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Computer Programming - Seneca Polytechnic College•2y ago

where is the difference between these two file modes ?

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 ?

67 Replies

.maverkOP•2y ago

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

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

Eakam•2y ago

w is writing only, w+ is reading and writing

.maverkOP•2y ago

how reading i don't understand ? when i use w+ the file will be erased even with only w where is the point here i am confused

Eakam•2y ago

If you use w, you can only write to the file If you use w+, you can write to the file and read from the file Using the file stream pointer that is returned

.maverkOP•2y ago

wait let address this little more better: the file will be erased before even reading it can you show me an example ? i used both w and w+ the file will be erased i cannot even read

#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☺

Eakam•2y ago

Do this: - Open a while in w mode - Write to it - Try to read from it All using the same file stream pointer that is returned

.maverkOP•2y ago

that doesn't work

Eakam•2y ago

Then try to do it using w+

.maverkOP•2y ago

No description

.maverkOP•2y ago

absolutely no output

CaramelCorn•2y ago

you need to rewind the pointer the pointer is still at the end of the write when you go to read, so it reads nothing otherwise yeah between w+ and w there will be no difference if you don't move the pointer around

.maverkOP•2y ago

very nice it works as expected

Eakam•2y ago

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

CaramelCorn•2y ago

yep it won't

Eakam•2y ago

I thought it would fail, but it seems to fail silently Like just returns -1

.maverkOP•2y ago

wait a second it works as expected but there is one thing confusing me

CaramelCorn•2y ago

yeah C doesn't have the same try->catch of newer langs so the return values are how you need to handle errors if it crashed outright that would be messy

Eakam•2y ago

I had forgotten, am too used to errors being thrown

CaramelCorn•2y ago

hah same

.maverkOP•2y ago

#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 am i correct ?

CaramelCorn•2y ago

nope

.maverkOP•2y ago

what ??? why

CaramelCorn•2y ago

the file stays opened, the pointer is a reference to the file you need to manually close the file when you're done the file is opened once in fopen then you perform operations on the open file

.maverkOP•2y ago

aaaah oky oky i understand but why hello world get erased ?

CaramelCorn•2y ago

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 you're just reading from this point: Hello World the is essentially where the pointer is when you then go to read from that point on

.maverkOP•2y ago

yes i understand this very well i understand the cursor never goes back to the begining after any operation 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

CaramelCorn•2y ago

oh really? that is odd shouldn't be happening I'd have to play around with it myself ig, but that behaviour doesn't make sense

Eakam•2y ago

Weird, it didn't disappear for me

CaramelCorn•2y ago

that's what I assume would be the case

Eakam•2y ago

No description

Eakam•2y ago

Maybe it disappears on windows?

CaramelCorn•2y ago

I'll test it on my desktop, it's running windows rn

.maverkOP•2y ago

what is odd ? why ? should the content of the file not be removed ?

CaramelCorn•2y ago

no it shouldn't be removed after that cose is run, not if the code is executed correctly

.maverkOP•2y ago

wait a minute is that a ub ?

CaramelCorn•2y ago

I'm getting an error on windows that causes a crash when I attempted to read from the end of the file, which causes the file to not be closed properly so it doesn't save the print

Eakam•2y ago

Yes, ubuntu using Windows subsystem for linux Same, it is because the file pointer is at the end of the file when you read

CaramelCorn•2y ago

yep

Eakam•2y ago

I managed to get it to save to the file by adding a rewind()

.maverkOP•2y ago

ok hhh this will cause some confusion

CaramelCorn•2y ago

yep yep, the crash would stop it from saving the write to the file

Eakam•2y ago

It is basically ubuntu running in a terminal in windows

.maverkOP•2y ago

i see

CaramelCorn•2y ago

we do love VMs

.maverkOP•2y ago

who doesn't carmen

Eakam•2y ago

I still don't know if there is an easier way to compile and run c on windows other than visual studio So I love ubuntu for this

CaramelCorn•2y ago

you can use mingw

Eakam•2y ago

Just gcc and run

CaramelCorn•2y ago

but yeah VS is definitely the easiest way on windows

Eakam•2y ago

Yea, I looked at it once and the whole process sounded like a lot It is funny though this particular code saves the file on linux but fails on windows

.maverkOP•2y ago

u can use gcc filename.c

Eakam•2y ago

That is what I did on ubuntu

.maverkOP•2y ago

but

Eakam•2y ago

Can you use gcc on windows?!

.maverkOP•2y ago

why don't u just use the extension with code runner ? or sublime with a system build i can share you can run c codes with f7 with one button

CaramelCorn•2y ago

via mingw as well u can use gcc on windows

Eakam•2y ago

Ohhhh, that's a thing

CaramelCorn•2y ago

but yeah

Eakam•2y ago

I have never used c with visual studio code so I didn't know Thanks

.maverkOP•2y ago

No description

.maverkOP•2y ago

hhhh with one click ooh yeah

Eakam•2y ago

Well it is one click with visual studio as well but I just hate going through the setup The code runner thing looks interesting

.maverkOP•2y ago

yeaah it can hide the file path too i use vs code when working with stdin and stdout

Eakam•2y ago

(Frantically looking back to see if I accidentally showed the entire file path in the screenshot) Ah, I have only used vs code with c++

.maverkOP•2y ago

it doesn't matter while no one has access to those sensitive data

Eakam•2y ago

Yea, the most you would get is my silly foder organization

.maverkOP•2y ago

i see

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