我想在一个函数内连接 2 个字符串。但是，我希望该函数也修改目标字符串(char 数组)。
到目前为止，我在下面得到了这段代码，但它显示了"分段错误"，我不知道如何解决这个问题。谢谢。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *my_strcat(char *dest, const char * src) {
char *tab = (char*) malloc(sizeof(char) * (strlen(*dest) + strlen(src) + 1));
if(NULL == tab)
return NULL;
strcpy(tab, dest);
strcpy(tab, src);
free(dest);
dest = (char*)malloc(sizeof(char) * (strlen(tab)+1));
strcpy(dest, tab);
return tab; }
int main(int argc, char **argv) {
char *dst = NULL, *src = NULL, *r = NULL;
int i;
src = malloc(sizeof(char) * 100);
strcpy(src, "fifty");
dst = malloc (sizeof (char *) * 100);
strcpy(src, "four");
r = my_strcat(dst, src);
printf("%sn", r);
printf("%sn", dst);
free(r);
free(dst);
free(src);
return 0; }
dst按main分配。若要修改函数中的分配，可以返回新指针或将指针传递给指针。由于返回另一个指针，因此必须传递指向指针char **dest的指针，并在函数中进行一些更改以适应该更改。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *my_strcat ( char **dest, const char * src) {
char *tab = malloc ( sizeof ( char) * (strlen ( *dest) + strlen ( src) + 1));//dereference dest
if ( NULL == tab)
return NULL;
strcpy ( tab, *dest);//dereference dst
strcat ( tab, src);
free ( *dest);//dereference dest
*dest = malloc ( sizeof ( char) * ( strlen ( tab) + 1));//dereference dest
strcpy ( *dest, tab);//dereference dest
return tab;
}
int main(int argc, char **argv) {
char *dst = NULL, *src = NULL, *r = NULL;
src = malloc ( sizeof ( char) * 100);
strcpy ( src, "fifty");
dst = malloc ( sizeof ( char) * 100);
strcpy ( dst, "four");
r = my_strcat ( &dst, src);//use address of dst
printf ( "%sn", r);
printf ( "%sn", dst);
free ( r);
free ( dst);
free ( src);
return 0;
}
你的代码中有几个问题。你可以使用valgrind，clang-tidy这样的工具来帮助你。
这是带有行号的代码
1 /* -*- compile-command: "gcc prog.c; ./a.out"; -*- */
2 #include <stdio.h>
3 #include <stdlib.h>
4 #include <string.h>
5
6 char *my_strcat(char *dest, const char *src)
7 {
8 char *tab = (char *)malloc(sizeof(char) * (strlen(*dest) + strlen(src) + 1));
9 if (NULL == tab)
10 return NULL;
11 strcpy(tab, dest);
12 strcpy(tab, src);
13 free(dest);
14 dest = (char *)malloc(sizeof(char) * (strlen(tab) + 1));
15 strcpy(dest, tab);
16 return tab;
17 }
18
19 int main(int argc, char **argv)
20 {
21 char *dst = NULL, *src = NULL, *r = NULL;
22 int i;
23 src = malloc(sizeof(char) * 100);
24 strcpy(src, "fifty");
25
26 dst = malloc(sizeof(char *) * 100);
27 strcpy(src, "four");
28
29 r = my_strcat(dst, src);
30 printf("%sn", r);
31 printf("%sn", dst);
32
33 free(r);
34 free(dst);
35 free(src);
36
37 return 0;
38 }
让我们使用 cland-tidy 来执行静态分析：
clang-tidy-7 prog.c --
打印一堆消息：
8 warnings generated. /home/picaud/Temp/prog.c:8:46: warning: 1st function call argument is an uninitialized value [clang-analyzer-core.CallAndMessage] char *tab = (char *)malloc(sizeof(char) * (strlen(*dest) + strlen(src) + 1)); ^ /home/picaud/Temp/prog.c:26:9: note: Storing uninitialized value dst = malloc(sizeof(char *) * 100); ^ /home/picaud/Temp/prog.c:29:7: note: Calling 'my_strcat' r = my_strcat(dst, src); ^ /home/picaud/Temp/prog.c:8:46: note: 1st function call argument is an uninitialized value char *tab = (char *)malloc(sizeof(char) * (strlen(*dest) + strlen(src) + 1)); ^ /home/picaud/Temp/prog.c:8:53: warning: incompatible integer to pointer conversion passing 'char' to parameter of type 'const char *';等。。。
