strings are in the array for indexing purposes. I want to pass it to this function below...

int builtin_cmd(char **argv)

but nothing seems to go through, I called the function like this.

    char *argv[MAXARGS];

    builtin_cmd(argv);

but once i get into the function builtin_cmd and try to print using

printf("%s\n", argv[0]);

it prints nothing.... but when i do this before calling the function...

char *argv[MAXARGS];
printf("%s\n", argv[0]);
//builtin_cmd(argv);

it will print the first C string in the array of c strings just fine. My thoughts are that I am passing the structure incorrecly and I would like to know the correct way to pass it to the function.

EDIT:

int builtin_cmd(char **argv);
int main()
{
    char *argv[128];
    //put 'quit' into argv[0]..
    printf("%s\n", argv[0]); //prints 'quit'
    builtin_cmd(argv);
}
int builtin_cmd(char **argv)
{
    printf("%s\n", argv[0]); //prints nothing
}

The problem is, again, that I cant seem to get argv into the function. It also compiles with no errors or warnings.

Answer 1

You haven't shown how you initialize the array, and that's the crucial part.

#include <stdio.h>

static void builtin_cmd(char **argv)
{
     while (*argv != NULL)
         printf("%s\n", *argv++);
}

int main(void)
{
    char *argv[] = { "abc", "def", "ghi", NULL };
    builtin_cmd(argv);
    return 0;
}

This code uses a sentinel value, the null pointer, to mark the end of the data. The alternative is to pass the number of strings in the array as a separate argument. When you use int main(int argc, char **argv), you actually have both mechanisms in use; argv[argc] == NULL so there is the count and then sentinel value.

Answer 2

You are trying to pass an array of charecter pointers to an function so your function declratation should be builtin_cmd(char* argv[]).Here's an sample example to pass array of pointers to an function

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void fill_strings(char *str[], int nstrings)
{
    int i;
    for ( i=0; i < nstrings; i++)
        strcpy(str[i], "hello world");
}

void print_strings(char *str[], int nstrings)
{
    int i;
    for (i=0; i < nstrings; i++)
        printf("string %d: %s\n", i, str[i]);
}

int main(void)
{
    char *index[10];
    int i;
    for(i=0; i<10; i++)
    {
        index[i] = malloc(20);
    }
    fill_strings(index, 10);
    print_strings(index, 10);

    return 0;
}

Answer 3

If you're passing your input on the command line you need to tell the compiler that you're using the parameters that are passed into main from the runtime.

int main(int argc, char **argv)
{

That should allow you to pass "quit" as the first parameter properly. Otherwise, as the others say, how you fill a "C vector" is important.