Error: Abort trap 6 - String obfuscation - c

Question

I am trying to write code for String Obfuscation but it gives me Abort trap: 6 error while running. Any ideas are much appreciated:
#include <stdio.h>
#include <math.h>
#include <string.h>
char* readLine_str_test_encrypt();
char* readLine_str_test_decrypt();
int main () { //body
printf("%s\n", readLine_str_test_encrypt());
printf("%s\n", readLine_str_test_decrypt());
}
char* readLine_str_test_decrypt() {
static unsigned char string[9] = {100, 115, 119, 114, 90, 127, 120, 115, 22};
static int i = 0;
for (; i < 9; ++i) {
string[i] ^= 22;
}
return (char*)string;
}
char* readLine_str_test_encrypt()
{
static unsigned char string[9] = "readLine";
char output[9];
char* output_start=output;
static int i =0;
for(; i < 9; ++i)
{
//string[i] = string[i]^22;
output_start += sprintf(output_start,"%d",string[i]^22);
}
return output_start;
}
My decrypt function is running successfully.

Answer 1

In the readLine_str_test_encrypt you are returning a pointer to variable output. This variable is a local variable and goes out of scope when the function exits.
Change it to
static char output[9];
and the error goes away.
Read more as to why you should not return a local variable here.

Answer 2

the posted code is:
including a header file those contents are not used
trying to 'sprintf()` a 3 char integer into a single byte
returning pointers to local variables
is not terminating the strings to be printed with a NUL byte
Note: size_t is a well known definition for unsigned long int
The following suggested code corrects all of the above problems and a few others.
#include <stdio.h> // printf()
#include <stdlib.h> // exit(), EXIT_FAILURE, malloc(), free()
//#include <math.h>
#include <string.h> // strlen()
char* readLine_str_test_encrypt( char *, size_t );
char* readLine_str_test_decrypt( char *, size_t );
int main ( void )
{ //body
char string[] = "readLine";
char * encryptedString = readLine_str_test_encrypt( string, strlen(string) );
// Note: the encrypted string may not be printable
printf("%s\n", encryptedString );
char * decryptedString = readLine_str_test_decrypt( encryptedString, strlen(string) );
printf( "%s\n", decryptedString );
free( encryptedString );
free( decryptedString );
} // end function: main
char* readLine_str_test_decrypt( char *encryptedString, size_t length)
{
char *string = NULL;
if( NULL == ( string = malloc( length +1) ) )
{// then malloc failed
perror( "malloc for decrypted string failed" );
exit( EXIT_FAILURE );
}
// implied else, malloc successful
for ( size_t i=0; encryptedString[i]; ++i )
{
string[i] = encryptedString[i] ^22;
}
string[length] = '\0';
return string;
} // end function: readLine_str_test_decrypt
char* readLine_str_test_encrypt( char *stringToEncrypt, size_t length )
{
char *output = NULL;
if( NULL == (output = malloc( length+1 ) ) )
{ // then, malloc failed
perror( "malloc for work area failed" );
exit( EXIT_FAILURE );
}
// implied else, malloc successful
for( size_t i=0; stringToEncrypt[i]; ++i)
{
output[i] = stringToEncrypt[i] ^22;
}
output[length] = '\0';
return output;
} // end function: readLine_str_test_encrypt
the output from the above code is:
dswrZxs
readLine