compilation error in c struct variable assignment

Question

Got a doubt in struct variable assignment.

struct udata
{
    char name[50];
    int num;
    char ch;
};

void main()
{
    struct udata a = {"ram", 20, 'a'};
    struct udata b;
    //b = {"ashok", 10, 'c'}; - illegal
    b = a;
}

In above code b = {"ashok", 10, 'c'}; is giving compilation error but its accepting b = a;. I hope both are similar kind of assignment, but I dont know why its not accepting first one. Can someone explain me why it is so ?

Note : I am compiling in a fedora gcc compiler.

haccks · Accepted Answer · 2014-07-12 21:22:53Z

4

Initializers can only be used at declaration time. If you want to initialize b after declaration, then you can do it by using a compound literal-- a C99 feature:

b =  (struct udata){"ashok", 10, 'c'};

GCC also support copound literals as an extension.

edited Jul 12, 2014 at 21:22

answered Jul 12, 2014 at 21:06

haccks

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Tim Čas · Accepted Answer · 2014-07-12 21:07:01Z

3

What you're trying to do cannot be done directly in standard C. The best standard and portable solution is to use a temporary:

const struct udata tmp = {"ashok", 10, 'c'};
b = tmp;

However, in practice, the following is often (but not always!) allowed by compilers (*note below):

b = (struct udata){...};

(* note: I believe at least MSVC does not support this syntax, and probably many others; but just throwing it out there. GCC, however, does support it)

answered Jul 12, 2014 at 21:07

Tim Čas

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Some programmer dude · Accepted Answer · 2014-07-12 21:06:11Z

2

That's how C is designed and specified to work. There's nothing you can do. If you have a structure variable, you can only initialize it by an initializer in the declaration or by later initializing the individual members.

In the future, when posting a question regarding compiler errors, please include the complete and unedited error log in the question.

answered Jul 12, 2014 at 21:06

Some programmer dude

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