Write int variable in file using write

Question

I'm currently trying to write an integer variable using write function in the file. This is my code:

int main(int argc, char** argv){
int fd, nbMult, i;
char buf[4];
if((fd = open("data", O_CREAT|O_RDWR, 0777))==-1){
    perror("ERROR\n");
    exit(EXIT_FAILURE); 
}

do{ 
    printf("Bla bla ");
    if( scanf("%d", &nbMult) !=1 ) while( (i = getchar()) != '\n' );
}while(nbMult<1);
sprintf(buf, "%d", nbMult);
if( write(fd, buf, sizeof(int)) == -1 ){
    perror("ERROR\n");
    exit(EXIT_FAILURE);
}

close(fd);
return 0; }

It creates the file, but when I try to open it, it shows me "cannot display ". When I remplace the name of the file by data.txt it works but it shows strange character in the file. I have tried also to convert the Integer variable to String using sprintf() and it's the same issue.

Do you understand the difference between binary and text files? — stark
– stark, Commented Feb 15, 2018 at 21:00
If you want the file to contain readable text instead of binary, then something like sprintf() or fprintf() is the way to go. Show us that code, and we'll show you where you went wrong. — Lee Daniel Crocker
– Lee Daniel Crocker, Commented Feb 15, 2018 at 21:02

einpoklum · Accepted Answer · 2018-02-15 22:05:07Z

3

The file does get created - and has sizeof(int) bytes. Its contents are the bytes making up the integer nbMult in memory.

The thing is, those are not the characters used to print nbMult on a terminal. For example, suppose you've typed in 1234 for nbMult. What you've typed in are actually the characters '1', '2', '3', '4' - whose numeric values are 49, 50, 51, 52 respectively (here's a program which illustates that). But the bytes you'll get in memory for nbMult, on a typical machine, are 210, 4, 0, 0, since:

* 210 ==  1234       % 2^8
*   4 == (1234/2^ 8) % 2^8
*   0 == (1234/2^16) % 2^8
*   0 == (1234/2^24) % 2^8

those are the "strange characters" you're seeing in the file.

edited Feb 15, 2018 at 22:05

answered Feb 15, 2018 at 21:05

einpoklum

137k86 gold badges445 silver badges913 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Anas M K Over a year ago

Thanks, I can see that now ^^ I appreciate your help

einpoklum Over a year ago

@AnasMK: If you found this answer helpful, you can accept it by pressing the "v" sign on the margin.

Collectives™ on Stack Overflow

Write int variable in file using write

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related