blocks|key|2985520|text|printf("%25s+%25s",phoneList[1].name,phoneList[1].phone);|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|2985521|上面的语句调用了未定义的行为。|unstyled|2985522|由于结构对象phoneList[1]的指针name和phone没有初始化，因此取消对它们的引用将调用UB。在你的例子中，他们抛出了垃圾值，但这也可能导致崩溃。|offset|length|style|CODE|2985523|为了适合读取文件内容并将其存储在结构对象中的情况，使用getline函数按行读取它们(假设所有细节都是按行存储的)，然后为char指针动态分配内存，然后将它们赋给读取值。但是，这种方法导致了大量的内存管理，这很容易出错。|2985524|entityMap^0|0|0|6|C|L|4|Q|5|0|R|7|1P|4|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|R|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|F|7|Z|8|@$I|10|J|11|K|L]|$I|12|J|13|K|L]]|9|@]|A|$]]|$1|O|3|-4|5|F|7|14|8|@]|9|@]|A|$]]]|P|$]]

<pre><code>printf("%s %s",phoneList[1].name,phoneList[1].phone);
</code></pre>

The above statement invokes undefined behaviour.

Since the pointers <code>name</code> &amp; <code>phone</code> of struct object <code>phoneList[1]</code> are not initialized dereferencing them invokes UB. In your case they are throwing out garbage values but it could have lead to a crash also.

To fit your case of reading the contents of file and storing it in the struct objects use <code>getline</code> function to read them row-wise(assuming that all the details are stored line-wise) and then dynamically allocate the memory for <code>char</code> pointers then assign them to the read value. But, this approach leads to lot of memory management which is error prone.

blocks|key|2985547|text|指针只有在它们产生的应用程序进程中才有意义。如果你将它们写到一个文件中，就像你在这里做的那样，你读回的值将是没有意义的--它们很可能指向未初始化的内存，或者指向正在被完全用于其他事情的内存。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2985548|您需要想出另一种将此数据写入文件的方法。|2985549|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Pointers are only meaningful in the application process that they originate from. If you write them to a file, as you're doing here, the values you read back will be meaningless — they will most likely point to uninitialized memory, or to memory which is being used for something else entirely.

You will need to come up with another way of writing this data to a file.

blocks|key|1491060|text|您遇到的问题是char*和char[]之间的模棱两可。当然，您可以为char*赋值字符串文字，但您需要了解清单结构的内容，以及如何将数据序列化和反序列化到文件中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1491061|保存来自一个进程的指针并将其读入另一个进程是没有意义的，因此您可能希望保存内容(指针所指向的内容)。您希望将两个值(name、phone)存储到文件中。由于您可能希望存储文字名称和文字电话，让我们考虑一下该文件可能是什么样子：|1491062|roast+duck%7C212-333-4444
peking+duck%7C411-511-61111
duck+soup%7C314-222-3333
free+duck%7C800-111-2222
...|code-block|syntax|javascript|1491063|您需要函数来序列化和反序列化数据。由于清单类型是指针，因此需要在读取这些值时为它们分配适当的空间，并且需要函数(方法)从文件读取序列化数据并将序列化数据写入文件。|1491064|读取(您将需要分配足够的空间)，|1491065|int
listing_read(FILE*fp,+LISTING*+listing)
{
++++char+name_buffer[100];
++++char+phone_buffer[100];

++++if(!fp)+return(-1);
++++if(!listing)+return(-2);

++++int+res+=+fscanf(fp,"%25s%7C%25s\n",name_buffer,phone_buffer);
++++if(+!res+)+{
++++++++//handle+error+here
++++}
++++//careful+here,+you+cannot+free+if+you+didn't+malloc/strdup
++++if(listing->name)+free(listing->name);
++++if(listing->phone)+free(listing->phone);

++++listing->name+=+strdup(name_buffer);
++++listing->phone+=+strdup(phone_buffer);
++++return(0);
}|1491066|编写(您需要提供适当的格式)，|1491067|int
listing_write(FILE*fp,+LISTING*+listing)
{
++++if(!fp)+return(-1);
++++if(!listing)+return(-2);

++++fprintf(fp,"%25s%7C%25s\n",listing->name,listing->phone);
++++return(0);
}|1491068|以下是修改代码所需方法，|1491069|//read+from+a+file
#include<stdio.h>

typedef+struct+
{
char*+name;
char*+phone;
}LISTING;

int+main(void)
{
++++LISTING+phoneList[14];++
++++FILE*+fp+=+NULL;
++++if(+!(fp+=+fopen("/media/Study/PhoneDirectory.dat","rb"))+)+{
++++++++printf("Error+opening+file!!!");
++++++++exit(1);
++++}

