Slurp file into buffer with C stdio

fslurp.c

long fslurp(char **buf, long *len, FILE *fp)
{
    size_t new_len;
    long size, start = ftell(fp);

    if (start == -1 || fseek(fp, 0, SEEK_END) == -1)
        return -1;

    if ((size = ftell(fp)) == -1)
        return -1;

    size -= start;

    if (fseek(fp, start, SEEK_SET) == -1)
        return -1;

    if (*buf) {
        if (!len) return -1;
        new_len = fread(*buf, 1, *len, fp);
        if (ferror(fp)) return -1;
        if (new_len < (size_t)*len)
            (*buf)[new_len] = '\0';
    } else {
        *buf = malloc(size + 1);
        new_len =fread(*buf, 1, size, fp);
        if (ferror(fp)) {
            free(*buf);
            *buf = NULL;
            return -1;
        }
        (*buf)[new_len] = '\0';
    }
    if (len) *len = new_len;

    return size;
}