# Linux Kernel開發學習日誌2020.1.31
###### tags: `linux2020` `C`
## Tree map

course week1
└── linux-kernel_rcf
    ├── c_structure
    └── file_IO
        └── fwrite() --> in Example3
        └── fread()  --> in Example4

## TODO:
* c file_IO-example3-reading and writing to a binary file.

To write into a
### Example3: Write to a binary file using ```fwrite()```

/*
 *  Created by unknowntpo at 2020.1.30 (Fri)
 *  @title      : Write to a binary file using fwrite()
 *  @ref        : https://www.programiz.com/c-programming/c-file-input-output
 *  @abstract   : write to a binary file using fwrite()
 *
 */

#include <stdio.h>
#include <stdlib.h>

struct threeNum
{
    int n1, n2, n3;
};

int main()
{
    int n;
    struct threeNum num;
    FILE *fptr;  // pointer to the file location

    if ((fptr = fopen("program.bin", "wb")) == NULL){
        printf("Error when opening file");

        // pointer exits if the file pointer returns NULL.
        exit(1);
    }

    for(n = 1; n < 5; ++n){
        num.n1 = n;
        num.n2 = 5*n;
        num.n3 = 5*n + 1;
        /* fwrite(addressData, sizeData, numbersData, pointerToFile) */
        fwrite(&num, sizeof(struct threeNum), 1, fptr);
    }
    fclose(fptr);

    return 0;
}

### Example4: Read from a binary file using ```fread()```

In this program, you read the same file program.bin and loop through the records one by one.

In simple terms,
* you read one ```threeNum``` record of ```threeNum```
* from the file pointed by ```*fptr```
* into the structure num.

You'll get the same records you inserted in Example 3.

/*
 *  Created by unknowntpo at 2020.1.30 (Fri)
 *  @title      : Read to a binary file using fread()
 *  @ref        : https://www.programiz.com/c-programming/c-file-input-output
 *  @abstract   : Read to a binary file using fread()
 *
 */

#include <stdio.h>
#include <stdlib.h>

struct threeNum
{
    int n1, n2, n3;
};    // struct variable ??

int main()
{
    int n;
    struct threeNum num;
    FILE *fptr;

    if ((fptr = fopen("program.bin", "rb")) == NULL){
        printf("Error! opening file.");
            // program exits if the file pointer returns NULL.
            exit(1);
    }

    for(n = 1; n < 5; ++n){     // ++n vs n++ ?
        fread(&num, sizeof(struct threeNum), 1, fptr);
        printf("n1: %d\tn2: %d\tn3: %d\n", num.n1, num.n2, num.n3);
    }
    // try without fclose --> nothing special happened.
    fclose(fptr);

    return 0;
}

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## ```strcpy()```: String Copy

The ```strcpy()``` function is defined in the ```string.h``` header file.
* Syntax:

    * dest: Pointer to the destination array where the content is to be copied.
    * src : string which will be copied.

**eg.**

#include <stdio.h>
#include <string.h>
int main()
{
char str1[10]= "awesome";
char str2[10];
char str3[10];
strcpy(str2, str1);
strcpy(str3, "well");
puts(str2);
puts(str3);
return 0;
}

Output:

awesome
well

## C Structures

ref : https://www.tutorialspoint.com/cprogramming/c_structures.htm

* struct variable ??