C decryption program - Heap corruption detected - c

Question

I'm trying to write a very simple code.
Basically, I get an "encrypted" string from the user, and my code decrypts it with the following rule:
"From the first letter of the string subtract one, from the second subtract 2, from the third subtract 3 and so on. Spaces stay unchanged"
I wrote this code:
char* q4(char* str_in)
{
char* str_out;
int str_out_length,i,temp;
str_out_length=strlen(str_in)+1; //+1 for null terminator
str_out=(char*)malloc(sizeof(char)*str_out_length);
str_out[str_out_length]='\0'; //null terminator at last index
for(i=0;i<str_out_length;i++)
{
if(str_in[i]!=' ')
{
temp=str_in[i];
str_out[i]=(char)(temp-i-1);
}
else
str_out[i]=str_in[i];
}
return str_out;
}
My code seems to return incorrect output, and not only that, I also have an error that reads "Heap Corruption detected".
For example, when the string i input is Btwlzx Dqqes, the output is Arthur <hgZg<= and an error. Basically the first word is correct. The second one is rubbish. And if I enter just Btwlzx, then the output is Arthur- and said error.
What am I doing wrong?

Answer 1

str_out_length=strlen(str_in)+1; //+1 for null terminator
str_out=(char*)malloc(sizeof(char)*str_out_length);
It seems that the terminator is placed one byte behind the allocated buffer. Try
str_out[str_out_length - 1]='\0';
And similarly, perhaps
for(i=0;i<str_out_length;i++)
should be
for(i=0;i<str_out_length-1;i++)
Or, maybe better, change
str_out_length=strlen(str_in)+1; //+1 for null terminator
str_out=(char*)malloc(sizeof(char)*str_out_length);
with
str_out_length=strlen(str_in);
str_out=(char*)malloc(sizeof(char)*(str_out_length+1)); //+1 for null terminator
and then use str_out_length without -1-correction.
Update (after reading the comment)
print_decrypted("Btwlzx Dqqes Eq|pj2 Tjhvqujs Iqoqjy bpg Eqfxtx Xcwwtt");
outputs
Arthur Conan Doyle, Sherlock Holmes and Doctor Watson
void print_decrypted(char* str)
{
int k = 0;
for (int i = 0; str[i] != '\0'; i++)
if (str[i] != ' ')
{
printf("%c", str[i] - k - 1);
k++;
}
else
{
printf(" ");
k = 0;
}
printf("\n");
}