Question

我写了一小段代码。我正在使用void *指针来为C的基本数据类型创建交换函数。我写了这个函数的两个版本，因为字符串与其他基本数据类型有点不同（但也许我错了）这个函数的版本适用于int，float和char，但是当我尝试将其用于字符串时，Valgrind报告了多个错误。这是主要的：

int main(){

 int dimA=0,dimB=0;
 char *v,*g,*str1="heygv",*str2="bag";

 v=malloc(strlen(str1)+1);
 strcpy(v,str1);
 g=malloc(strlen(str2)+1);
 strcpy(g,str2);

 dimA=strlen(v);
 dimB=strlen(g);

 printf("\nX : %s %p ,Y : %s %p \n",v,v,g,g);
 swapStr(v,g,dimA+1,dimB+1);
 printf("\nX : %s %p ,Y : %s %p \n",v,v,g,g);

 free(v);
 free(g);

return 191;

}

这是交换功能，它可以正常交换字符串的值：

void* swapStr(void* a,void* b,int dimA,int dimB){

void* temp=malloc(dimA);
void* ptr;

    memcpy(temp,a,dimA);

    if(dimB>dimA){

        ptr=realloc(a,dimB);

        if(ptr==NULL){
            printf("\nError 1 realloc in swapStr\n");
            return;
        }else a=ptr;

    }

    memcpy(a,b,dimB);

    if(dimA>dimB){

        ptr=realloc(b,dimA);

        if(ptr==NULL){
            printf("\nError 2 realloc in swapStr\n");
            return;
        }else b=ptr;

    }

    memcpy(b,temp,dimA);

free(temp);
temp=NULL;
}

这里是Valgrind的报告：

X : heygv 0x51f2040 ,Y : bag 0x51f2090 
--5756-- REDIR: 0x4ec9080 (libc.so.6:memcpy@@GLIBC_2.14) redirected to 0x4a2472e (_vgnU_ifunc_wrapper)
--5756-- REDIR: 0x4ed1430 (libc.so.6:__memcpy_sse2_unaligned) redirected to 0x4c2e0d0 (memcpy@@GLIBC_2.14)
--5756-- REDIR: 0x4eb9ab0 (libc.so.6:realloc) redirected to 0x4c2ba52 (realloc)
--5756-- REDIR: 0x4eb8bb0 (libc.so.6:free) redirected to 0x4c2ac62 (free)

==5756== Invalid read of size 1
==5756==    at 0x4E7EC15: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2090 is 0 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== Invalid read of size 1
==5756==    at 0x4EAEBC1: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2092 is 2 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== Invalid read of size 1
==5756==    at 0x4EAEBD4: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2091 is 1 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== Invalid read of size 1
==5756==    at 0x4EC406B: __GI_mempcpy (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4EAEB0F: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2090 is 0 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== Invalid read of size 2
==5756==    at 0x4EC407B: __GI_mempcpy (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4EAEB0F: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2091 is 1 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
X : bag 0x51f2040 ,Y : bag 0x51f2090 
==5756== Invalid free() / delete / delete[] / realloc()
==5756==    at 0x4C2ACE9: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x40089D: main (swapVoid.c:49)
==5756==  Address 0x51f2090 is 0 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== 
==5756== HEAP SUMMARY:
==5756==     in use at exit: 6 bytes in 1 blocks
==5756==   total heap usage: 4 allocs, 4 frees, 22 bytes allocated
==5756== 
==5756== Searching for pointers to 1 not-freed blocks
==5756== Checked 65,256 bytes
==5756== 
==5756== 6 bytes in 1 blocks are definitely lost in loss record 1 of 1
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== LEAK SUMMARY:
==5756==    definitely lost: 6 bytes in 1 blocks
==5756==    indirectly lost: 0 bytes in 0 blocks
==5756==      possibly lost: 0 bytes in 0 blocks
==5756==    still reachable: 0 bytes in 0 blocks
==5756==         suppressed: 0 bytes in 0 blocks
==5756== 
==5756== ERROR SUMMARY: 11 errors from 7 contexts (suppressed: 0 from 0)
==5756== 
==5756== 1 errors in context 1 of 7:
==5756== Invalid free() / delete / delete[] / realloc()
==5756==    at 0x4C2ACE9: free (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x40089D: main (swapVoid.c:49)
==5756==  Address 0x51f2090 is 0 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== 
==5756== 1 errors in context 2 of 7:
==5756== Invalid read of size 2
==5756==    at 0x4EC407B: __GI_mempcpy (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4EAEB0F: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2091 is 1 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== 
==5756== 1 errors in context 3 of 7:
==5756== Invalid read of size 1
==5756==    at 0x4EC406B: __GI_mempcpy (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4EAEB0F: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2090 is 0 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== 
==5756== 1 errors in context 4 of 7:
==5756== Invalid read of size 1
==5756==    at 0x4EAEBC1: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2092 is 2 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== 
==5756== 2 errors in context 5 of 7:
==5756== Invalid read of size 1
==5756==    at 0x4EAEBD4: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E7EBB5: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2091 is 1 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== 
==5756== 4 errors in context 6 of 7:
==5756== Invalid read of size 1
==5756==    at 0x4E7EC15: vfprintf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x4E88748: printf (in /usr/lib64/libc-2.20.so)
==5756==    by 0x400885: main (swapVoid.c:46)
==5756==  Address 0x51f2090 is 0 bytes inside a block of size 4 free'd
==5756==    at 0x4C2BB1C: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==5756==    by 0x400959: swapStr (swapVoid.c:77)
==5756==    by 0x400860: main (swapVoid.c:45)
==5756== 
==5756== ERROR SUMMARY: 11 errors from 7 contexts (suppressed: 0 from 0)

