我刚开始学习C，来自php。我还不明白如何使用malloc和free。
1）在下面的示例代码中，我可以将“免费”放在哪里？
valgrind ./reverse Text ONe
==3124== Memcheck, a memory error detector
==3124== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==3124== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==3124== Command: ./reverse Text ONe
==3124==
==3124== Invalid write of size 1
==3124== at 0x40060F: reverse (in /localServer/temp/C/reverse)
==3124== by 0x400654: main (in /localServer/temp/C/reverse)
==3124== Address 0x5203044 is 0 bytes after a block of size 4 alloc'd
==3124== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3124== by 0x4005C6: reverse (in /localServer/temp/C/reverse)
==3124== by 0x400654: main (in /localServer/temp/C/reverse)
==3124==
==3124== Invalid read of size 1
==3124== at 0x4E88CC0: vfprintf (vfprintf.c:1632)
==3124== by 0x4E8F898: printf (printf.c:33)
==3124== by 0x400682: main (in /localServer/temp/C/reverse)
==3124== Address 0x5203044 is 0 bytes after a block of size 4 alloc'd
==3124== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==3124== by 0x4005C6: reverse (in /localServer/temp/C/reverse)
==3124== by 0x400654: main (in /localServer/temp/C/reverse)
==3124==
1 Text txeT
2 ONe eNO
==3124==
==3124== HEAP SUMMARY:
==3124== in use at exit: 7 bytes in 2 blocks
==3124== total heap usage: 3 allocs, 1 frees, 1,031 bytes allocated
==3124==
==3124== LEAK SUMMARY:
==3124== definitely lost: 7 bytes in 2 blocks
==3124== indirectly lost: 0 bytes in 0 blocks
==3124== possibly lost: 0 bytes in 0 blocks
==3124== still reachable: 0 bytes in 0 blocks
==3124== suppressed: 0 bytes in 0 blocks
==3124== Rerun with --leak-check=full to see details of leaked memory
==3124==
==3124== For counts of detected and suppressed errors, rerun with: -v
==3124== ERROR SUMMARY: 4 errors from 2 contexts (suppressed: 0 from 0)
2）这是valgrind的输出。什么意思错误“无效写入大小1”和“无效读取大小1”？
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:orientation="vertical"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/single_message_view">
<LinearLayout
android:orientation="horizontal"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
<TextView
android:layout_width="wrap_content"
android:layout_weight="1"
android:layout_height="wrap_content"
android:text="author"
android:id="@+id/sender_data"
android:layout_marginEnd="5dp"
android:textColor="@color/colorWhite" />
<TextView
android:layout_width="wrap_content"
android:layout_weight="1"
android:textAlignment="textEnd"
android:layout_height="wrap_content"
android:text="date"
android:textColor="@color/colorWhite"
android:id="@+id/send_date" />
</LinearLayout>
<LinearLayout
android:layout_marginEnd="15dp"
android:layout_marginStart="15dp"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:minWidth="100dp"
android:orientation="vertical">
<RelativeLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
android:id="@+id/message_content"
android:background="@drawable/msg_haze"
>
<TextView
android:layout_margin="10dp"
android:layout_alignParentTop="true"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:textColor="@color/colorWhite"
android:id="@+id/message_text"
android:text="content" />
<LinearLayout
android:layout_margin="10dp"
android:minWidth="100dp"
android:layout_below="@id/message_text"
android:id="@+id/tags_container"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content">
<android.support.v7.widget.RecyclerView
android:layout_marginTop="5dp"
android:layout_marginBottom="5dp"
android:id="@+id/message_tags"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layoutManager="LinearLayoutManager"
tools:listitem="@layout/item_tag" />
</LinearLayout>
<ImageView
android:layout_marginBottom="10dp"
android:layout_marginEnd="10dp"
android:paddingBottom="10dp"
android:layout_alignParentEnd="true"
android:layout_alignParentBottom="true"
android:layout_width="15dp"
android:layout_height="15dp"
android:id="@+id/message_state" />
<LinearLayout
android:id="@+id/attachment_holder"
android:layout_marginTop="10dp"
android:padding="5dp"
android:visibility="gone"
android:orientation="vertical"
android:layout_alignParentBottom="true"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/attachments_haze">
<android.support.v7.widget.RecyclerView
android:id="@+id/attachments"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layoutManager="GridLayoutManager"/>
<ImageView
android:id="@+id/attachment"
android:visibility="gone"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</LinearLayout>
</RelativeLayout>
</LinearLayout>
谢谢。
答案 0 :(得分：3)
在使用反向返回的指针后，你应该在循环中释放，如下所示。
int main (int argc, char** argv) {
char *reverseStrPtr;
if (argc > 1) {
for (int i = 1; i < argc; ++i) {
reverseStrPtr = reverse(argv[i]);
printf("%d\t%s\t%s\n", i, argv[i], reverseStrPtr);
free(reverseStrPtr);
}
}
return 0;
}
此外，valgrind错误基于下面的行reverse。
t[count] = '\0'; /* Add end of string */
简单地说，当你分配count-many字节时，你的索引范围是[0, count)。因此，尝试访问计数等同于访问(count+1)位，这是访问冲突。
您需要纠正的是您分配的内存量，即(count+1)个字节。需要额外的字节来放置终止'\0'字符。
char* reverse(char* text) {
int count = strLen(text);
char* t = malloc(count+1); // +1 for the terminating '\0'
for (int i = count; i > 0; --i) { t[count - i] = text[i-1];}
t[count] = '\0'; /* Add end of string */
return t;
}
答案 1 :(得分：1)
您需要保留从reverse(argv[i])和free返回的指针。
一种方法是将main略微调整为
int main (int argc, char** argv) {
char* s;
// if (argc > 1) { Comment out the redundant check
for (int i = 1; i < argc; ++i) {
printf("%d\t%s\t%s\n", i, argv[i], s = reverse(argv[i]));
free(s);
}
//}
return 0;
}
表达式内的赋值不是每个人的口味（因为它们可以混淆），但它们在这样的实例中运行良好。我应该习惯于在C中习惯这样的事情。
此外，t[count] = '\0';超出了您的缓冲区 - 其行为是 undefined 。您需要malloc(1 + count);