问题描述：在系统中执行回收命令（gdb --batch --pid pidof mysqld --ex ‘call malloc_trim(0)’;）报错’malloc_trim’ has unknown return type; cast the call to its declared return type

一 系统环境

[root@localhost ~]# nkvers 
############## Kylin Linux Version #################
Release:
Kylin Linux Advanced Server release V10 (Sword)Kernel:
4.19.90-24.4.v2101.ky10.x86_64Build:
Kylin Linux Advanced Server
release V10 (SP2) /(Sword)-x86_64-Build09/20210524
#################################################

二 问题现象

[root@localhost ~]# gdb --batch --pid `pidof mysqld` --ex 'call malloc_trim(0)';
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[New LWP 2945]warning: Expected absolute pathname for libpthread in the inferior, but got target:/usr/lib64/libpthread.so.0.warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.warning: Expected absolute pathname for libpthread in the inferior, but got target:/usr/lib64/libpthread.so.0.warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.
0x00007fb2130629d9 in poll () from target:/usr/lib64/libc.so.6
'malloc_trim' has unknown return type; cast the call to its declared return type
[Inferior 1 (process 2914) detached]

三 分析原因

在C语言中，malloc_trim函数通常没有返回值，也就是说，它的返回类型是void。因此，malloc_trim函数不应该被强制转换为int类型。这样的转换在标准C语言中是没有意义的，因为malloc_trim不会返回一个整数值。
然而，一些特定的C库实现（如GNU C库）可能会为malloc_trim提供一个非标准的返回值，例如返回释放的内存字节数。在这种情况下，将malloc_trim的返回值强制转换为int可能是为了与这些非标准实现兼容。
总的来说，call (int)malloc_trim(0)和call malloc_trim(0)在标准C语言中的行为应该是相同的，因为malloc_trim的标准定义不包含返回值。但是，如果您的代码依赖于某个特定C库的非标准扩展，那么这两种调用方式可能会有所不同：

• call (int)malloc_trim(0)：这种调用方式尝试将malloc_trim的任何返回值（如果有）强制转换为int类型。如果malloc_trim实际上返回了一个整数值，那么这个值将被转换为int类型。
• call malloc_trim(0)：这种调用方式忽略了malloc_trim的任何返回值。这是标准C语言中推荐的调用方式，因为它遵循了malloc_trim的标准定义。

需要将malloc_trim函数指定为int整数类型

四 解决方法

将gdb --batch --pid pidof mysqld --ex ‘call malloc_trim(0)’;命令修改为gdb --batch --pid pidof mysqld --ex ‘call (int)malloc_trim(0)’;

[root@localhost ~]# gdb --batch --pid `pidof mysqld` --ex 'call (int)malloc_trim(0)';
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[New LWP 2945]warning: Expected absolute pathname for libpthread in the inferior, but got target:/usr/lib64/libpthread.so.0.warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.warning: Expected absolute pathname for libpthread in the inferior, but got target:/usr/lib64/libpthread.so.0.warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.
0x00007fb2130629d9 in poll () from target:/usr/lib64/libc.so.6
$1 = 1
[Inferior 1 (process 2914) detached]