将值分配给char数组 [英] Assigning values to char arrays
问题描述
大家好，
这里是一个基本问题。假设我已经声明了两个变量，
char * a，** b;
然后我可以给一个像
a =" hello world" ;;
问题是，我应该如何为b分配值？一个简单的
b [0] =" string";
导致分段错误。
非常感谢。
问候，Emyl。
emyl< ; kw **** @ yahoo.com写道：
大家好，
这里是一个基本问题。假设我已经声明了两个变量，
char * a，** b;
b是一个指向char指针的指针。
>
然后我可以给一个像一个像
a =" hello world" ;;
的问题是，我应该如何赋值给b'一个简单的
b [0] =" string" ;;
* b =" s";
>
导致分段错误。
答案非常感谢。
问候，Emyl。
emyl说：
大家好，
这里是一个基本问题。假设我已经声明了两个变量，
char * a，** b;
然后我可以给一个像
a =" hello world" ;;
问题是，我应该如何为b分配值？一个简单的
b [0] =" string";
导致分段错误。
非常感谢。
您的定义：
char * a，** b;
为名为a的指向char的指针保留足够的存储空间，并将名为b的指针指向char的指针保留足够的存储空间。
a =" ; hello world" ;;
为这个指向char的指针赋值，有问题的值是给定第一个字符的
地址字符串文字。
但是b [0] =" string" ;;是一个问题，不是因为
语法有什么问题，而是因为你做了一个不正确的假设。
b是一个指针-to-pointer-to-char，但你没有指向任何指向char的指针，所以它目前是不确定的。 B [0] ="串英寸;
*不是*试图给b一个值。这是试图给出价值
b [0]。但是b [0]没有意义，除非b有一个有意义的值。
你可以用几种方式给出有意义的价值，但最明显的是b $ b b b是为它分配一些新的内存：
#include< stdlib.h>
/ *为某些指针分配内存-char * /
char ** cpalloc（size_t n）
{
char ** ptr = malloc（n * sizeof * ptr） ;
if（ptr！= NULL）
{
while（n--）
{
ptr [n] = NULL;
}
}
返回ptr;
}
#include< stdio.h>
int main（无效）
{
char * a，** b;
a =&它的口袋里有什么东西？;
b = cpalloc（2）;
if（b！= NULL）
{
b [0] =" string";
b [1 ] =没有;
printf（"％s \ n"，a）;
printf（"％s或％s \ n，b [0]， b [1]）;
免费（b）;
}
返回0;
}
小心。上面写的cpalloc函数不为字符串分配存储空间
，仅用于指向char的指针集合。指向char
的指针足以指向一个字符串，但不能用于存储它。
-
Richard Heathfield < http://www.cpax.org.uk>
电子邮件：-http：// www。 + rjh @
谷歌用户：< http://www.cpax.org.uk/prg/writings/googly.php>
Usenet是一个奇怪的放置" - dmr 1999年7月29日
emyl写道：
>
here这是一个基本问题。假设我已经声明了两个变量，
char * a，** b;
然后我可以给出类似值的值
a =" ; hello world" ;;
问题是，我应该如何为b分配值？一个简单的
b [0] =" string";
导致分段错误。
" char ** b;"声明指向char的指针。您可以使用b =& a;来初始化它。 （如果声明是
存在）。那么** b是[0]。
但请注意，你初始化< a =" hello world" ;;>
会留下一个指向不可修改的字符串。
-
Chuck F（cinefalconer at maineline dot net）
可用于咨询/临时嵌入式和系统。
< http://cbfalconer.home.att.net>
-
发表于来自 http://www.teranews.com 的免费Usenet帐户
Hi all,
here''s an elementary question. Assume I have declared two variables,
char *a, **b;
I can then give a value to a like
a="hello world";
The question is, how should I assign values to b? A simple
b[0]="string";
results in a segmentation fault.
Answers greatly appreciated.
Regards, Emyl.
emyl <kw****@yahoo.comwrites:
Hi all,
here''s an elementary question. Assume I have declared two variables,
char *a, **b;b is a pointer to a char pointer.
>
I can then give a value to a like
a="hello world";
The question is, how should I assign values to b? A simple
b[0]="string";*b = "s";
>
results in a segmentation fault.
Answers greatly appreciated.
Regards, Emyl.
emyl said:
Hi all,
here''s an elementary question. Assume I have declared two variables,
char *a, **b;
I can then give a value to a like
a="hello world";
The question is, how should I assign values to b? A simple
b[0]="string";
results in a segmentation fault.
Answers greatly appreciated.Your definition:
char *a, **b;
reserves sufficient storage for a pointer-to-char named a, and a
pointer-to-pointer-to-char named b.
a="hello world";
assigns a value to this pointer-to-char, the value in question being the
address of the first character in the given string literal.
But b[0]="string"; is a problem, not because there''s anything wrong with
the syntax, but because you''ve made an incorrect assumption.
b is a pointer-to-pointer-to-char, but you haven''t pointed it to any
pointers-to-char, so it is currently indeterminate. b[0]="string"; is
*not* an attempt to give a value to b. It is an attempt to give a value to
b[0]. But b[0] is meaningless unless b has a meaningful value.
You can give b a meaningful value in any of several ways, but the most
obvious is to allocate some fresh memory for it:
#include <stdlib.h>
/* allocate memory for some pointers-to-char */
char **cpalloc(size_t n)
{
char **ptr = malloc(n * sizeof *ptr);
if(ptr != NULL)
{
while(n--)
{
ptr[n] = NULL;
}
}
return ptr;
}
#include <stdio.h>
int main(void)
{
char *a, **b;
a = "what has it got in its pocketses?";
b = cpalloc(2);
if(b != NULL)
{
b[0] = "string";
b[1] = "nothing";
printf("%s\n", a);
printf("%s or %s\n", b[0], b[1]);
free(b);
}
return 0;
}
Be careful. The cpalloc function written above does not allocate storage
for strings, only for a collection of pointers to char. A pointer to char
is sufficient for pointing at a string, but not for storing it.
--
Richard Heathfield <http://www.cpax.org.uk>
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999
emyl wrote:>
here''s an elementary question. Assume I have declared two variables,
char *a, **b;
I can then give a value to a like
a="hello world";
The question is, how should I assign values to b? A simple
b[0]="string";
results in a segmentation fault."char **b;" declares a pointer to a pointer to char. You could
initialize it with "b = &a;" (provided the a declaration is
present). Then **b is a[0].
However note that your initialization of <a = "hello world";>
leaves a pointing to an unmodifiable string.
--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>
--
Posted via a free Usenet account from http://www.teranews.com
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