Stuck halfway through a function

Hi all,

I'm trying to create a function that finds a key=value pair in a file and returns the value part for the given key. EG, if test.conf contains the text "port=10", I need a function to which I can hand "port" and get "10" returned to me.

I've got this far:-

Code:

---

char * getStringFromConf(char searchkey) {
        char c[120];
        FILE *f;
        f = fopen(CONF_FILE_PATH, "r");
        while ( fgets(c, 120, f) != NULL ) {
                char *sep = "=";
                char *key, *value;
                key = strtok(c, sep);
                value = strtok(NULL, sep);
                }
        }

        fclose(f);
}

---

... and the core of this works fine outside of a function (I wrote a quick and dirty test for that).

I'm unsure where to go from here in terms of matching the key variable with searchkey, and then returning the value variable as an appropriate type.

Can the experts here help?

Many thanks.

Your function returns char*, but I see no return.

Or if you decided to snip for brevity, I bet you're returning a pointer to your local array (or a pointer to somewhere within that array).

Both of which are wrong.

And the searchkey argument should likely be a const char *, and you want to compare that to the key read from the file...

Solution 1: Take 4 arguments, 2 buffers, 2 sizes of buffers.
Solution 2: Take 2 pointer-to-pointers and allocate enough memory via malloc. The function that calls getStringFromConf will have to free them.

Quote:

---

Your function returns char*, but I see no return.

---

Yup, as mentioned I haven't got that far. So taking the above into account:-

Code:

---

char * getStringFromConf(const char * searchkey) {
        char c[120];
        FILE *f;
        f = fopen(CONF_FILE_PATH, "r");
        while ( fgets(c, 120, f) != NULL ) {
                char *sep = "=";
                char *key, *value;
                key = strtok(c, sep);
                value = strtok(NULL, sep);
                }
        }

        if ( strcmp(key, searchkey) == 0 ) {
              // Got our required value
        }
        fclose(f);
}

---

Does that look about right? What's the best way for me to store and then return the value?

Thanks.

int getStringFromConf(const char * searchkey, char *place_for_result, size_t resultSize );

The return result is a boolean true / false indicating the success or failure of you trying to read the file.
If success, you copy what you find to place_for_result, up to at most resultSize characters.

In that respect, a little bit like how fgets() returns strings to you.

Quote:

---

Originally Posted by trillianjedi

Does that look about right? What's the best way for me to store and then return the value?
Thanks.

---

Quote:

---

Originally Posted by Elysia

Solution 1: Take 4 arguments, 2 buffers, 2 sizes of buffers.
Solution 2: Take 2 pointer-to-pointers and allocate enough memory via malloc. The function that calls getStringFromConf will have to free them.

---

Copy result into the buffers instead of retuning it.
Pay attention to Salem.

Thanks Salem/Elysia - sorry, I was being slow earlier (late night ;) ).

Now I have:-

Code:

---

int getStringFromConf(const char * searchkey, char *resultValue, size_t resultSize) {
        int result = 0;
        char c[120];
        FILE *f;
        f = fopen(CONF_FILE_PATH, "r");
        if (f != NULL)  {

                while ( fgets(c, 120, f) != NULL ) {
                        char *sep = "=";
                        char *key, *value;
                        key = strtok(c, sep);
                        value = strtok(NULL, sep);
                        if ( (value != NULL) && (strcmp(key, searchkey) == 0) ) {
                                //strcpy(resultValue, value); <-- causes a bus error
                                resultSize = strlen(value);
                                printf("Value : &#37;s", value);
                                printf("Size : %d", resultSize);
                                result = 1;
                        }
                }

                fclose(f);

        } else {
                result = -1;
        }
                                                                     
        return result;
}

---

But the strcpy I'm trying to do (currently commented out) is causing a bus error. I'm not sure why as I believe the pointer to char is the correct type for it (according to the man page)?

Thanks...

What's the calling code for getStringFromConf?

Ah, you might have just put me on the right track with your question. My test calling app looks like this (header includes removed for brevity):-

Code:

---

int main(void)
{

char * s;
size_t i;

        if ( getStringFromConf("host", s, i) == 0 ) {
                printf("%s", s);
        }

return 0;

}

---

I guess I need to hand the function a pointer to the searchkey string rather than a string? I'm unclear on the correct syntax here as it's been a long time for me since doing any C.

I tried declaring a char * key for the key variable and then using strcpy(key, "host") but that's also causing the bus error.

Thanks for your help so far - it's much appreciated.

No, the error is that s is not initialized.
http://cpwiki.sourceforge.net/A_pointer_on_pointers
Plus, resultSize is supposed to be the size of the buffer, not an uninitialized value.
And you can just pass a buffer defined on the stack, along with sizeof(variable).

Never pass an argument that is uninitialized, unless you are passing its address.
Uninitialized variables contain garbage and will therefore most likely not make your program work correctly.

Thanks Elysia. Just trying to get my head around this:-

Quote:

---

And you can just pass a buffer defined on the stack, along with sizeof(variable).

---

So, something like:-

char key[20];
char value[20];

.... and called as:-

i = getStringFromConf(&key, &value, sizeof(value))

?

Thanks

Nay, don't pass an array with &. They are automatically passed as pointers.
Otherwise it is more correct. Don't forget to check if your buffer is large enough to hold the data before you copy it into the buffer.
Moreover, you can return the required size or take size as a pointer and update it with the required buffer to hold all the data.

Many thanks - all working now.

Quote:

---

Don't forget to check if your buffer is large enough to hold the data before you copy it into the buffer.

---

Right. So I would look at the strlen of value and see if there's enough room in the buffer (plus 1 for the null terminator) before copying.

If buffer isn't big enough, I understand updating the size_t if I've got a pointer to it, but I'm unclear on how the memory for the buffer is actually allocated? Would I need to allocate that manually ?

Thanks.

The calling function would have to do something about it. Most likely it would have to dynamically allocate big enough store afterwards.

Thanks.

Is it considered bad form to just brute force allocate enough memory in the first place (eg, 80 chars) ?

No, not really (although too much is usually a bad thing(TM)). Memory is plenty and brute force allocation is faster. If you think it's enough, then most of the times, it's fine.
But if you don't know that it may be enough, dynamic may be better.
But in C, it's better to brute force to get rid of complexity of having to call free. In a big application, it can be difficult to know where to free the memory.

This reminds of a function I saw once, which had to assemble a string from some input parameters and then write it out to a file or something. Because the code had to be fast, the programmer created a 100 char array to assemble the string in. The code then checked whether the final string would be longer than 100 chars and if so, would allocate memory on the heap instead. The final string didn't have to be passed anywhere though, so there were no free issues.
Not that I condone that kind of thing ;)

QuantumPete