在结构中设置一个 const char 指针

Question

我正在尝试编写一个程序，我可以在其中创建令牌，使每个令牌都有一个常量类型和值。

token.h:

#ifndef TOKEN_H
#define TOKEN_H
typedef struct{
    const char *type;
    const char *value;
}token;

token *gen_token(const char *r, const char *val);

#endif

token.c:

#include <stdlib.h>
#include <string.h>
#include "lib/token.h"

token *gen_token(const char *type, const char *val){
    token o = {type, val}, *out = &o;
    return out;
}

main.c:

#include <stdio.h>
#include <stdlib.h>
#include "lib/token.h"

int main(int argi, char *argv[]){
    const char th[] = "line", va[] = ":";

    token *t = gen_token(th, va);

    printf("(%s, %s)\n", t->type, t->value);

    return 0;
}

当我运行正常时，似乎没有错误发生，但是当valgrind查看它时，它会抱怨很多。这是我的日志：

==4086== Memcheck, a memory error detector
==4086== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==4086== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==4086== Command: ./a lol
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4E7FD4F: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Use of uninitialised value of size 8
==4086==    at 0x4C2EBE2: strlen (vg_replace_strmem.c:458)
==4086==    by 0x4E80D77: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Use of uninitialised value of size 8
==4086==    at 0x4C2EBF4: strlen (vg_replace_strmem.c:458)
==4086==    by 0x4E80D77: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4EAAEEB: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1307)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Use of uninitialised value of size 8
==4086==    at 0x4EAAEF1: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1309)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4EAAF03: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1307)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Use of uninitialised value of size 8
==4086==    at 0x4EAAF0D: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1309)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4C32EB3: is_overlap (vg_replace_strmem.c:137)
==4086==    by 0x4C32EB3: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4C32EB9: is_overlap (vg_replace_strmem.c:140)
==4086==    by 0x4C32EB9: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4C32EC7: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4C32ECC: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Conditional jump or move depends on uninitialised value(s)
==4086==    at 0x4C32F64: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Use of uninitialised value of size 8
==4086==    at 0x4C32F70: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086== 
==4086== Use of uninitialised value of size 8
==4086==    at 0x4C32F7E: __GI_mempcpy (vg_replace_strmem.c:1525)
==4086==    by 0x4EAAE23: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1327)
==4086==    by 0x4E80BBA: vfprintf (vfprintf.c:1637)
==4086==    by 0x4E871F8: printf (printf.c:33)
==4086==    by 0x1087AB: main (main.c:11)
==4086==  Uninitialised value was created by a stack allocation
==4086==    at 0x108785: main (main.c:10)
==4086==  (line, :)
==4086== 
==4086== HEAP SUMMARY:
==4086==     in use at exit: 0 bytes in 0 blocks
==4086==   total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==4086== 
==4086== All heap blocks were freed -- no leaks are possible
==4086== 
==4086== For counts of detected and suppressed errors, rerun with: -v
==4086== ERROR SUMMARY: 36 errors from 14 contexts (suppressed: 0 from 0)

如果对我的代码提出任何帮助 and/or 批评，我将不胜感激。

Answer 1

函数中

token *gen_token(const char *type, const char *val){
    token o = {type, val}, *out = &o;
    return out;
}

变量out指向局部变量o，配置在函数returns之后。这纯属偶然，您可以在调用函数后读取这些值。因此关于

的警告

Uninitialised value was created by a stack allocation

相反，您需要分配内存并将值复制到其中。注意你需要记住在退出你的程序之前释放分配的内存，特别是如果你想让 valgrind 开心:)

Answer 2

您正在返回变量的地址 automatic storage duration in gen_token function. For this kind of storage duration in c:

The storage is allocated when the block in which the object was declared is entered and deallocated when it is exited by any means (goto, return, reaching the end).

注意： 使用您从 gen_token 函数返回的地址是 未定义的行为.

我建议您在示例中使用 opaque pointer 技术，如下所示：

token.h

#ifndef TOKEN_H
#define TOKEN_H

struct token; /* incomplete type, but you can use pointers to it */
typedef struct token token;

/* allocates memory for 'token' and initializes it */
token* gen_token(const char* r, const char* val);
/* deallocates the specified memory */
void delete_token(token* ptr);
/* returns the value of 'type' member */
const char* get_type(token* ptr);
/* returns the value of 'value' member */
const char* get_value(token* ptr);

#endif

token.c

#include <stdlib.h>
#include "token.h"

/* the 'struct token' is defined here in the source file */
struct token {
    const char* type;
    const char* value;
};

token *gen_token(const char* type, const char* val) {
    token* out = (token*)malloc(sizeof(token));
    out->type = type;
    out->value = val;
    return out;
}
void delete_token(token* ptr) {
    free(ptr);
}
const char* get_type(token* ptr) {
    return ptr == NULL ? "" : ptr->value;
}
const char* get_value(token* ptr) {
    return ptr == NULL ? "" : ptr->type;
}

在上面的示例中，gen_token 为 token 对象分配内存，初始化它，returns 分配内存块的地址。 c中的这种存储时长是分配的存储时长：

The storage is allocated and deallocated on request, using dynamic memory allocation functions.

示例用法：

#include <stdio.h>
#include <stdlib.h>
#include "token.h"

int main() {
    token* t = gen_token("line", ":");
    printf("(%s, %s)\n", get_type(t), get_value(t));
    return 0;
}

还有一件重要的事情。 main 函数中的以下变量是具有自动存储持续时间的常量数组：

const char th[] = "line", va[] = ":";

您可以存储指向数组第一个元素的指针，但如果按如下方式进行，您将得到 dangling pointers:

token* create_token() {
    const char th[] = "line", va[] = ":";
    return gen_token(th, va);
} /* from this point the created 'token' object will store dangling pointers */

在结构中设置一个 const char 指针

Set a const char pointer in a struct

c

string

struct

valgrind