c - 在父进程中调用 fork() 后子进程内存泄漏,为什么?
问题描述
Mac OS X 10.13.4。运行此程序会触发 Valgrind 中的内存泄漏信号并泄漏系统调用:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
char *ft_strcpy(char *s1, const char *s2)
{
size_t cur;
cur = -1;
while (s2[++cur] != '\0')
s1[cur] = s2[cur];
s1[cur] = '\0';
return (s1);
}
void ptr_test()
{
char *ptr;
ptr = (char *)malloc(sizeof(char) * 15);
printf("FIRST: %p\n", ptr);
ft_strcpy(ptr, "Hello, World!");
printf("SECOND: %p\n\n", ptr);
free(ptr);
}
int main(void)
{
pid_t pid;
pid = fork();
while(1)
ptr_test();
if (pid != 0)
wait(NULL);
return (0);
}
泄漏系统调用:
Process: main [98746]
Path: /nfs/2018/p/patrisor/Desktop/ptr/main
Load Address: 0x1095a2000
Identifier: main
Version: ???
Code Type: X86-64
Parent Process: main [98687]
Date/Time: 2019-09-22 05:28:32.273 -0700
Launch Time: 2019-09-22 05:28:23.881 -0700
OS Version: Mac OS X 10.13.4 (17E199)
Report Version: 7
Analysis Tool: /usr/bin/leaks
Physical footprint: 280K
Physical footprint (peak): 280K
----
leaks Report Version: 3.0
Process 98746: 176 nodes malloced for 27 KB
Process 98746: 4 leaks for 4128 total leaked bytes.
Leak: 0x7f9d9ad00040 size=16 zone: DefaultMallocZone_0x1095a7000
Leak: 0x7f9d9ad00050 size=16 zone: DefaultMallocZone_0x1095a7000
Leak: 0x7f9d9b800800 size=2048 zone: DefaultMallocZone_0x1095a7000
Leak: 0x7f9d9b801000 size=2048 zone: DefaultMallocZone_0x1095a7000
瓦尔格林:
==1388== LEAK SUMMARY:
==1388== definitely lost: 32 bytes in 2 blocks
==1388== indirectly lost: 4,096 bytes in 2 blocks
==1388== possibly lost: 72 bytes in 3 blocks
==1388== still reachable: 215 bytes in 7 blocks
==1388== suppressed: 27,726 bytes in 170 blocks
==1388==
==1388== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 13 from 13)
最初我怀疑这是 free() 无法使用正确的分配地址的问题。但是,当我手动扫描进程的内存使用情况时,运行时它的使用情况并没有显着增加。后来我怀疑是子进程没有正确结束,可能是创建了僵尸进程。泄漏发生在开始的某个地方,即调用 free 的地方。是函数本身有问题,还是……?我似乎很失落。
有什么猜测吗?
解决方案
以下包含对代码问题的评论:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
/*
* strongly suggest:
* #include <string.h> and
* eliminating this function and in ptr_test()
* strcpy( ptr, "Hello, World! );
*/
char *ft_strcpy(char *s1, const char *s2)
{
size_t cur;
/* this causes an implicit conversion to unsigned
* so the value will be seen as a VERY LARGE number
* suggest initialize to 0 then use:
* while( s2[cur++] != '\0' )
*/
cur = -1;
while (s2[++cur] != '\0')
s1[cur] = s2[cur];
s1[cur] = '\0';
return (s1);
}
void ptr_test()
{
char *ptr;
/*
* returned value from malloc has type `void*`
* which can be assigned to any pointer
* casting just clutters the code,
* making it more difficult to understand, debug, etc
* suggest removing that cast.
*/
/* sizeof( char ) is defined in the c standard as 1
* multiplying anything by 1 has no effect
* suggest removing that expresson
*/
/*
* when calling any of the heap allocation functions
* always check (!=NULL) the returned value
* to assure the operation was successful
* if not successful, call 'perror( "malloc failed" );
* to output your error message and the text reason
* the system thinks the error occurred to 'stderr'
*/
ptr = (char *)malloc(sizeof(char) * 15);
printf("FIRST: %p\n", ptr);
ft_strcpy(ptr, "Hello, World!");
printf("SECOND: %p\n\n", ptr);
free(ptr);
}
int main(void)
{
pid_t pid;
pid = fork();
/*
* this runs 'ptr_test()' FOREVER
* and in the fork() error condition FOREVER
* and in the child process FOREVER
* and in the parent process FOREVER
*
* note: 'exit()' and EXIT_SUCCESS are from
* the header file: `stdlib.h`
* Suggest:
* if( !pid )
* { // then child process
* for( int i=0; i<1024; i++ )
* ptr_test();
* exit( EXIT_SUCCESS );
*/
while(1)
ptr_test();
/* there are three types of returned values from 'fork()'
* <0 means an error occurred
* ==0 means in the child process
* >0 means in the parent process
*
* a test for '!=0' will 'catch' the error AND the parent
* however, there is no child process for the parent to 'wait()'
* strongly suggest testing for each condition separately
* I.E.
* if( pid < 0 )
* //handle error
* perror( "fork failed" );
* else if( !pid )
* // handle child process ...
* exit( EXIT_ SUCCESS );
* else
* // handle parent process
* wait( NULL );
*/
if (pid != 0)
/* the prototype for 'wait()' is missing
* strongly suggest adding the header files:
* #include <sys/types.h>
* #include <sys/wait.h>
*/
wait(NULL);
return (0);
}
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