C:连接动态分配的字符串

Question

我到处搜索适用的答案，但找不到。我很清楚我无法解决这个问题是因为对 C 不熟悉。

我正在研究一个将一个动态分配的字符串连接到另一个字符串的函数，它似乎在内存泄漏和 valgrind 领域之外工作；然而，当我用 valgrind 运行它时，很明显我有一些明显的内存泄漏，我就是找不到。这似乎是我如何使用 realloc 的问题。

代码如下:

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

void dynamCat(char *dest, const char *src) {

    size_t len = strlen(dest)  + strlen(src) + 1;

    printf("strLen: %ld\n", len);

    char *tmp = realloc(dest, len);

    if(tmp == NULL){
        fprintf(stderr, "strAppend: realloc messed up\n");
        free(tmp);
        return;
    } else {
       dest = tmp;
    }

    strcat(dest, src);
}

int main (int argc, char *argv[]) {

        char *one = malloc(4);
        strcpy(one, "tee");

        char *two = malloc(4);
        strcpy(two, "hee");

        printf("one: %s\n", one);
        printf("two: %s\n", two);

        dynamCat(one, two);

        printf("one: %s\n", one);
        printf("two: %s\n", two);

        //freeee
        free(one);
        free(two);


        return 0;

}

命令行输出:

$ ./dynamCat
one: tee
two: hee
strLen: 7
one: teehee
two: hee

这就是坏事来的地方。

在命令行上使用 valgrind 输出:

==28804== Memcheck, a memory error detector
==28804== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==28804== Using Valgrind-3.16.1 and LibVEX; rerun with -h for copyright info
==28804== Command: ./dynamCat
==28804==
==28804== Invalid read of size 1
==28804==    at 0x4841C72: strlen (vg_replace_strmem.c:459)
==28804==    by 0x48D9407: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c040 is 0 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid read of size 1
==28804==    at 0x4841C84: strlen (vg_replace_strmem.c:459)
==28804==    by 0x48D9407: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c041 is 1 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid read of size 1
==28804==    at 0x48460D0: mempcpy (vg_replace_strmem.c:1536)
==28804==    by 0x48ED211: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib/libc-2.33.so)
==28804==    by 0x48D915A: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c042 is 2 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid read of size 1
==28804==    at 0x48460DE: mempcpy (vg_replace_strmem.c:1536)
==28804==    by 0x48ED211: _IO_file_xsputn@@GLIBC_2.2.5 (in /usr/lib/libc-2.33.so)
==28804==    by 0x48D915A: __vfprintf_internal (in /usr/lib/libc-2.33.so)
==28804==    by 0x48C463E: printf (in /usr/lib/libc-2.33.so)
==28804==    by 0x1092EE: main (dynamCat.c:37)
==28804==  Address 0x4a3c040 is 0 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804== Invalid free() / delete / delete[] / realloc()
==28804==    at 0x483F9AB: free (vg_replace_malloc.c:538)
==28804==    by 0x109312: main (dynamCat.c:41)
==28804==  Address 0x4a3c040 is 0 bytes inside a block of size 4 free'd
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==  Block was alloc'd at
==28804==    at 0x483E77F: malloc (vg_replace_malloc.c:307)
==28804==    by 0x10926D: main (dynamCat.c:26)
==28804==
==28804==
==28804== HEAP SUMMARY:
==28804==     in use at exit: 7 bytes in 1 blocks
==28804==   total heap usage: 4 allocs, 4 frees, 4,111 bytes allocated
==28804==
==28804== 7 bytes in 1 blocks are definitely lost in loss record 1 of 1
==28804==    at 0x4840D7B: realloc (vg_replace_malloc.c:834)
==28804==    by 0x1091FA: dynamCat (dynamCat.c:11)
==28804==    by 0x1092D6: main (dynamCat.c:35)
==28804==
==28804== LEAK SUMMARY:
==28804==    definitely lost: 7 bytes in 1 blocks
==28804==    indirectly lost: 0 bytes in 0 blocks
==28804==      possibly lost: 0 bytes in 0 blocks
==28804==    still reachable: 0 bytes in 0 blocks
==28804==         suppressed: 0 bytes in 0 blocks
==28804==
==28804== For lists of detected and suppressed errors, rerun with: -s
==28804== ERROR SUMMARY: 9 errors from 6 contexts (suppressed: 0 from 0)

非常感谢您的帮助。我对 C 语言还是很陌生，非常感谢您的帮助。

Answer 1

如果realloc返回一个空指针，不要释放它的返回值；将空指针传递给 free 是没有意义的:

    char *tmp = realloc(dest, len);

    if(tmp == NULL){
        fprintf(stderr, "strAppend: realloc messed up\n");
        free(tmp); // This does nothing.

dest 是一个参数，因此分配给它只会改变参数。它不会改变原始参数:

       dest = tmp; // This changes only the dest inside dynamCat.

要给调用者一个新地址，您必须将其作为指针返回:

char *dynamCat(char *dest, const char *src)
{
    …
    dest = something;
    …
    return dest;
}

或者你必须把它放入调用者传递的指针中:

void dynamCat(char **dest, const char *src)
{
    …
    char *tmp = realloc(*dest, len);
    …
        *dest = tmp;
    }

    strcat(*dest, src);
}

在后一种情况下，当调用 dynamCat 时，您必须将指针的地址传递给它:

        dynamCat(&one, two);

在前一种情况下，您必须从dynamCat 的返回值中获取新指针:

    one = dynamCat(one, two);