CODEWITHSUNDEEP
c
simon_xia
simon_xia

Reputation: 2544

incompatible types when a function returns a union

here is code pieces:

ip.h

typedef union _ip_t{
    struct _dot_ip {
        unsigned char f4;
        unsigned char f3;
        unsigned char f2;
        unsigned char f1;   //the first field
    }dot_ip;
    unsigned int int_ip;
}ip_t;

ip.c

ip_t
get_mask(int sub_len)
{
    assert(sub_len > 0 || sub_len < 32);
    ip_t ret;
    ret.int_ip = ~((1 << (32 - sub_len)) - 1);
    return ret;
}

main.c

ip_t mask;
mask = get_mask(24);

then the error:

error: incompatible types when assigning to type ‘ip_t’ from type ‘int’

mask = get_mask(24);

I can't figure out where is wrong, any help will be appreciated

PS: gcc verison: gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2

Upvotes: 0

Views: 112

Answers (1)

AnT stands with Russia
AnT stands with Russia

Reputation: 320747

No declaration of your your function is visible in main.c. In main.c the function is completely unknown to the compiler. Your compiler assumed that it returns int. The rest follows.

Such behavior of the compiler is C89/90-specific. It has been outlawed in C99 language specification. Modern C compilers are not supposed to let you call undeclared functions.

Add a prototype of your get_mask function to ip.h

ip_t get_mask(int sub_len);

to tell the compiler that get_mask actually returns ip_t.

Since your are using gcc, I suspect that the compiler actually issued an additional diagnostic message informing you about get_mask being undeclared. Did you just ignore that message?

Upvotes: 3

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