c - Flex yy_scan_string 内存泄漏

Question

描述

这是一个为了引入更大解决方案中的问题而创建的示例。我必须使用 flex 和 yy_scan_string() 。我在 Flex 中遇到内存泄漏问题(代码如下)。在此示例中，内存泄漏被标记为“仍然可访问”，但在原始解决方案中，它们被标记为“丢失的内存”。

我认为这个问题是由flex内部分配的内存中的某个地方，我不知道如何正确释放它，并且我找不到该问题的任何教程/文档。

文件.lex

%option noyywrap                   

%{                    
#include <stdio.h>
%}

%%
. printf("%s\n", yytext);
%%

int main() {
    printf("Start\n");
    yy_scan_string("ABC");
    yylex();
    printf("Stop\n");
    return 0;
}

bash$ flex file.lex
bash$ gcc lex.yy.c
bash$ ./a.out
Start
H
a
l
l
o

W
o
r
l
d
Stop
bash$ valgrind --leak-check=full --show-leak-kinds=all ./a.out
==6351== Memcheck, a memory error detector
==6351== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6351== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==6351== Command: ./a.out
==6351==
==6351== error calling PR_SET_PTRACER, vgdb might block
Start
A
B
C
Stop
==6351==
==6351== HEAP SUMMARY:
==6351==     in use at exit: 77 bytes in 3 blocks
==6351==   total heap usage: 4 allocs, 1 frees, 589 bytes allocated
==6351==
==6351== 5 bytes in 1 blocks are still reachable in loss record 1 of 3
==6351==    at 0x483577F: malloc (vg_replace_malloc.c:299)
==6351==    by 0x10AE84: yyalloc (in ./a.out)
==6351==    by 0x10ABE5: yy_scan_bytes (in ./a.out)
==6351==    by 0x10ABBC: yy_scan_string (in ./a.out)
==6351==    by 0x10AEE2: main (in /home/./a.out)
==6351==
==6351== 8 bytes in 1 blocks are still reachable in loss record 2 of 3
==6351==    at 0x483577F: malloc (vg_replace_malloc.c:299)
==6351==    by 0x10AE84: yyalloc (in ./a.out)
==6351==    by 0x10A991: yyensure_buffer_stack (in ./a.out)
==6351==    by 0x10A38D: yy_switch_to_buffer (in ./a.out)
==6351==    by 0x10AB8E: yy_scan_buffer (in ./a.out)
==6351==    by 0x10AC68: yy_scan_bytes (in ./a.out)
==6351==    by 0x10ABBC: yy_scan_string (in ./a.out)
==6351==    by 0x10AEE2: main (in ./a.out)
==6351==
==6351== 64 bytes in 1 blocks are still reachable in loss record 3 of 3
==6351==    at 0x483577F: malloc (vg_replace_malloc.c:299)
==6351==    by 0x10AE84: yyalloc (in ./a.out)
==6351==    by 0x10AAEF: yy_scan_buffer (in ./a.out)
==6351==    by 0x10AC68: yy_scan_bytes (in ./a.out)
==6351==    by 0x10ABBC: yy_scan_string (in ./a.out)
==6351==    by 0x10AEE2: main (in ./a.out)
==6351==
==6351== LEAK SUMMARY:
==6351==    definitely lost: 0 bytes in 0 blocks
==6351==    indirectly lost: 0 bytes in 0 blocks
==6351==      possibly lost: 0 bytes in 0 blocks
==6351==    still reachable: 77 bytes in 3 blocks
==6351==         suppressed: 0 bytes in 0 blocks
==6351==
==6351== For counts of detected and suppressed errors, rerun with: -v
==6351== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Answer 1

flex文档中提到了答案，但是你必须仔细阅读才能找到它。 (见下文。)

如果您生成可重入扫描仪，则您负责创建和销毁所需的每个扫描仪对象，并且扫描仪对象管理扫描仪实例所需的所有内存。但即使不使用可重入接口(interface)，也可以使用yylex_destroy来管理内存。在传统的不可重入接口(interface)中，没有 scanner_t 参数，因此 yylex_destroy 的原型(prototype)很简单

int yylex_destroy(void);

(虽然它有一个返回值，应该是状态代码，但它永远不会返回错误。)

如果您愿意，您可以调用yylex_init，它在不可重入接口(interface)中也不接受任何参数，但与可重入接口(interface)不同的是，没有必要调用它。

摘自手册memory management章节:

Flex allocates dynamic memory during initialization, and once in a while from within a call to yylex(). Initialization takes place during the first call to yylex(). Thereafter, flex may reallocate more memory if it needs to enlarge a buffer. As of version 2.5.9 Flex will clean up all memory when you call yylex_destroy See faq-memory-leak.

<小时/>

示例:

$ cat clean.l
%option noinput nounput noyywrap nodefault
%{                    
#include <stdio.h>
%}

%%
.|\n ECHO;
%%

int main() {
    printf("Start\n");
    yy_scan_string("ABC");
    yylex();
    printf("Stop\n");
    yylex_destroy();
    return 0;
}

$ flex -o clean.c clean.l
$ gcc -Wall -o clean clean.c
$ valgrind --leak-check=full --show-leak-kinds=all ./clean
==16187== Memcheck, a memory error detector
==16187== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==16187== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==16187== Command: ./clean
==16187== 
Start
ABCStop
==16187== 
==16187== HEAP SUMMARY:
==16187==     in use at exit: 0 bytes in 0 blocks
==16187==   total heap usage: 4 allocs, 4 frees, 1,101 bytes allocated
==16187== 
==16187== All heap blocks were freed -- no leaks are possible
==16187== 
==16187== For counts of detected and suppressed errors, rerun with: -v
==16187== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)