第一行说明 dest 未初始化，您可以将其追溯到：
26 dst = malloc(sizeof(char *) * 100);
27 strcpy(src, "four");
这当然是一个错误，必须由
26 dst = malloc(sizeof(char *) * 100);
27 strcpy(dst, "four");
现在您可以重新运行叮叮当当的整洁。第一条消息是：
/home/picaud/Temp/prog.c:8:53: warning: incompatible integer to pointer conversion passing 'char' to parameter of type 'const char *'; remove * [clang-diagnostic-int-conversion] char *tab = (char *)malloc(sizeof(char) * (strlen(*dest) + strlen(src) + 1)); ^~~~~
立即指向另一个错误，替换：
8 char *tab = (char *)malloc(sizeof(char) * (strlen(*dest) + strlen(src) + 1));
由
8 char *tab = (char *)malloc(sizeof(char) * (strlen(dest) + strlen(src) + 1));
你也有
/home/picaud/Temp/prog.c:26:9: warning: Result of 'malloc' is converted to a pointer of type 'char', which is incompatible with sizeof operand type 'char *' [clang-analyzer-unix.MallocSizeof]
dst = malloc(sizeof(char *) * 100);
^
因此，您必须替换：
dst = malloc(sizeof(char *) * 100);
由
dst = malloc(sizeof(char) * 100);
重新运行叮叮当当仍然显示一些问题：
/home/picaud/Temp/prog.c:27:3: note: Call to function 'strcpy' is insecure as it does not provide bounding of the memory buffer. Replace unbounded copy functions with analogous functions that support length arguments such as 'strlcpy'. CWE-119
/home/picaud/Temp/prog.c:31:3: warning: Use of memory after it is freed [clang-analyzer-unix.Malloc]
printf("%sn", dst);
^
/home/picaud/Temp/prog.c:26:9: note: Memory is allocated
dst = malloc(sizeof(char) * 100);
^
/home/picaud/Temp/prog.c:29:7: note: Calling 'my_strcat'
r = my_strcat(dst, src);
^
/home/picaud/Temp/prog.c:9:3: note: Taking false branch
if (NULL == tab)
^
/home/picaud/Temp/prog.c:13:3: note: Memory is released
free(dest);
^
/home/picaud/Temp/prog.c:29:7: note: Returning; memory was released via 1st parameter
r = my_strcat(dst, src);
^
/home/picaud/Temp/prog.c:31:3: note: Use of memory after it is freed
printf("%sn", dst);
我让你检查这一切。在我这边，遵循这些警告，我为您的代码获得了这个新版本：
/* -*- compile-command: "gcc prog.c; ./a.out"; -*- */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *my_strcat(char *dest, const char *src)
{
char *tab = (char *)malloc(sizeof(char) * (strlen(dest) + strlen(src) + 1));
if (NULL == tab)
return NULL;
strcpy(tab, dest);
strcpy(tab+strlen(dest), src);
return tab;
}
int main(int argc, char **argv)
{
char *dst = NULL, *src = NULL, *r = NULL;
int i;
src = malloc(sizeof(char) * 100);
strcpy(src, "fifty");
dst = malloc(sizeof(char) * 100);
strcpy(dst, "four");
r = my_strcat(dst, src);
printf("%sn", r);
printf("%sn", dst);
free(r);
free(dst);
free(src);
return 0;
}
运行时，此代码将打印：
gcc prog.c; ./a.out
fourfifty
four
您可以使用 valgrind 来检查是否有内存泄漏：
valgrind ./a.out
==4023== Memcheck, a memory error detector
==4023== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==4023== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==4023== Command: ./a.out
==4023==
fourfifty
four
==4023==
==4023== HEAP SUMMARY:
==4023== in use at exit: 0 bytes in 0 blocks
==4023== total heap usage: 4 allocs, 4 frees, 1,234 bytes allocated
==4023==
==4023== All heap blocks were freed -- no leaks are possible
==4023==
==4023== For counts of detected and suppressed errors, rerun with: -v
==4023== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
好学！