compressing the lines of a file using dynamic memory - c

Question

i need to make a function that returns a compressed line with the following formats,
input:
pprrrinnnttttfff
output:
p2r3i1n3t4f3
and if the new string is larger than the original, return the original, can someone tell what is wrong with my code?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *comprimir(char *s);
int main(){
FILE* input;
char *lineptr = NULL;
size_t len=0, c;
input =fopen ("input.dat", "r");
while ((c = getline(&lineptr, &len, input))!= -1){
lineptr = comprimir(lineptr);
printf("%s", lineptr );
}
fclose(input);
}
char* comprimir (char *s){
int len1 = strlen(s), len2=0;
char *str, *in, *mystr;
mystr =(char*) calloc(len1*2, sizeof(char));
strcpy(mystr, s);
for (str =mystr, in=mystr; *str; str++){
len2 += 2;
if (len2 >= len1) {
free(mystr);
return s;
}
int count =1;
in[0] = str[0]; printf("%s",in[0] ); in++;
if (len2 > len1) return s;
while (str[0] == str[1]){
count++;
str++;
}
in[0] = '0' + count;
in++; printf("%s", in[0] );
if (len2 > len1) return s;
}
strcpy(s, in);
free(mystr);
return s;
}

Answer 1

sample to fix
char* comprimir (char *s){
int len1 = strlen(s), len2=0;
char *out, *in, *mystr;
mystr =malloc(len1 + 2);//2*len1 not required, +2 : "r" -> "r1"(len + NUM_len + NUL
//`s` not copy to mystr, avoid overwriting
for (out = mystr, in=s; *in;){
int count = 1;
*out++ = *in++;//Pre-increment `in` to reduce code.
while (in[-1] == in[0]){
count++;
in++;
}
int num_len = sprintf(out, "%d", count);//OK even count is more than 10
len2 += 1 + num_len;
if (len2 >= len1) {
free(mystr);
return s;
}
out += num_len;
}
*out = 0;//terminate by '\0'
strcpy(s, mystr);
free(mystr);
return s;
}