An unexplained overflow in C - c

Question

I'm writing code to find the longest row in the input stream and print it out. However, after I defined an int called max_count = 0, I always found an overflow, which displayed max_count as 1633771873. I've initialized that variable, so I don't know where the problem is. You probably do not need to figure out all of the functions, but each of them has its comment.
Here is my code:
#include <stdio.h>
#define DEFAULT 10
int getline(char line[], int limit);
void copy(char from[], char to[]);
int enlarge(int lim, char s[]);
main()
{
int i;
int max_count = 0;
char line[DEFAULT];
char maxline[DEFAULT];
while ((i = getline(line, DEFAULT)) != 0) {
if (i > max_count) { // where weird thing happend (max_count=1633771873)
max_count = i;
copy(line, maxline);
}
}
if (max_count > 0) {
printf("maxline: %s", maxline);
} else {
printf("No maxline");
}
return 0;
}
/*get a row from input stream and return its length*/
int getline(char s[], int lim)
{
int i, c;
for (i = 0; ((c = getchar()) != EOF) && (c != '\n'); ++i) {
if (i == lim - 1) {
lim = enlarge(lim, s);
}
s[i] = c;
}
if (c == '\n') {
s[i] = c;
++i;
}
if (i == lim) {
enlarge(lim, s);
}
s[i] = '\0';
return i;
}
/*copy an array to another */
void copy(char from[], char to[])
{
int i = 0;
while (from[i] != '\0') {
to[i] = from[i];
++i;
}
}
/*expand an array twice as its capacity*/
int enlarge(int lim, char s[])
{
s[lim - 1] = '\0';
lim *= 2;
char temp[lim];
copy(s, temp);
s = temp;
return lim;
}
This is the console window:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
^Z
maxline:
--------------------------------
Process exited after 15.19 seconds with return value 3221225477

Answer 1

You have a buffer with space for 10 characters:
#define DEFAULT 10
char line[DEFAULT];
You enter 37 characters including a newline:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
Your getline function tries to store them all in line (by the way, enlarge doesn't do anything useful).
The first 10 characters fit into line. The other 27 characters and the terminating '\0' overwrite other random variables which come after line in memory.
That is why max_count holds the ASCII codes for aaaa.

Answer 2

Your enlarge function isn't doing what you think it is.
int enlarge(int lim, char s[])
{
s[lim - 1] = '\0';
lim *= 2;
char temp[lim];
copy(s, temp);
s = temp;
return lim;
}
You're creating a new array temp within the scope of the function. You then copy the address of the start of the array to s. Since s is a parameter to the function, modifying s won't be reflected in the calling function. So after this function returns s in getline is unchanged.
Even if you were to fix this by either returning a char * or changing the function to accept a char ** and assigning temp to the dereferenced pointer, you would be returning the address of a variable local to enlarge. That variable goes out of scope when the function returns and so the pointer would be invalid.
The only way you can change the size of an array is if you first allocate it dynamically with malloc and then later use realloc to change its size.
Also, getline is the name of a function on POSIX systems. You should change the name to something else.