filling the content of union produce error

Question

I try to fill the content of a union with something, however I receive an error. here's the code:

struct command
{
    int type;
    int *input;
    int *output;
    union{
        struct command *command[2];
        char **word;
    }u;
};

typedef struct command *command_t;

command_t read_command(){
    command_t main1 = NULL;
    command_t main2 = NULL;
    main1->u->word = (char**)malloc(1);
    main1->u->word[0] = (char*)malloc(1);
    //some other code in here
}

I got an error in "main1->u->word = (char**)malloc(1);" line saying: "invalid type argument of â->â (have âunion â)"

any suggestion? thx

Use `main1->u.word`, you'll also have to use `malloc` to get `main1`. — Bryan Olivier, Jul 01 '13 at 20:34

Johannes Overmann · Answer 1 · 2013-07-01T20:39:06.940

0

It needs to be main1->u.word = .... (dot instead of arrow after u.)

But your code contains multiple other errors which the compiler will not find:

You cannot assign to anything in main1 and main2 since these pointers are NULL pointers. Allocate some memory for them first.

The first malloc just allocates a single byte, but your char* needs 4 or 8 bytes. To make this work use:

main1->u.word = (char**)malloc(sizeof(char*));

assuming you want to allocate an array of char* of length 1.

The second malloc also allocates just a single byte. How much data do you want to store here?

edited Jul 01 '13 at 20:39

answered Jul 01 '13 at 20:32

Johannes Overmann

4,914
22
38

by allocating some memory first, do u mean something like "command_t main1 = (command_t)malloc(sizeof(command_t));"? – Jessica Herlian Jul 01 '13 at 21:21

score 0 · Answer 2 · edited May 23 '17 at 10:32

main1->u is an union, and not a pointer-to-union, therefore you can't use the -> operator on it.

Furthermore, you don't allocate enough space for the pointer (malloc(1) returns a pointer to a memory block which is 1 byte large, and I doubt that that's what sizeof(char **) is). You are also casting the return value of malloc(), which is wrong. What you want is

main1->u.word = malloc(sizeof(*main->u.word));
main1->u.word[0] = malloc(LENGTH_OF_STRING + 1); // for the NUL terminator

instead.

filling the content of union produce error

2 Answers2