Valgrind "Invalide write of size 1"

Question

Below I wrote a function_getstr(), to get string in input without using scanf. Running valgrind, however, results "Invalid write of size 1" error. This happens for each input. What causes this error?

char *_getstr(){
    char *str = NULL;
    int bufsize = 0, c = 0;
    do {
        c = getchar();
        if (c != '\n'){
            str = realloc(str, (bufsize + 1)*sizeof(char));
            str[bufsize++] = c;
        }
    } while (c != '\n');
    str[bufsize] = '\0';
    return str;
}

In main I do that:

char *s1 = _getstr();

The output of valgrind when I insert the input is this:

Insert of the first string: hello
==6830== Invalid write of size 1
==6830==    at 0x109294: _getstr (changeJOIN.c:43)
==6830==    by 0x10919B: main (changeJOIN.c:15)
==6830==  Address 0x4a419b4 is 0 bytes after a block of size 4 alloc'd
==6830==    at 0x4839D7B: realloc (vg_replace_malloc.c:826)
==6830==    by 0x109264: _getstr (changeJOIN.c:39)
==6830==    by 0x10919B: main (changeJOIN.c:15)

Answer 1

When you add the terminating null byte, str points to bufsize bytes, so str[bufsize] = '\0'; writes one element past the end of allocated memory. This is what valgrind it complaining about.

You need to allocate one more byte before adding the null terminator.

str = realloc(str, (bufsize + 1)*sizeof(char));
str[bufsize] = '\0';