C Initialise struct using = { { '?', '?', '?', '?' } };

Question

What I am trying to do is initialize the struct with a large about of hex data stored in a separate part of the device's memory, eventually this data will be written to memory through other means but for now I want to manually initialize the struct to use in my project.

The project I am working on has the following lines of code (that compile in Atmel studio).

typedef struct configData_t
{
    uint8_t version[4]; // ASCII

    uint8_t numIn;
    uint8_t numOut;
    uint8_t numKey;
    uint8_t numTest;
    uint8_t numAuto;

    controlModuleConfig_t homeConfig;

    inputModuleConfig_t  inConfig  [MAX_IN];
    outputModuleConfig_t outConfig [MAX_OUTPUT];
    keypadModuleConfig_t keyConfig [MAX_KEY];
    notificationConfig_t testConfig [MAX_TEST];
    autoFunctionConfig_t autoConfig [MAX_AUTO];

    precheckConfig_t precheckConfig;
    sleepConfig_t    sleepConfig;

    uint16_t audioCrc16;
    uint16_t configCrc16;
}
configData_t;

const __attribute__((__section__(".application_footer_data"))) 
configData_t theConfigData = { { '?', '?', '?', '?' } };

__attribute__ ((section(".application_footer_data"))) 
const unsigned char configBuffer[28672] = { /* Lots of data e.g. 0x31, 0x30, 0x33,...*/}

I have not seen a struct initialized this way before, how is theConfigData struct being initialized with the { { '?', '?', '?', '?' } } statement?

The inner braces will delineate the elements an array, and the outer braces the structure of which it is the (first) member. Anyway, voting to close this as incomplete because it asks why some code doesn't work without showing all the code... — underscore_d, Apr 12 '18 at 15:14
what makes you believe `configBuffer` contents should appear in `theConfigData` struct? — mfro, Apr 12 '18 at 15:49

score 1 · Accepted Answer · answered Apr 12 '18 at 15:24

1

The line

configData_t theConfigData = { { '?', '?', '?', '?' } };

will initialize each element of version data member to ASCII value of '?'. Rest of the member of the struct will be initialized to '0'.

answered Apr 12 '18 at 15:24

haccks

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Will the configBuffer[28672] set the rest of the struct data? – Aaron Thompson Apr 12 '18 at 15:29
1

Do you see the problem of asking 2 questions in 1 post? People read your title and the 99% of the post that focus on #1 and don't notice that you throw #2 in there at the end. I would recommend moving that to another, new post. – underscore_d Apr 12 '18 at 15:43

score 0 · Answer 2 · answered Apr 12 '18 at 15:25

The outer set of braces denotes the initializer for the struct. Since the first element of the struct is an array of uint8_t, the second set of braces initializes the elements of that array.

The remaining elements, which have no explicit initializer, are implicitly initialized to all 0 values.

C Initialise struct using = { { '?', '?', '?', '?' } };

2 Answers2