++++fseek(fp,0,SEEK_SET);
++++if(+listing_read(fp,&phoneList[0])+>=+0+)+{
++++++++printf("%25s+%25s",phoneList[0].name,phoneList[0].phone);
++++}
++++fclose(fp);

++++return+0;
}|1491070|这是如何写文件的变化，|1491071|//Write+to+file
#include<stdio.h>

typedef+struct+
{
char*+name;
char*+phone;
}LISTING;

int+main(void)
{
++++LISTING+phoneList[14];+++
++++FILE*+fp+=+NULL;

++++if(+!(fp+=+fopen("/media/Study/PhoneDirectory.dat","wb"))+)+{
++++++++printf("error,+cannot+write+file\n");
++++++++exit(1);
++++}

++++phoneList[0].name+=+"Santosh";
++++phoneList[0].phone+=+"9657681798";

++++if(+listing_write(fp,&phoneList[0])>=0)+{
++++++++printf("inserted");
++++}
++++fclose(fp);

++++return+0;
}|1491072|请注意，在编写程序时，需要将字符串"Santosh“和"9657681798”分配给列表成员的姓名和电话。虽然这样做是合法的，但您需要更好地理解C语言在这里的作用。C获取这些C字符串常量的地址，并将这些地址分配给honelist1.name和honelist1.phone成员指针。|1491073|考虑一下，如果你完成了这项任务，|1491074|++++phoneList[0].name+=+"Santosh";
++++phoneList[0].phone+=+"9657681798";|1491075|您已将指向常量字符串的指针分配给结构成员。|1491076|但如果要分配空间(例如，使用strdup())，|1491077|++++phoneList[0].name+=+strdup("Santosh");
++++phoneList[0].phone+=+strdup("9657681798");|1491078|您已经为字符串分配了空间，为这些成员元素分配了独立的位置。这更有可能是你想要做的。|1491079|1491080|请注意，我使用了phonelist，因为C具有从零开始的数组。|1491081|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1K|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|1L|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|1M|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|1N|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|1O|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|1P|8|@]|9|@]|A|$]]|$1|Q|3|R|5|F|7|1Q|8|@]|9|@]|A|$G|H]]|$1|S|3|T|5|6|7|1R|8|@]|9|@]|A|$]]|$1|U|3|V|5|F|7|1S|8|@]|9|@]|A|$G|H]]|$1|W|3|X|5|6|7|1T|8|@]|9|@]|A|$]]|$1|Y|3|Z|5|F|7|1U|8|@]|9|@]|A|$G|H]]|$1|10|3|11|5|6|7|1V|8|@]|9|@]|A|$]]|$1|12|3|13|5|6|7|1W|8|@]|9|@]|A|$]]|$1|14|3|15|5|F|7|1X|8|@]|9|@]|A|$G|H]]|$1|16|3|17|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|18|3|19|5|6|7|1Z|8|@]|9|@]|A|$]]|$1|1A|3|1B|5|F|7|20|8|@]|9|@]|A|$G|H]]|$1|1C|3|1D|5|6|7|21|8|@]|9|@]|A|$]]|$1|1E|3|-4|5|6|7|22|8|@]|9|@]|A|$]]|$1|1F|3|1G|5|6|7|23|8|@]|9|@]|A|$]]|$1|1H|3|-4|5|6|7|24|8|@]|9|@]|A|$]]]|1I|$]]

The problem you have is equivocating between char* and char[]. You can certainly assign a string literal to a char*, but you need to understand what the contents of a LISTING structure contain, and how you want to serialize and deserialize data to a file.

It does not make sense to save pointers from one process and read them into another process, so you probably want to save the contents (what a pointer points at). You want to store two values, (name, phone) to the file. Since you likely want to store the literal name and literal phone, let us consider what the file might look like:

<pre><code>roast duck|212-333-4444
peking duck|411-511-61111
duck soup|314-222-3333
free duck|800-111-2222
...
</code></pre>

You need functions to serialize and deserialize your data. Since your LISTING type is pointers, you will need to allocate appropriate space for those values, as you read them, and you need functions (methods) to read serialized data from a file and write serialized data to a file.

