CODEWITHSUNDEEP
clinuxunixfile-io
user1159517
user1159517

Reputation: 6350

What is the use of open system call in *nix

What is open()? What does this do? 

if (open("/dev/null", O_RDWR) < 0) {
  die("error=open: %m");
}

Upvotes: 1

Views: 198

Answers (3)

rashok
rashok

Reputation: 13484

open("filename", O_RDWR) is equivalent to fopen("filename", "w")

/dev/null - This is used to truncate any unwanted streams. Consider that you are running a command in terminal, which will print both stdout and stderr in the terminal. If you want to truncate errors, we can run the command like cmd 2>/dev/null. If you want to see only the compiler warnings during compilation, we can run like make 1> /dev/null.

Upvotes: 1

Duck
Duck

Reputation: 27572

it opens a device known as /dev/null that discards anything written to it. Basically a waste basket.

So the code is opening that device in read/write mode. If the open fails it calls a function die which prints the error literal you see along with the system error message returned from strerror(errno) and exits the program. The literal you see passed to die is probably just a format string for printf.

From man 3 printf:

m (Glibc extension.) Print output of strerror(errno). No argument is required.

There doesn't seem to be a glibc function called die(); it is presumably a function defined by the author of the code snippet, and it presumably exits the function after reporting the error. However, we can only make educated guesses.

Upvotes: 2

GrandMarquis
GrandMarquis

Reputation: 1913

You should look at man 2 open in a terminal window, or Google for it.

The open() function is used to open a file and assign a file descriptor. If open() fails, it returns a negative value.

Upvotes: 1

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