Question

I want to create a global 22 character (+ NULL) string in C. I am initialising it like this:

#define SVN_REVISION     "1143"
#define SERIAL_NUMBER    "PST3201109270001"
char general_info[23] = SVN_REVISION SERIAL_NUMBER SAMPLING_FREQUENCY;

So far, this works fine. general_info is initialised with the concatenation of the two strings. Now, for the last two characters, I want them to be a 16-bit integer. But I can’t simply do this:

#define SVN_REVISION          "1143"
#define SERIAL_NUMBER         "PST3201109270001"
#define SAMPLING_FREQUENCY    (UINT16)500
char general_info[23] = SVN_REVISION SERIAL_NUMBER SAMPLING_FREQUENCY SAMPLING_FREQUENCY;

What can I do?

Answer 1

If you mean you want to embed 16-bit integer 500 as binary data in the end of the string, you can use raw hex bytes:

/* Little-endian unsigned 16-bit integer with the value 500 */
#define SAMPLING_FREQUENCY    "\xf4\x01"

Beware, though, endianness is architecture dependent, so you might want to add an extra macro to correctly order the bytes depending on the architecture:

#ifdef ... /* little endian */
#define BYTES2(a,b) b a
#else ... /* big endian */
#define BYTES2(a,b) a b
#endif
#define SAMPLING_FREQUENCY    BYTES16("x01", "\xf4")

EDIT: If you want to have a human-readable number, you can’t have it at compile-time (at least not in C), but you can initialize it at runtime:

#define SVN_REVISION     "1143"
#define SERIAL_NUMBER    "PST3201109270001"
#pragma pack(push, 1)
union {
    struct {
        char svn_rev_serial[20];
        uint16_t freq;
        char nul;
    };
    char general_info[23];
} u = {{
    SVN_REVISION SERIAL_NUMBER,
    500,
    '\0'
}};
#pragma pack(pop)

This uses more non-standard (but well-supported) constructs, like #pragma pack and writing to one union member and reading from another.