Reading (you will need to allocate enough space),

<pre><code>int
listing_read(FILE*fp, LISTING* listing)
{
 char name_buffer[100];
 char phone_buffer[100];

 if(!fp) return(-1);
 if(!listing) return(-2);

 int res = fscanf(fp,"%s|%s\n",name_buffer,phone_buffer);
 if( !res ) {
 //handle error here
 }
 //careful here, you cannot free if you didn't malloc/strdup
 if(listing-&gt;name) free(listing-&gt;name);
 if(listing-&gt;phone) free(listing-&gt;phone);

 listing-&gt;name = strdup(name_buffer);
 listing-&gt;phone = strdup(phone_buffer);
 return(0);
}
</code></pre>

Writing (you will need to provide proper formatting),

<pre><code>int
listing_write(FILE*fp, LISTING* listing)
{
 if(!fp) return(-1);
 if(!listing) return(-2);

 fprintf(fp,"%s|%s\n",listing-&gt;name,listing-&gt;phone);
 return(0);
}
</code></pre>

Here is how you need to modify your code,

<pre><code>//read from a file
#include&lt;stdio.h&gt;

typedef struct 
{
char* name;
char* phone;
}LISTING;

int main(void)
{
 LISTING phoneList[14]; 
 FILE* fp = NULL;
 if( !(fp = fopen("/media/Study/PhoneDirectory.dat","rb")) ) {
 printf("Error opening file!!!");
 exit(1);
 }

 fseek(fp,0,SEEK_SET);
 if( listing_read(fp,&amp;phoneList[0]) &gt;= 0 ) {
 printf("%s %s",phoneList[0].name,phoneList[0].phone);
 }
 fclose(fp);

 return 0;
}
</code></pre>

And here is how writing the file would change,

<pre><code>//Write to file
#include&lt;stdio.h&gt;

typedef struct 
{
char* name;
char* phone;
}LISTING;

int main(void)
{
 LISTING phoneList[14]; 
 FILE* fp = NULL;

 if( !(fp = fopen("/media/Study/PhoneDirectory.dat","wb")) ) {
 printf("error, cannot write file\n");
 exit(1);
 }

 phoneList[0].name = "Santosh";
 phoneList[0].phone = "9657681798";

 if( listing_write(fp,&amp;phoneList[0])&gt;=0) {
 printf("inserted");
 }
 fclose(fp);

 return 0;
}
</code></pre>

Note that in you writing program you assign the string literals "Santosh" and "9657681798" to the LISTING members name and phone. Though legal to do, you need a better understanding of what C does here. C takes the address of these C-string constants and assigns those addresses to the phonelist[1].name and phonelist[1].phone member pointers.

Consider that if you did this assignment,

<pre><code> phoneList[0].name = "Santosh";
 phoneList[0].phone = "9657681798";
</code></pre>

You have assigned the pointers to constant strings to your structure members.

But if you were to allocate space (for example, using strdup()),

<pre><code> phoneList[0].name = strdup("Santosh");
 phoneList[0].phone = strdup("9657681798");
</code></pre>

You have allocated space for the strings, assigning independent locations for these member elements. Which is is more likely what you want to do.

<hr>

Note that I used phonelist[0] since C has zero-based arrays.

I tried to write and read from a file with pointers in structures. But when I read from file I see some garbage value. I am using GCC 4.7.2 on Linux. Need some help.

Read:

<pre><code>//read from a file

#include&lt;stdio.h&gt;

typedef struct 
{
 char* name;
 char* phone;
}LISTING;

int main(void)
{
 LISTING phoneList[14]; 
 FILE * fp = NULL;

 fp = fopen("/media/Study/PhoneDirectory.dat","rb"); 

 if(fp == NULL)
 printf("Error opening file!!!"); 

 fseek(fp,0,SEEK_SET);

 if(fread(&amp;phoneList[1],sizeof(LISTING),1,fp)==1)
 printf("%s %s",phoneList[1].name,phoneList[1].phone);

 fclose(fp);

 return 0;
}
</code></pre>

And write:

<pre><code>//Write to file

#include&lt;stdio.h&gt;

typedef struct 
{
 char* name;
 char* phone;
}LISTING;

int main(void)
{
 LISTING phoneList[2]; 
 FILE * fp = NULL;

 fp = fopen("/media/Study/PhoneDirectory.dat","wb");

 phoneList[1].name = "Santosh";
 phoneList[1].phone = "9657681798";

 if(fwrite(&amp;phoneList[1],sizeof(LISTING),1,fp)==1)
 printf("inserted");

 fclose(fp);

 return 0;
}
</code></pre>

using pointers in structures to read from file

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我尝试使用结构中的指针对文件进行写入和读取。但是当我从文件中读取时，我看到了一些无用的值。我在Linux上使用的是GCC 4.7.2。我需要一些帮助。阅读：//read from a file#include<stdio.h>typedef struct { char* name; char* phone;...

问在结构中使用指针读取文件
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