[英]Segmentation fault while reading binary file in C
Im a beginner in C. I would like to write and read a binary file , and i reached this so far: 我是C语言的初学者。我想写和读一个二进制文件 ,到目前为止,我达到了:
I can my struct to a binary file, and can read. 我可以将结构体转换为二进制文件,并且可以读取。
Problem one: I can only read integers, somehow strings are printed as garbage or random characters. 问题一:我只能读取整数,以某种方式将字符串打印为垃圾或随机字符。
Problem two: If i run my program, i add some entries to my binary file, then i print all the entries it is working fine (except Problem one), but after i terminate my program and run AGAIN i got Segmentation Fault while i try to read the file. 问题二:如果我运行程序,则将一些条目添加到二进制文件中,然后我将所有条目打印正常(问题一除外),但是在我终止程序并再次运行后,我尝试时遇到了分段错误读取文件。
Please help me, i just cannot move forward. 请帮助我,我只是无法前进。
/* Our structure */
struct rec
{
int max,cost;
char *name;
};
struct rec addNewEntry()
{
//init
char name[256];
int max;
int cost;
//input
printf("Type name: \n");
scanf("%s" , &name) ;
printf("Type guests limit: \n");
scanf("%d", &max);
printf("Type price: \n");
scanf("%d", &cost);
//create record
struct rec record;
record.name = name;
record.max = max;
record.cost = cost;
return record;
}
int main()
{
FILE *ptr_myfile;
//////////////////////////MENU////////////////////////////////
int option=-1;
while(option!=3)
{
printf("\n=== MENU === \n");
printf("\n1. Print all entries");
printf("\n2. Add new entry");
printf("\n3. Exit");
printf("\n");
printf("\nType menu option:");
scanf("%d", &option);
if(option == 1)
{
printf("\n...Printing all entries\n");
int f=open("stadionok.db",O_RDONLY);
if (f<0){ perror("Error at opening the file\n");exit(1);}
struct rec my_record;
while (read(f,&my_record,sizeof(my_record))){ //use write for writing
printf("name: %s \n",my_record.name);
printf("max: %d \n",my_record.max);
printf("cost: %d \n",my_record.cost);
}
close(f);
}
else if(option ==2)
{
printf("\n...Type a new entry\n");
//OPEN AND CHECK
ptr_myfile=fopen("stadionok.db","a");
if (!ptr_myfile)
{
printf("Unable to open file!");
return 1;
}
//TYPE A NEW ENTRY
struct rec new_stad = addNewEntry();
//WRITE TO FILE
fwrite(&new_stad, sizeof(struct rec), 1, ptr_myfile);
//CLOSE
fclose(ptr_myfile);
printf("Done.\n");
}
}
return 0;
}
EDIT: 编辑:
I modified just as you suggested and now i got: error: incompatible types in assignment 我按照您的建议进行了修改,现在我得到: 错误:分配中的类型不兼容
at: 在:
char name[256];
//input
printf("Type name: \n");
scanf("%s" , &name) ;
struct rec record;
record.name = name; //HERE
The reason you get garbage strings is that you are writing a char pointer into the file. 您获得垃圾字符串的原因是您正在将char 指针写入文件。 The simple way to do what you want is to use a char array with a fixed length inside your struct.
执行所需操作的简单方法是在结构内部使用固定长度的char数组。 like so:
像这样:
struct rec
{
int max,cost;
char name[1024];
};
It is also the reason for the crash. 这也是坠机的原因。 When you read the data and trying to print the string the
printf()
probably tries to read from a memory block that is not valid, because the memory addresses changed. 当您读取数据并尝试打印字符串时,
printf()
可能会尝试从无效的内存块中读取数据,因为内存地址已更改。
If you want to write strings with dynamic size into the file it gets a bit more complicated. 如果要将动态大小的字符串写到文件中,它将变得更加复杂。
Then you should not write the struct itself, but the values of it and the size of the string. 然后,您不应该编写该结构本身,而应该编写它的值和字符串的大小。
PseudoCode Example: 伪代码示例:
write(max);
write(cost);
write(strlen(name)); // strlen + 1 if you want to include \0
write(name);
max = readInt();
cost = readInt();
nameLen = readInt();
allocatememory(nameLen);
name = read_n_bytes(namelen);
To your edit: if rec.name is still a pointer (char*) you have to assign a pointer to it, not an array. 编辑:如果rec.name仍然是指针(char *),则必须为其分配一个指针,而不是数组。
rec.name = &name;
if your rec.name is an array with a fixed size now: 如果您的rec.name现在是具有固定大小的数组:
strncpy(rec.name, name, sizeof(rec.name)); // copy the string to rec.name
rec.name[strlen(name)] = '\0'; // add a binary 0 to the end of the string
Note: if you are reading more than 1 struct (in a loop), you should not use char name[256];
注意:如果您要读取1个以上的结构(在循环中),则不应使用
char name[256];
, because every rec.name would be pointing to name which is overwritten everytime you read a new value. ,因为每个rec.name都指向每次读取新值时都会被覆盖的name。 Use the following construct for every entry:
对每个条目使用以下构造:
char* name; //define name as a pointer
name = malloc(256); // allocate a new memory block and assign a pointer to it to name
scanf("%s" , name); // No & needed because name is already a pointer
rec.name = name; // No & needed because name is already a pointer
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