将结构读/写到C中的文件

先决条件：C语言中的结构

对于写入文件，使用fprintf和putc可以很容易地将字符串或int写入文件，但是在写入struct的内容时可能会遇到困难。当您要写入和读取数据块时， fwrite和fread使任务变得更容易。

1. fwrite：以下是fwrite函数的声明

   size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)
   ptr - This is pointer to array of elements to be written
   size -  This is the size in bytes of each element to be written
   nmemb - This is the number of elements, each one with a size of size bytes
   stream - This is the pointer to a FILE object that specifies an output stream
   

   // C program for writing 
   // struct to file
   #include 
   #include 
   #include 
     
   // a struct to read and write
   struct person 
   {
       int id;
       char fname[20];
       char lname[20];
   };
     
   int main ()
   {
       FILE *outfile;
         
       // open file for writing
       outfile = fopen ("person.dat", "w");
       if (outfile == NULL)
       {
           fprintf(stderr, "\nError opend file\n");
           exit (1);
       }
     
       struct person input1 = {1, "rohan", "sharma"};
       struct person input2 = {2, "mahendra", "dhoni"};
         
       // write struct to file
       fwrite (&input1, sizeof(struct person), 1, outfile);
       fwrite (&input2, sizeof(struct person), 1, outfile);
         
       if(fwrite != 0) 
           printf("contents to file written successfully !\n");
       else 
           printf("error writing file !\n");
     
       // close file
       fclose (outfile);
     
       return 0;
   }
   

   输出：

   gcc demowrite.c
   ./a.out
   contents to file written successfully!
   

2. fread：以下是fread函数的声明

   size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)
   ptr - This is the pointer to a block of memory with a minimum size of size*nmemb bytes.
   size - This is the size in bytes of each element to be read.
   nmemb - This is the number of elements, each one with a size of size bytes.
   stream - This is the pointer to a FILE object that specifies an input stream.

   // C program for reading 
   // struct from a file
   #include 
   #include 
     
   // struct person with 3 fields
   struct person 
   {
       int id;
       char fname[20];
       char lname[20];
   };
     
   // Driver program
   int main ()
   {
       FILE *infile;
       struct person input;
         
       // Open person.dat for reading
       infile = fopen ("person.dat", "r");
       if (infile == NULL)
       {
           fprintf(stderr, "\nError opening file\n");
           exit (1);
       }
         
       // read file contents till end of file
       while(fread(&input, sizeof(struct person), 1, infile))
           printf ("id = %d name = %s %s\n", input.id,
           input.fname, input.lname);
     
       // close file
       fclose (infile);
     
       return 0;
   }
   

   输出：

   gcc demoread.c
   ./a.out
   id = 1   name = rohan sharma
   id = 2   name = mahendra dhoni
   

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