c-gcc, 78 bytes

Result

AC

Code [DL]

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

stdin

8903688996674279813650198393250749750453640635847423933234109513396716454871667210602412187027518691

stdout

0000000001111111111122222222333333333333444444444455555555666666666666777777777788888888899999999999

stderr

/volume/CODE.c:1:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
 f(char*a,char*b){return*a-*b;}s[100];main(){gets(s);qsort(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[100];main(){gets(s);qsort(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:1:38: warning: return type defaults to ‘int’ [-Wimplicit-int]
 f(char*a,char*b){return*a-*b;}s[100];main(){gets(s);qsort(s,100,1,f);puts(s);}
                                      ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:45: warning: implicit declaration of function ‘gets’ [-Wimplicit-function-declaration]
 f(char*a,char*b){return*a-*b;}s[100];main(){gets(s);qsort(s,100,1,f);puts(s);}
                                             ^
/volume/CODE.c:1:53: warning: implicit declaration of function ‘qsort’ [-Wimplicit-function-declaration]
 f(char*a,char*b){return*a-*b;}s[100];main(){gets(s);qsort(s,100,1,f);puts(s);}
                                                     ^
/volume/CODE.c:1:70: warning: implicit declaration of function ‘puts’ [-Wimplicit-function-declaration]
 f(char*a,char*b){return*a-*b;}s[100];main(){gets(s);qsort(s,100,1,f);puts(s);}
                                                                      ^
/tmp/cck5QuqO.o: In function `main':
CODE.c:(.text.startup+0xc): warning: the `gets' function is dangerous and should not be used.

strace