prog.c: In function 'main':
prog.c:17:2: error: unknown type name 'fstring'
fstring str = "1234";
^
prog.c:17:16: warning: initialization makes integer from pointer without a cast [-Wint-conversion]
fstring str = "1234";
^
prog.c:19:26: warning: implicit declaration of function 'StringSubstr' [-Wimplicit-function-declaration]
fprintf(stdout, "%d\n", StringSubstr(str, 1));
^
prog.c:16:6: warning: unused variable 'n' [-Wunused-variable]
int n;
^