c-gcc, 112 bytes

Result

AC

Code [DL]

char l[100],i,j;main(){for(;i<100;i++){read(0,&j,1);l[j]++;}for(i=48;i<58;i++)for(j=0;j<l[i];j++)write(1,&i,1);}

stdin

4643985618726536078809814721217612182781847507643491813990088049728987140066984805351058489380651839

stdout

0000000000011111111111122222233333334444444444555555566666666677777777788888888888888888889999999999

stderr

/volume/CODE.c:1:17: warning: return type defaults to ‘int’ [-Wimplicit-int]
 char l[100],i,j;main(){for(;i<100;i++){read(0,&j,1);l[j]++;}for(i=48;i<58;i++)for(j=0;j<l[i];j++)write(1,&i,1);}
                 ^
/volume/CODE.c: In function ‘main’:
/volume/CODE.c:1:40: warning: implicit declaration of function ‘read’ [-Wimplicit-function-declaration]
 char l[100],i,j;main(){for(;i<100;i++){read(0,&j,1);l[j]++;}for(i=48;i<58;i++)for(j=0;j<l[i];j++)write(1,&i,1);}
                                        ^
/volume/CODE.c:1:54: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char l[100],i,j;main(){for(;i<100;i++){read(0,&j,1);l[j]++;}for(i=48;i<58;i++)for(j=0;j<l[i];j++)write(1,&i,1);}
                                                      ^
/volume/CODE.c:1:90: warning: array subscript has type ‘char’ [-Wchar-subscripts]
 char l[100],i,j;main(){for(;i<100;i++){read(0,&j,1);l[j]++;}for(i=48;i<58;i++)for(j=0;j<l[i];j++)write(1,&i,1);}
                                                                                          ^
/volume/CODE.c:1:98: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
 char l[100],i,j;main(){for(;i<100;i++){read(0,&j,1);l[j]++;}for(i=48;i<58;i++)for(j=0;j<l[i];j++)write(1,&i,1);}
                                                                                                  ^

strace