Calling a function to return a string inside another function call in c

Question

I am currently working on a program with the Dante Brooklyn II board from audinate. I am trying to use a void return function from the Dante API - foo() - that needs to have 3 various structs passed in as well as 2 strings. I have another function get_value_by_key() which returns the string value that I need. I initially tried calling on the program like so:

    foo(struct, struct, struct, get_value_by_key(key1), get_value_by_key(key2);

When I run it this way, the program twists the returned strings of get_value_by_key() and crashes the entire program. I have, however, found an alternative that works, but it is longer and I would like to not waste the extra processing power. The alternative is here:

    char value1[15], value2[15];
    strcpy(value1, get_value_by_key(key1));
    strcpy(value2, get_value_by_key(key2));
    foo(struct, struct, struct, value1, value2);

I cant seem to decipher why the Brooklyn board shuts down with the first command, and not the second. Any help at all would be much appreciated. This is the code for get_value_key(). Pretty simple logic here:

    char * get_value_by_key(command_t command, char key[50]){
         int i;
         char value[50];
         for(i = 0; i < NUM_PARAMETERS; i++){
             if(strcmp(user_command.command_parameter[i].key, key) == 0){
                 strcpy(value, user_command.command_parameter[i].value);
             }
         }
         return value;
     }

Answer 1

Your get_value_by_key function is broken -- it is returning a pointer to a local variable (value) which is destroyed when the function returns. So you get undefined behavior when you try to read the string -- it might still be there but might have been overwritten by something else.

Answer 2

This looks like a mistake:

char * value1, value2; /* value1 is a char*, value2 is a char */
strcpy(value1, get_value_by_key(key1));
strcpy(value2, get_value_by_key(key2));

You are copying data into value1 and value2, but you have not allocated memory for the strings - just the pointer and the char. NOTE: only value1 is a char*, and value2 is a single char. If you want to declare both a char*, you need to write:

char *value1, *value2;

The code below should work without crashing (assuming get_value_by_key() returns a char*), but I don't know how you want to allocate the space. On the stack, the heap or in static RAM ?

char value1[128], value2[128];
strcpy(value1, get_value_by_key(key1));
strcpy(value2, get_value_by_key(key2));

EDIT: (edited after new info in the comments)

Please post the code for get_value_by_key.

I am betting it uses a static buffer to hold the string/char-array it returns a pointer to. That would mean the content gets destroyed after the second call and that could be a problem.

EDIT2:

If that is indeed the real source, then you are returning a pointer to something allocated on the stack - strictly verboten!!

See returning a local variable from function in C