where is the difference between these two file modes ?

.maverk · 2023-09-02T23:22:21.996Z

where is the difference between ``w`` and ``w+`` in c ? they both erase the file where is the point of using ``w+`` in c language ?

.

.maverkOP•9/2/23, 11:23 PM

i have checked :

https://en.cppreference.com/w/c/io/fopen#File_access_flags

https://en.cppreference.com/w/c/io/fopen#File_access_flags

https://en.cppreference.com/w/c/io/fopen#File_access_flags

https://en.cppreference.com/w/c/io/fopen#File_access_flags

but it is still not clear to me where the point of using

w+

w+

is

E

Eakam•9/2/23, 11:37 PM

w is writing only, w+ is reading and writing

EEakam w is writing only, w+ is reading and writing

.

.maverkOP•9/2/23, 11:41 PM

how reading i don't understand ?

.

.maverkOP•9/2/23, 11:41 PM

when i use w+ the file will be erased

.

.maverkOP•9/2/23, 11:41 PM

even with only w

.

.maverkOP•9/2/23, 11:42 PM

where is the point here i am confused

E

Eakam•9/2/23, 11:42 PM

If you use w, you can only write to the file

E

Eakam•9/2/23, 11:42 PM

If you use w+, you can write to the file and read from the file

E

Eakam•9/2/23, 11:42 PM

Using the file stream pointer that is returned

.

.maverkOP•9/2/23, 11:42 PM

wait let address this little more better:

the file will be erased before even reading it

.

.maverkOP•9/2/23, 11:43 PM

can you show me an example ?

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.maverkOP•9/2/23, 11:43 PM

i used both w and w+ the file will be erased i cannot even read

.

.maverkOP•9/2/23, 11:44 PM

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w+");

    char buffer[992];


    fscanf(file, "%s", buffer);

    printf("%s", buffer);
    
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w+");

    char buffer[992];


    fscanf(file, "%s", buffer);

    printf("%s", buffer);
    
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w+");

    char buffer[992];


    fscanf(file, "%s", buffer);

    printf("%s", buffer);
    
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w+");

    char buffer[992];


    fscanf(file, "%s", buffer);

    printf("%s", buffer);
    
}

result:

░dVJf☺

░dVJf☺

░dVJf☺

░dVJf☺

E

Eakam•9/2/23, 11:45 PM

Do this:

Open a while in
```
w
```
```
w
```
mode
Write to it
Try to read from it

E

Eakam•9/2/23, 11:45 PM

All using the same file stream pointer that is returned

.

.maverkOP•9/2/23, 11:45 PM

that doesn't work

E

Eakam•9/2/23, 11:46 PM

Then try to do it using

w+

w+

.

.maverkOP•9/2/23, 11:46 PM

.

.maverkOP•9/2/23, 11:46 PM

absolutely no output

C

CaramelCorn•9/2/23, 11:51 PM

you need to rewind the pointer

C

CaramelCorn•9/2/23, 11:51 PM

the pointer is still at the end of the write when you go to read, so it reads nothing

C

CaramelCorn•9/2/23, 11:53 PM

otherwise yeah between w+ and w there will be no difference if you don't move the pointer around

CCaramelCorn the pointer is still at the end of the write when you go to read, so it reads no...

.

.maverkOP•9/3/23, 12:05 AM

very nice it works as expected

E

Eakam•9/3/23, 12:09 AM

shouldn't allow you to read the file using the same file pointer

C

CaramelCorn•9/3/23, 12:10 AM

yep it won't

E

Eakam•9/3/23, 12:10 AM

I thought it would fail, but it seems to fail silently

E

Eakam•9/3/23, 12:10 AM

Like just returns -1

.

.maverkOP•9/3/23, 12:11 AM

wait a second

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.maverkOP•9/3/23, 12:11 AM

it works as expected but there is one thing confusing me

C

CaramelCorn•9/3/23, 12:11 AM

yeah C doesn't have the same try->catch of newer langs so the return values are how you need to handle errors

C

CaramelCorn•9/3/23, 12:11 AM

if it crashed outright that would be messy

E

Eakam•9/3/23, 12:12 AM

I had forgotten, am too used to errors being thrown

EEakam I had forgotten, am too used to errors being thrown

C

CaramelCorn•9/3/23, 12:12 AM

hah same

.

.maverkOP•9/3/23, 12:12 AM

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w");

    fprintf(file, "hello world");


    char buffer[443];
    fscanf(file, "%s", buffer);
    printf("%s", buffer);
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w");

    fprintf(file, "hello world");


    char buffer[443];
    fscanf(file, "%s", buffer);
    printf("%s", buffer);
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w");

    fprintf(file, "hello world");


    char buffer[443];
    fscanf(file, "%s", buffer);
    printf("%s", buffer);
}

#include <stdio.h>

int main(){
    FILE *file = fopen("test.txt", "w");

    fprintf(file, "hello world");


    char buffer[443];
    fscanf(file, "%s", buffer);
    printf("%s", buffer);
}

there are 2 operations going on here right ?

it opens the file twice

first opening a file and writing to it hello world

and then opening the file again for reading

.

.maverkOP•9/3/23, 12:12 AM

am i correct ?

C

CaramelCorn•9/3/23, 12:12 AM

nope

.

.maverkOP•9/3/23, 12:12 AM

what ??? why

C

CaramelCorn•9/3/23, 12:12 AM

the file stays opened, the pointer is a reference to the file

C

CaramelCorn•9/3/23, 12:12 AM

you need to manually close the file when you're done

C

CaramelCorn•9/3/23, 12:13 AM

the file is opened once in fopen

C

CaramelCorn•9/3/23, 12:13 AM

then you perform operations on the open file

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.maverkOP•9/3/23, 12:13 AM

aaaah oky oky i understand

.

.maverkOP•9/3/23, 12:13 AM

but

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.maverkOP•9/3/23, 12:13 AM

why hello world get erased ?

C

CaramelCorn•9/3/23, 12:14 AM

it doesn't get erased, the file pointer is pointing to the end of the last thing you wrote at the end of fprintf, C doesn't reset the file pointer after each operation, which is actually very helpful

C

CaramelCorn•9/3/23, 12:15 AM

you're just reading from this point: Hello World_

C

CaramelCorn•9/3/23, 12:15 AM

the _ is essentially where the pointer is when you then go to read from that point on

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.maverkOP•9/3/23, 12:16 AM

yes i understand this very well

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.maverkOP•9/3/23, 12:16 AM

i understand the cursor never goes back to the begining after any operation

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.maverkOP•9/3/23, 12:17 AM

but what i don't understand is my hello world gets erased and i don't find it in the file when i look into it

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