Fopen()中 r + 与 w + 的区别

There are 2 differences, unlike r+, w+ will:

• create the file if it does not already exist
• first truncate it, i.e., will delete its contents

r = read mode only
r+ = read/write mode
w = write mode only
w+ = read/write mode, if the file already exists override it (empty it)

So yes, if the file already exists w+ will erase the file and give you an empty file.

The main difference is w+ truncate the file to zero length if it exists or create a new file if it doesn't. While r+ neither deletes the content nor create a new file if it doesn't exist.

Try these codes and you will understand:

#include <stdio.h>
int main()
{
FILE *fp;


fp = fopen("test.txt", "w+");
fprintf(fp, "This is testing for fprintf...\n");
fputs("This is testing for fputs...\n", fp);
fclose(fp);
}

and then this

#include <stdio.h>
int main()
{
FILE *fp;


fp = fopen("test.txt", "w+");
fclose(fp);
}

If you will open test.txt, you will see that all data written by the first program has been erased.
Repeat this for r+ and see the result.
Here is the summary of different file modes:

Both r+ and w+ can read and write to a file. However, r+ doesn't delete the content of the file and doesn't create a new file if such file doesn't exist, whereas w+ deletes the content of the file and creates it if it doesn't exist.

r+ The existing file is opened to the beginning for both reading and writing. w+ Same as w except both for reading and writing.

w+

#include <stdio.h>
int main()
{
FILE *fp;
fp = fopen("test.txt", "w+");  //write and read mode
fprintf(fp, "This is testing for fprintf...\n");


rewind(fp); //rewind () function moves file pointer position to the beginning of the file.
char ch;
while((ch=getc(fp))!=EOF)
putchar(ch);


fclose(fp);
}

output

This is testing for fprintf...

test.txt

This is testing for fprintf...

w and r to form w+

#include <stdio.h>
int main()
{
FILE *fp;


fp = fopen("test.txt", "w"); //only write mode
fprintf(fp, "This is testing for fprintf...\n");
fclose(fp);
fp = fopen("test.txt", "r");
char ch;
while((ch=getc(fp))!=EOF)
putchar(ch);
fclose(fp);
}

output

This is testing for fprintf...

test.txt

This is testing for fprintf...

r+

test.txt

This is testing for fprintf...

#include<stdio.h>
int main()
{
FILE *fp;
fp = fopen("test.txt", "r+");  //read and write mode
char ch;
while((ch=getc(fp))!=EOF)
putchar(ch);
rewind(fp); //rewind () function moves file pointer position to the beginning of the file.
fprintf(fp, "This is testing for fprintf again...\n");
fclose(fp);
return 0;
}

output

This is testing for fprintf...

test.txt

This is testing for fprintf again...

r and w to form r+

test.txt

This is testing for fprintf...

#include<stdio.h>
int main()
{
FILE *fp;
fp = fopen("test.txt", "r");
char ch;
while((ch=getc(fp))!=EOF)
putchar(ch);
fclose(fp);


fp=fopen("test.txt","w");
fprintf(fp, "This is testing for fprintf again...\n");
fclose(fp);
return 0;
}

output

This is testing for fprintf...

test.txt

This is testing for fprintf again...

a+

test.txt

This is testing for fprintf...

#include<stdio.h>
int main()
{
FILE *fp;
fp = fopen("test.txt", "a+");  //append and read mode
char ch;
while((ch=getc(fp))!=EOF)
putchar(ch);
rewind(fp); //rewind () function moves file pointer position to the beginning of the file.
fprintf(fp, "This is testing for fprintf again...\n");
fclose(fp);
return 0;
}

output

This is testing for fprintf...

test.txt

This is testing for fprintf...
This is testing for fprintf again...

a and r to form a+

test.txt

This is testing for fprintf...

#include<stdio.h>
int main()
{
FILE *fp;
fp = fopen("test.txt", "a");  //append and read mode
char ch;
while((ch=getc(fp))!=EOF)
putchar(ch);
fclose(fp);
fp=fopen("test.txt","r");
fprintf(fp, "This is testing for fprintf again...\n");
fclose(fp);
return 0;
}

output

This is testing for fprintf...

test.txt

This is testing for fprintf...
This is testing for fprintf again...

This diagram will be faster to read next time. Maybe someone else will find that helpful too. This is clearly explained the difference between.