C:连接动态分配的字符串

Question

我到处寻找合适的答案，但找不到。我很清楚我无法解决这个问题是由于对C.不熟悉

我正在开发一个函数，将一个动态分配的字符串连接到另一个字符串上，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的一个问题。

这是代码：

#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: %ldn", len);
char *tmp = realloc(dest, len);
if(tmp == NULL){
fprintf(stderr, "strAppend: realloc messed upn");
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: %sn", one);
printf("two: %sn", two);
dynamCat(one, two);
printf("one: %sn", one);
printf("two: %sn", two);
//freeee
free(one);
free(two);

return 0;
}

命令行上的输出：

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

这就是坏东西的来源。

命令行上带有valgrind的输出：

==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)

非常感谢你的帮助。我对C还很陌生，非常感谢你的帮助。

Answer 1

如果realloc返回空指针，则不释放其返回值；将空指针传递给free:没有意义

char *tmp = realloc(dest, len);
if(tmp == NULL){
fprintf(stderr, "strAppend: realloc messed upn");
free(tmp); // This does nothing.

dest是一个参数，因此分配给它只会更改参数。它不会改变最初的论点：

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;
}

或者您必须将其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，必须向它传递指针的地址：

dynamCat(&one, two);

在前一种情况下，必须从dynamCat:的返回值中获取新指针

one = dynamCat(one, two);

Answer 2

realloc函数可以更改对象的地址。当物体变得更大时，这样做的必要性是显而易见的：原始位置可能没有空间。收缩对象时可能也是如此。

抽象地说，当我们调用realloc(x, newsize)时，指针x变得不确定。我们必须捕获返回的指针y = realloc(x, newsize)，然后在以前引用x的所有地方使用它。

在您的程序中，连接函数返回void，而main一直引用旧字符串。

你的dynamCat应该是这样的：

/* dest is destroyed: use the return value
* in place of dest!
*/
char *dynamCat(char *dest, const char *src) {
size_t len = strlen(dest)  + strlen(src) + 1;
printf("strLen: %ldn", len);
char *tmp = realloc(dest, len);
if(tmp == NULL){
fprintf(stderr, "strAppend: realloc messed upn");
/* better handling needed here, too */
}
strcat(tmp, src);
return tmp;
}