c - How to calculate the length of output that sprintf will generate? -

goal: serialize data json.

issue: cant know beforehand how many chars long integer is.

i thought way using sprintf()

size_t length = sprintf(no_buff, "{data:%d}",12312); char *buff = malloc(length); snprintf(buff, length, "{data:%d}",12312); //buff passed on ... 

of course can use stack variable char a[256] instead of no_buff.

question: there in c utility disposable writes unix /dev/null? smth this:

#define forget_about_this ... size_t length = sprintf(forget_about_this, "{data:%d}",12312); 

p.s. know can length of integer through log ways seems nicer.

since c simple language, there no such thing "disposable buffers" -- memory management on programmers shoulders (there gnu c compiler extensions these not standard).

cant know beforehand how many chars long integer is.

there easier solution problem. snprintf knows!

on c99-compatible platforms call snprintf null first argument:

ssize_t bufsz = snprintf(null, 0, "{data:%d}",12312); char* buf = malloc(bufsz + 1); snprintf(buf, bufsz + 1, "{data:%d}",12312);  ...  free(buf); 

in older visual studio versions (which have non-c99 compatible crt), use _scprintf instead of snprintf(null, ...) call.