I have three uint32_t that when combined, they will generate a unique key. I have to do this about 100M or more & potentially several times a day and store that in a key-value database. I'd like to keep the key to the least amount of bytes possible. I'm doing it in the following way but I'm curious if there's is a faster way to do this.
char *key = xmalloc(snprintf(NULL, 0, "%" PRIu32 "-%" PRIu32 "-%" PRIu32,num1,num2,num3) + 1);
sprintf(key, "%" PRIu32 "-%" PRIu32 "-%" PRIu32, num1,num2,num3);
Converting to decimal representation is rather costly. You can get faster conversion if you use hexadecimal:
sprintf(key, "%" PRIx32 "-%" PRIx32 "-%" PRIx32, num1, num2, num3);
As @AKX mentioned, use a fixed sized buffer. Since the string is (presumably) copied into the database, you shouldn't worry about it taking more space than necessary in the DB:
char key[32];
snprintf(key, sizeof(key), "%" PRIx32 "-%" PRIx32 "-%" PRIx32, num1, num2, num3);
The database engine doesn't know that you over-allocated your buffer. It will allocate it's own memory based on the actual length of the string rather than the size of the buffer.
Implement your own hexadecimal formatting. snprintf needs to parse its format string and interpret it against the argument list at runtime. This has a non-negligible overhead for such tasks like yours. Instead you can make your own int32-to-hex conversion that's specialized for your task. I'd use "abcdefghijklmnop" for the digits instead of the traditional "0123456789abcdef".
Does your key-value database require text-encoded keys? If not, you can try a binary encoding for your keys (e.g. take a look at SQLite4 varint encoding for inspiration).