Trouble with dangling pointers and character arrays in C

Question

main(){
   char *cmd1[20] = {NULL};
   int x = parse_command(cmd1);
   printf("%s\ ",cmd1[0]);
}

parse_command(char *inTempString){
   char tempString[256];
   (call to function that assigns a string to tempString)
   cmd1[0] = tempString;
}

There is a problem when printing out the cmd1[0] within main. I am pretty sure that it is a dangling pointer error. I don't really know how to go about fixing it. Any help would be greatly appreciated. Thank you.

Answer 1

You're declaring cmd1 in main as a char** -- that is, a pointer to pointer to char. However, you then pass it to parse_command, which you've defined as taking a char*; a pointer to char.

This only compiles because of automatic conversion of pointer-to-anything to pointer-to-char. That's a historical artifact of old versions of C that used 'char*' in place of 'void*'; in your case, it just means that the compiler is ignoring the type error that you made rather than reporting it to you.

Answer 2

Yeah, you can't do that.

char tempString[256];

declares a variable on the stack in the function parse_command, that variable goes out of scope and pointers to it cease to be valid when parse_command returns.

You need to allocate the command string on the heap, so that it will still be valid when parse_command returns. This is one way.

parse_command(char *inTempString){
   char tempString[256];
   (call to function that assigns a string to tempString)

   int cb = strlen(tempString)+1;
   cmd1[0] = (char *)malloc(cb);
   strcpy(cmd1[0], tempString);
}

You should also call free(cmd[0]) before main exits.

In addition to that, this code doesn't compile. You can't reference cmd1[0] from inside the parse_command function. You should be getting a type mismatch when you try and pass cmd1 into parse_command, If you mean to return a char* from parse_command then it should be declared to take a char** as an argument, more like this.

parse_command(char **pcmd){
   char tempString[256];
   (call to function that assigns a string to tempString)

   int cb = strlen(tempString)+1;
   pcmd[0] = (char *)malloc(cb);
   strcpy(pcmd[0], tempString);
}

Answer 3

Something like this will work:

parse_command(char **inTempString){
        static char tempString[256];
        strcpy(tempString,"some string you want to copy");
        inTempString[0] = tempString;
}

• In your code the tempString would not exist once the function returns. You need to keep it alive even after the function returns. You can allocate the space dynamically and de-allocate later or you can declare it as static.
• Also you need to change the type argument inTempString from char* to char**.

Answer 4

You are trying to access cmd1 variable that is inside main function from parse_command.

I would say that at least the cmd1[0] will look like an integer because it is not declared withing that method.

The cmd1 is declared as array of char* but the parameter to method is char* which might be a pointer to char array but not pointer to array of char*.

The best way to copy char array into another char array is to use either memcpy,strcpy or similar methods that accept pointers to src, dest and size to be copied.