C getln (getline substitute using fgetc)

#define SZINIT 120

/** getln reads up to n characters from stream into s, (if n=0 no limit).
 *  Reading continues until n characters read (or newline or EOF reached).
 *  s is initially reallocated n (or SZINIT, if n=0) + 1 bytes. (no overrun).
 *  If n=0, s is reallocated as needed to read an entire line. A final call
 *  to realloc trims the amount of memory allocated only that required. On
 *  success, the length of s is returned, -1 otherwise.
 */
ssize_t getln (char **s, size_t *n, FILE *stream)
{
#ifndef SZINIT
#define SZINIT 1024
#endif
    int c = 0;
    size_t maxc = *n ? *n : SZINIT;
    ssize_t nchr = 0;

    if (!(*s = realloc (*s, (maxc + 1) * sizeof **s)))
        return -1;

    while ((size_t)nchr < maxc && (c = fgetc (stream)) != EOF && c != '\n')
    {
        (*s)[nchr++] = c;   /* assign char to s, increment nchr */

        if ((size_t)nchr == maxc)   /* test if limit is reached */
        {
            if (*n) {  /* if reading only *n chars, flush stdin */
                while ((c = fgetc (stream)) != '\n' && c != EOF);
                break;
            }
            else {     /* realloc as required, adding SZINIT */
                void *tmp = realloc (*s, (maxc + SZINIT) * sizeof **s);
                if (!tmp)
                    return -1;  /* return failure on short read */
                *s = tmp;
                maxc += SZINIT;
            }
        }
    }
    (*s)[nchr] = 0; /* nul-terminate */

    /* final realloc to trim allocation to exact amount.
     * validate: update s only on success. (optional)
     */
    void *tmp = realloc (*s, (nchr + 1) * sizeof **s);
    if (tmp)
        *s = tmp;

    return nchr || c != EOF ? nchr : -1;
}