In fopen("myfile", "r+") what is the difference between the "r+" and "w+" open mode? I read this:
"r"Open a text file for reading."w"Open a text file for writing, truncating an an existing file to zero length, or creating the file if it does not exist.
"r+"Open a text file for update (that is, for both reading and writing)."w+"Open a text file for update (reading and writing), first truncating the file to zero length if it exists or creating the file if it does not exist.
I mean the difference is that if I open the file with "w+", the file will be erased first?
The answer is
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);
}
Then open the file test.txt and see the what happens. You will see that all data written by the first program has been erased.
Repeat this for r+ and see the result. Hope you will understand.
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.
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...
r+ The existing file is opened to the beginning for both reading and writing. w+ Same as w except both for reading and writing.
This diagram will be faster to read next time. Maybe someone else will find that helpful too. This is clearly explained the difference between. 
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.
Source: Stackoverflow.com