c-gcc, 74 bytes

Result

AC

Code [DL]

f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}

stdin

7857664389434430600512649968566605764753124019309938951401895143675971208945400873716335355079747091

stdout

0000000000001111111112223333333333344444444444455555555555566666666666777777777778888888999999999999

stderr

/volume/CODE.c:1:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
 f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}
 ^
/volume/CODE.c:1:31: warning: data definition has no type or storage class
 f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}
                               ^
/volume/CODE.c:1:31: warning: type defaults to ‘int’ in declaration of ‘s’ [-Wimplicit-int]
/volume/CODE.c:1:36: warning: return type defaults to ‘int’ [-Wimplicit-int]
 f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}
                                    ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:43: warning: implicit declaration of function ‘qsort’ [-Wimplicit-function-declaration]
 f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}
                                           ^
/volume/CODE.c:1:49: warning: implicit declaration of function ‘gets’ [-Wimplicit-function-declaration]
 f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}
                                                 ^
/volume/CODE.c:1:66: warning: implicit declaration of function ‘puts’ [-Wimplicit-function-declaration]
 f(char*a,char*b){return*a-*b;}s[1];main(){qsort(gets(s),100,1,f);puts(s);}
                                                                  ^
/tmp/ccVYWTM7.o: In function `main':
CODE.c:(.text.startup+0xc): warning: the `gets' function is dangerous and should not be used.

strace