CODEWITHSUNDEEP
c
Sirop4ik
Sirop4ik

Reputation: 5243

error: expression must have struct or union type in c when passing struct as param in function

I have such a struct

struct Request
{
    char command[COMMAND_LENGHT];
    char firstSetName[SET_NAME_LENGTH];
    char secondSetName[SET_NAME_LENGTH];
    char resultSetName[SET_NAME_LENGTH];
    int input[sizeof(int) * 4];
};

and I have such a method

int parseToReadRequest(char * command, struct Request ** request)
{
    printf("Command is : %s\n", command);

    *request.firstSetName = "firs";

    return 0;
}

As far as I understood when you need to pass a struct as a param you need to mark it with **

but anyway when I try to assign a struct value firstSetName I get an error

Expression must have struct or union type

What am I doing wrong?

EDIT I get such error

enter image description here

Upvotes: 1

Views: 2087

Answers (2)

dbush
dbush

Reputation: 223739

In this expression:

*request.firstSetName = "firs";

The member access operator . has the highest precedence, so it assumes that request is a struct or union type. However it is not. It is a pointer-to-pointer-to struct.

If your intent is to modify an instance of a struct, you don't need a double pointer. Just a single will do:

int parseToReadRequest(char * command, struct Request *request)

Then, you would need to either put parenthesis around *request to ensure the pointer is first dereferenced before using .:

(*request).firstSetName = "firs";

Or you can use the pointer-to-member operator -> which is cleaner:

request->firstSetName = "firs";

However there is another issue here. You're attempting to assign a value to an array. An array can't be assigned to directly. What you want instead is to use the strcpy function which is used to copy strings:

strcpy(request->firstSetName, "firs");

Upvotes: 4

Guillaume Petitjean
Guillaume Petitjean

Reputation: 2718

You have to pass the structure by reference, ie. pass the pointer to it. You need to pass a double pointer only if you need to modify the pointer itself (for example dynamically allocate a structure inside the function).

The function should be: int parseToReadRequest(char * command, struct Request * request)

And inside the function you should access the structure fields like this: request->firstSetName

Upvotes: 1

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