error while initializing array of structures

Question

I have my struct like below , there can be n number of vendor which can contain n number of test struct. I am trying to initialize this structure . This is a sample code I am trying , later I want to make it using macros and load the structure like X-macros.

I am also using flexible structure concept as I do not know how many test structs for a vendor are going to be. The data would be in a file , the struct needs to load all that is there. I have created a minimal sample code for SO. Below is my code.

#include <stdio.h>

typedef struct test{
int a;
int b;
int c;
}test;

typedef struct vendor{
int size;
test t[0];
}vendor;

vendor v[]={

{.size = 1, .t[] = {{1,2,3},}}
};

int main()
{
return 0;
}

I get this error -

a.c:16: error: expected expression before ‘]’ token
a.c:16: error: array index in initializer not of integer type
a.c:16: error: (near initialization for ‘v[0].t’)
a.c:16: error: extra brace group at end of initializer
a.c:16: error: (near initialization for ‘v[0]’)
a.c:16: error: extra brace group at end of initializer
a.c:16: error: (near initialization for ‘v[0]’)
a.c:16: warning: excess elements in struct initializer
a.c:16: warning: (near initialization for ‘v[0]’)

I have tried without flexible struct , no luck so far. any suggestions on how to init this struct ?

Standard C prohibits arrays of size 0; if you want a FAM (flexible array member), you use [] instead of [0] in the structure. But you then can't statically allocate an array of structures containing a FAM — again in standard C. GCC has extensions that override the rules of the standard; these are not necessarily good features to use. With an ordinary array member, you'd use .t = { { 1, 2, 3 }, … }; to provide the initializers. Standard C says you can't provide an initializer for a structure with a FAM. — Jonathan Leffler
– Jonathan Leffler, Commented Mar 3, 2019 at 19:10

dbush · Accepted Answer · 2019-03-03 19:03:42Z

3

The .t[]= syntax in the initializer is invalid. When using a designated initializer, you only need to specify the name of the member:

.t={1, 2, 3}

However, this still won't work with a flexible array member.

The size of a struct with a flexible array member doesn't include space for the flexible array member, so you can't created a static or automatic instance of it. You need to allocate memory for the struct dynamically:

vendor *v;

void init()
{
    v = malloc(sizeof(vendor) + 1 * sizeof(test));
    v.size = 1;
    v.t = (test){1, 2, 3};
}

int main()
{
    init();
    return 0;
}

Also, because of the variable size, a struct with a flexible array member cannon be a member of an array.

answered Mar 3, 2019 at 19:03

dbush

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3 Comments

CodeTry Over a year ago

Thankyou, i was getting too much adventurous with flexible structure, i guess.

CodeTry Over a year ago

How to do this without FM and malloc , any suggestions ? Sole objective is load all tests for all vendors given in a data file.

dbush Over a year ago

@CodeTry Depends on your use case. Make a new question detailing what you're trying to do and your attempt at solving it.

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