%m length specifier allocates as many memory as needed to place input

scanf

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#ifdef __GLIBC__

#if !(__GLIBC_PREREQ(2, 7))                                                                                                         
#error %m is not available
#endif

#elif defined(_POSIX_VERSION)

#if _POSIX_VERSION < 200809L
#error %m is not available
#endif

#else
#error Your C-library is not supported.
#endif

int main(void)
{
    char *s;
    scanf("%ms", &s);
    printf("%s\n", s);
    free(s);
    return 0;
}