C strings: Random characters appearing in memory after using strcpy and strcat

Question

I'm trying to remove the file extension from a file so I can rename the file with the substring "opt.s". I do this with following simple example program:

 char in[5] = "hel.s";
 char test[40];
 strncpy(test,in,strlen(in)-2);
 char result[50];
 strcpy(result,strcat(test,"opt.s"));
 printf(" %s ",result);

The output i get differs but usually looks something like the following:

 helþ(opt.s 

So basically random numbers appear in between "hel" and "opt.s". What is the cause of this? Did I use strcpy wrong or is strcat causing the problem?

Answer 1

This is incorrect:

char in[5] = "hel.s";

The string takes six, not five, characters (the sixths one is for null terminator). Without it the string is unterminated, causing strlen, strcat, and other functions that work on terminated strings to fail with undefined behavior.

To fix, remove 5 from the declaration, and let the compiler decide on the length for you:

char in[] = "hel.s";

In addition, your use of strncpy is invalid: it does not implicitly append null terminators

No null-character is implicitly appended at the end of destination if source is longer than num. Thus, in this case, destination shall not be considered a null terminated C string (reading it as such would overflow).

This line

strncpy(test,in,strlen(in)-2);

always produces an unterminated string, causing subsequent call of strcat to fail. To fix this, you need to add a terminator yourself:

size_t len = strlen(in)-2;
memcpy(test, in, len); // Do not use strncpy
test[len] = '\0';

Answer 2

char in[5] = "hel.s";

in is NOT a string: it does not have a terminating '\0' char. You cannot use in with string functions.

strncpy(test, in, strlen(in) - 2); // invalid use of strlen

However, strncpy() is not a string function (it doesn't need a terminating NUL in the src array; it doesn't necessarily write terminating NUL in the dst array).
If it worked as I think you expected, the resulting test array is also NOT a string ... and you cannot use it with string functions.

strcpy(result, strcat(test, "opt.s")); // invalid use of strcat