How do you convert a struct into a char array?

Question

I am a little bit confused on how to convert a struct to a char[] in C.

My CDMA modem doesn't support sending variables - it only understands ASCII characters. I need to do the conversion operation.

Let's say that I have an sMSG struct like this:

struct sMSG
{
    int a;
    int b[];
    char c[];
    double d;
    float f;
};

So, I have to make a string like char str[] = "sMSG_converted_into_ASCII_chars";

I'm wondering if somebody will help me out on this, please.

Thanks in advance.

Solution

First you need to copy the data of the struct into a byte array

int len = sizeof(struct sMSG);
unsigned char * raw = malloc(len);
memcpy(raw, &msg, len);

Now use a function to convert the byte array into Base64 text or just hexadecimal representation (2 chars/byte). Since you use the embedded tag, the latter might be the easiest to implement.

#define TOHEX(x) (x > 9 ? (x - 10 + 'A') : (x + '0'));
char * text = malloc(2 * len + 1);
for (int i = 0; i < len; i++)
{
    text[2 * i + 0] = TOHEX(raw[i] >> 4);
    text[2 * i + 1] = TOHEX(raw[i] & 0xF);
}
text[2 * len] = '\0';

free(raw);
free(text);

Comments

[Shahbaz]

I was gonna say sprintf it into a large enough array, but this solution is better! The sprintf solution has a problem handling field separators.

[moooeeeep]

will the data those arrays point to be copied using memcpy this way (or will rather the pointers be copied)?

[ArjunShankar]

whether this 'just works' really depends on who 'sends' messages to whom, what the sizes of different data types are, endian-ness, etc etc. You can't just read this on a different machine and 'memcpy' it back into the right structure.

[huysentruitw]

@ArjunShankar: that's correct, especially floats and double could even raise more trouble (if both platforms use a different notation)

[uɐɪ]

This assumes (BIG ASSUMPTION) that there are no empty bytes in the data structure that you are memcpy'ing. If the data elements are not assigned to a single contiguous set of memory locations then you will have extra padding bytes in the transmitted data. The values in these locations are undefined and could have undesirable effects.