错误似乎与free（）（或realloc？）的错误用法有关，但是为什么呢？另一个奇怪的事实是，如果我在没有valgrind的情况下运行这个程序，它可以工作，相反现在你看到字符串是没有正确交换。

关于如何创建可以交换至少基本数据类型的交换函数，每个可能的建议都被广泛接受。

Answer 1

您需要将指向char*的指针传递给swapStr()，因为如果重新分配指针，则需要字符串指针（char*）指向新分配的内存。

例如

void* swapStr(char **ap,char** bp,int dimA,int dimB){
    char *a = *ap;
    char *b = *bp;
    // ... rest of the function, including realloc
    *ap = a;
    *bp = b;
 }

swapStr(&a, &b, dimA, dimB);

这确保了如果重新分配了a或b指针，则呼叫者的指针也会更新。

当指向char*数组时，首选使用正确类型的指针（void*）而不是char。

Answer 2

首先void*函数不返回指针（编译器应警告你的东西）。

main()函数不知道swapStr()

中重新分配的指针

swapStr(v,g,dimA+1,dimB+1);
printf("\nX : %s %p ,Y : %s %p \n",v,v,g,g);

尝试使用函数

中的双*指针

void swapStr(void **a, void **b, int dimA, int dimB)
...
swapStr(&v,&g,dimA+1,dimB+1);

这样函数就可以将重新分配的指针值写入main()中的变量。

最后并非所有返回路径free(temp)。这是对代码的重复。我左边的函数类型为void**，因为您希望它也可以交换其他数据类型。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void fatal(char *msg) {
    printf("%s\n", msg);
    exit (1);
}

void swapStr(void **a, void **b, int dimA, int dimB){
    void *temp = malloc(dimA);
    if (temp == NULL)
        fatal("Error allocating memory for temp\n");
    memcpy(temp, *a, dimA);

    if(dimB>dimA){
        *a = realloc(*a, dimB);
        if (*a == NULL)
            fatal("Error reallocating memory for a\n");
    }
    if(dimA>dimB){
        *b = realloc(*b, dimA);
        if (*b == NULL)
            fatal("Error reallocating memory for b\n");
    }

    memcpy(*a, *b, dimB);
    memcpy(*b, temp, dimA);
    free(temp);
}

int main(void){
    int dimA=0, dimB=0;
    char *str1="heygv", *str2="bag";
    void *v, *g;

    v = malloc(strlen(str1)+1);
    if (v == NULL)
        fatal("Error allocating memory for v\n");
    strcpy(v, str1);
    g = malloc(strlen(str2)+1);
    if (g == NULL)
        fatal("Error allocating memory for g\n");
    strcpy(g, str2);

    dimA = strlen(v);
    dimB = strlen(g);
    printf("\nX : %s %p ,Y : %s %p \n", (char*)v, v, (char*)g, g);

    swapStr(&v, &g, dimA+1, dimB+1);
    printf("\nX : %s %p ,Y : %s %p \n", (char*)v, v, (char*)g, g);

    free(v);
    free(g);

    return 0;
}

使用free（）和realloc（）的错误

2 个答案: