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There's nothing wrong with the other answers, but if you'd prefer a solution more portable than asprinf() and a bit less cumbersome than Jerry Coffin's vsnprintf() helper function solution, consider this variable length macro solution:

#define FOO_ASPRINTF(_str, ...) /* replace FOO with your codebase's prefix */ \
do { \
    int byte_c = snprintf(NULL, 0, __VA_ARGS__); \
    (_str) = malloc(byte_c + 1); \
    if (!(_str)) { \
        FOO_MALLOC_ERR; /* should depend on your codebase's malloc() policy */ \
    } \
    sprintf(_str, __VA_ARGS__); \
} while (0)

Which in OP's case would be invoked like this:

char * buff = NULL;
CG_ASPRINTF(buff,
    "%lld %f %f %f %f %f %f %f %f %f %f",
    state->last_update_time,
    p->health,
    p->max_health,
    p->satiation,
    p->max_satiation,
    s->milliseconds_per_tick,
    s->hunger_pain_per_tick,
    s->hunger_per_tick,
    s->satiated_heal_per_tick,
    s->pain_per_wrong_answer,
    s->satiation_per_right_answer
);

Some people might be allergic to macros, but in this case it allows easy implementation, avoids any overhead, and is perfectly portable.