Question

Hi guys I got some problems with using a function with a reference to a string as argument. I read that you should use e double pointer for this, but I can’t get it to work.
This is (partly) my code.

enum errCode { ERR_NONE = 0, ERR_EMPTY, ERR_FULL, ERR_MEM, ERR_INIT, ERR_COMMAND, ERR_UNDEFINED };
typedef enum errCode ErrCode;

typedef enum {
    no = 0, add, del, src, show, exit
} Command;

int main(void) {
    char stringval[50];
    char stringval2[50];
    ErrCode err;
    Command currentCommand = no;

    printf("Enter a command\n");

    if (fgets(stringval, 50, stdin) != NULL) {
        char *p;
        if ((p = strchr(stringval, '\n')) != NULL)
            *p = '\0';
    }

    ErrHandler(
            extractCommand(&currentCommand, stringval, &stringval2)
            );

    printf("stringval 2 = %s.\n", stringval2);

    return 0;
}

ErrCode extractCommand(Command *command, char *inputString, char **outputString) {

    char *strTemp;
    char *strTemp2;

    //Get the first word of the string
    strTemp = strtok(inputString, " ");

    strTemp2 = strtok(NULL, " ");
    *outputString = strTemp2;

    //Check if it equals a command
    if (strcmp(strTemp, "exit") == 0) {
        *command = exit;
        return ERR_NONE;
    } else if (strcmp(strTemp, "add") == 0) {
        *command = add;
        return ERR_NONE;
    } else if (strcmp(strTemp, "del") == 0) {
        *command = del;
        return ERR_NONE;
    } else if (strcmp(strTemp, "src") == 0) {
        *command = src;
        return ERR_NONE;
    } else if (strcmp(strTemp, "show") == 0) {
        *command = show;
        return ERR_NONE;
    } else {
        *command = no;
        printf("%s", strTemp);
        return ERR_COMMAND;
    }
}

This is what my output looks like:

Enter a command
add this is a test
stringval 2 = z˜ˇøÀo‡èK‡èT¯ˇø.

I obviously want to have the second word of the inputted string, but I’m doing something wrong.
Thx for the help!

Answer 1

stringVal2 is not initialised and is never populated: that is reason junk is being printed. There is no need in this case to pass a char**, passing a char* will work. However, this:

outputString = strTemp2;

does not copy the content of strTemp2 to outputString: it makes outputString point to the same address as strTemp2: use strcpy().

---

A double pointer, char** for example, is commonly passed to a function when that function allocates a new buffer for the argument (which is not the case in the posted code):

char* buf = NULL;
my_alloc(&buf);

void my_alloc(char** p)
{
    *p = malloc(10);
}