How to initialize this array of structures of array of structures?

Question

I have the following code, which is not working as expected. It compiles, but throws a lot of warnings and segfaults when executed:

#include <stdio.h>

enum output {
    A,
    B,
    C,
    D,
};

struct translation {
    char *from;
    enum output to;
};

struct dictionary {
    struct translation *foo;
    struct translation *bar;
};

enum language {
    ONE,
    ANOTHER,
};

struct dictionary languages[] = {
        [ONE] = {
                .foo = {
                        {"LF", A},
                        {"LLF", C},
                        {"RRF", D},
                },
                .bar = {
                        {"L", B},
                },
        },
        [ANOTHER] = {
                .foo = {
                        {"FF", B},
                        {"RRF", D},
                },
                .bar = {
                        {"LF", B},
                        {"R", C},
                        {"RR", D},
                },
        },
};

int main(void)
{
        printf("%s\n", languages[ONE].foo[0].from);
        return 0;
}

I am probably initializing languages the wrong way.

I would like to have that languages array in which I can access different dictionaries by language: languages[ONE]
I would like to access then different translation tables with the dictionary field: languages[ONE].foo
All translation tables accessed with a language+field pair may have different array lengths, as shown in the code example

Is that even possible? What am I doing wrong?

When compiling with gcc I get this (cropped) output:

asdf.c:27:17: warning: braces around scalar initializer
                 .foo = {
                 ^
asdf.c:27:17: note: (near initialization for ‘languages[0].foo’)
asdf.c:28:25: warning: braces around scalar initializer
                         {"LF", A},
                         ^
asdf.c:28:25: note: (near initialization for ‘languages[0].foo’)
asdf.c:28:26: warning: initialization of ‘struct translation *’ from incompatible pointer type ‘char *’ [-Wincompatible-pointer-types]

[...]

The same warnings/notes repeat for multiple parts of the code.

@FiddlingBits Actually, that could be a workaround! :-) I tried with [] (no size) but that was illegal. You can post that as an answer if you want. Although I will wait before accepting to see if there is a way of making this work without having to specify a dictionary table size "big enough" to hold all translations. — Peque
– Peque, Commented Nov 19, 2018 at 21:35

Fiddling Bits · Accepted Answer · 2018-11-19 22:05:09Z

3

Here are two things you can do:

Allocate memory for struct translation *foo; and struct translation *bar; (you can also use malloc to dynamically allocate memory). For example:

struct dictionary
{
    struct translation foo[10];
    struct translation bar[10];
};

Use a compound literal in your definition:

struct dictionary languages[] = {
    [ONE] = {
            .foo = (struct translation []){
                    {"LF", A},
                    {"LLF", C},
                    {"RRF", D},
            },
            .bar = (struct translation []){
                    {"L", B},
            },
    },
    [ANOTHER] = {
            .foo = (struct translation []){
                    {"FF", B},
                    {"RRF", D},
            },
            .bar = (struct translation []){
                    {"LF", B},
                    {"R", C},
                    {"RR", D},
            },
    },
};

Note

As mentioned by @M.M, adding the qualifier const before struct dictionary is a good idea if its values won't change during runtime.

edited Nov 19, 2018 at 22:05

answered Nov 19, 2018 at 21:37

Fiddling Bits

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9 Comments

Peque Over a year ago

Love the latter <3. I am learning a lot of C with this project. :-D

user7860670 Over a year ago

something like struct translation foo[10]; (leaving some of them uninitialized) is a very poor idea while compound literals are feature of C99 language

Fiddling Bits Over a year ago

@VTT Sure. I mentioned it as an option, not a recommendation.

M.M Over a year ago

@VTT why is it a poor idea in your opinion?

M.M Over a year ago

@VTT not necessarily; in some systems memory is cheap and other concerns such as code clarity, portability, ease of development, ease of runtime access etc. may have a higher priority

|

user7860670 · Accepted Answer · 2018-11-19 21:41:58Z

Just initialize each array separately:

#include <stdio.h>

enum output
{
    a
,   b
,   c
,   d
};

struct translation
{
    char const * from;
    enum output  to;
};

struct dictionary
{
    struct translation * foo;
    struct translation * bar;
};

enum language
{
    one
,   another
,   languages_count
};

struct translation one_language_foo_translations[] =
{
    {"LF" , a}
,   {"LLF", c}
,   {"RRF", d}
};

struct translation one_language_bar_translations[] =
{
    {"L", b}
};

struct translation another_language_foo_translations[] =
{
    {"FF" , b}
,   {"RRF", d}
};

struct translation another_language_bar_translations[] =
{
    {"LF", b}
,   {"R" , c}
,   {"RR", d}
};

struct dictionary languages[languages_count] =
{
    {one_language_foo_translations, one_language_bar_translations}
,   {another_language_foo_translations, another_language_bar_translations}
};

int main(void)
{
    printf("%s\n", languages[one].foo[0].from);
    return 0;
}

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