realloc() invalid old size

Question

I am doing an exercise for fun from KandR C programming book. The program is for finding the longest line from a set of lines entered by the user and then prints it.

Here is what I have written (partially, some part is taken from the book directly):-

#include <stdio.h>
#include <stdlib.h>

int MAXLINE =  10;
int INCREMENT = 10;

void copy(char longest[], char line[]){
    int i=0;

    while((longest[i] = line[i]) != '\0'){
        ++i;
    }
}

int _getline(char s[]){
    int i,c;

    for(i=0; ((c=getchar())!=EOF && c!='\n'); i++){
        if(i == MAXLINE - 1){
            s = (char*)realloc(s,MAXLINE + INCREMENT);

            if(s == NULL){
                printf("%s","Unable to allocate memory");
                //  goto ADDNULL;
                exit(1);
            }

            MAXLINE = MAXLINE + INCREMENT;
        }
        s[i] = c;
    }

    if(c == '\n'){
        s[i] = c;
        ++i;
    }

ADDNULL:
    s[i]= '\0';
    return i;
} 

int main(){
    int max=0, len;

    char line[MAXLINE], longest[MAXLINE];

    while((len = _getline(line)) > 0){
        printf("%d", MAXLINE);
        if(len > max){
            max = len;
            copy(longest, line);
        }
    }

    if(max>0){
        printf("%s",longest);
    }

    return 0;
}

The moment I input more than 10 characters in a line, the program crashes and displays:-

*** Error in `./a.out': realloc(): invalid old size: 0x00007fff26502ed0 ***
======= Backtrace: =========
/lib64/libc.so.6[0x3d6a07bbe7]
/lib64/libc.so.6[0x3d6a07f177]
/lib64/libc.so.6(realloc+0xd2)[0x3d6a0805a2]
./a.out[0x400697]
./a.out[0x40083c]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x3d6a021b45]
./a.out[0x400549]

I also checked realloc invalid old size but could not follow the logic of passing a pointer to a pointer to the function modifying the array.

Answer 1

You get an invalid old size error when your code writes the memory that malloc/realloc allocated for "housekeeping information". This is where they store the "old" allocated size. This also happens when the pointer that you pass to realloc has not been properly initialized, i.e. it's neither a NULL nor a pointer previously returned from malloc/calloc/realloc.

In your case, the pointer passed to realloc is actually an array allocated in automatic memory - i.e. it's not a valid pointer. To fix, change the declarations of line and longest as follows:

char *line = malloc(MAXLINE), *longest = malloc(MAXLINE);

To avoid memory leaks, make sure that you call free(line) and free(longest) at the end of your program.