Here are five possible code completions for the given function:

**Option 1: Using malloc and free**
```c
buf = malloc(bufsize);
if (!buf) {
    return -ENOMEM;
}
for (i = 0; i < len; i++) {
    ret = fun(buf, priv, size);
    if (ret) {
        free(buf);
        return ret;
    }
    pg += size;
}
free(buf);
return ret;
```

**Option 2: Using alloca and no free**
```c
buf = alloca(bufsize);
if (!buf) {
    return -ENOMEM;
}
for (i = 0; i < len; i++) {
    ret = fun(buf, priv, size);
    if (ret) {
        return ret;
    }
    pg += size;
}
return ret;
```

**Option 3: Using a stack-allocated buffer**
```c
char buf[bufsize];
for (i = 0; i < len; i++) {
    ret = fun(buf, priv, size);
    if (ret) {
        return ret;
    }
    pg += size;
}
return ret;
```

**Option 4: Using a dynamically allocated buffer with error handling**
```c
buf = malloc(bufsize);
if (!buf) {
    return -ENOMEM;
}
try {
    for (i = 0; i < len; i++) {
        ret = fun(buf, priv, size);
        if (ret) {
            throw ret;
        }
        pg += size;
    }
} catch (int e) {
    free(buf);
    return e;
}
free(buf);
return ret;
```

**Option 5: Using a dynamically allocated buffer with goto error handling**
```c
buf = malloc(bufsize);
if (!buf) {
    return -ENOMEM;
}
for (i = 0; i < len; i++) {
    ret = fun(buf, priv, size);
    if (ret) {
        goto error;
    }
    pg += size;
}
free(buf);
return ret;

error:
free(buf);
return ret;
```

Note that the best option depends on the specific requirements of your code, such as performance, memory constraints, and error handling.