char myip[INET_ADDRSTRLEN];
fscanf(file, "%16s", myip);

#define _STRIFY(x) #x
char myip[INET_ADDRSTRLEN];
fscanf(file, "%" _STRIFY(INET_ADDRSTRLEN) "s", myip);

unknown conversion character type 'N' format

char format[14];
sprintf(format, "%%%ds", INET_ADDRSTRLEN-1);
fscanf(file, format, myip);

#define stringify_1(x...)  #x
#define stringify(x...)    stringify_1(x)

char myip[INET_ADDRSTRLEN];
fscanf(file, "%" stringify(INET_ADDRSTRLEN) "s", myip);