Passing strings to and from functions

[JSchaenzle]

I am trying to pass a string to a Mid() function, return a portion of the string, and then convert that portion to an int. My attempt looks like this..

char * ReceivedStr;
char * Mid(char * inString, int Start, int Length );


main()
{
int i;
int dis;
char * Distance;
ReceivedStr = "Move125";

Distance = Mid(ReceivedStr, 4, 3);
dis = atoi(Distance);
}

//This function should return the middle section of a given string…
char * Mid(char * inString, int Start, int Length )
{
char outString[50];
int i, iOut;
iOut = 0;
for (i = Start; i < Start + Length; i++){
outString[iOut] = inString[i];
iOut++;
}
outString[Length] = '\0';
return outString;
}

Right before the mid function returns the value of outString is "152", but for some reason the line Distance = Mid(ReceivedStr, 4, 3); does not set Distance to "152".
Could someone please explain what I'm doing wrong?

[Banfa]

You are returning a pointer to local data. That is the function Mid returns a pointer to its own internal buffer outString. However as soon as Mid returns the data allocated to outString is released (normally back to the stack).

Never, ever return a pointer to local data. Either pass a pointer with a location to write to in the function parameter list or dynamically allocate the return buffer, but then you will have to free it too.

You could declare outString static, this makes the variable persist even after the function Mid has returned. However this is very poor practice and as soon as you get to writing multi-threaded applications you would have to use a different scheme because that is not re-entrant.

[JSchaenzle]

...I see. That makes sense. Thank you!

[newb16]

You return address of the temporary (allocated on stack during function call) variable outString. Its content then may be destroyed in any time, e.g. in call to atoi. Allocate memory for it using malloc or make the calling function pre-alocate buffer.