System Call for touch

tolosenc

I'm sure many of you are familiar with the "touch" command. Essentially "touch program.c" creates a blank file called program.c in the current directory.

I've been playing around with system calls lately, and was wondering if there was one that did this? I've been operating off the following code in lieu:

int touch(char* fn)
{
    FILE *fp;
    fp = fopen(fn, "w");

    if(fp == NULL)
    {
        return -1;
    }

    fclose(fp);

    return 0;
}

djmarkus

Touch does this...

open("boom", O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
dup2(3, 0) = 0
close(3) = 0


So I assume if there was a system call that did it, touch would be using it

schrodinger

Which operating system...?

[-0-]

There's two ways of doing this:
1. use the open system call
2. use the creat system call

int touch(char* fn)
{
    int fd = open(fn, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);

    if(fd == -1) { return -1; }

    close(fd);

   return 0;
}

Since creat() is now considered obsolete, you should use open() for more control. The above open call will open new or existing files for reading and writing, truncating to zero bytes if the file already exists; file permissions read+write for owner, nothing for all others. So essentially if you do "touch bleh" and bleh already exists it will zero it out. Granted, touch doesn't behave like this (it simply won't let you touch a file if it already exists), but you can change the open flags for that functionality if you wish - just use O_EXCL and O_CREAT exclusively and it will error if the file exists.

This is applicable for the Linux kernel. I highly recommend the following book: http://www.amazon.com/Linux-Programming-Interface-System-Handbook/dp/1593272200

[-0-]

[-0-] wrote: »

There's two ways of doing this:
1. use the open system call
2. use the creat system call

int touch(char* fn)
{
    int fd = open(fn, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);

    if(fd == -1) { return -1; }

    close(fd);

   return 0;
}

Since creat() is now considered obsolete, you should use open() for more control. The above open call will open new or existing files for reading and writing, truncating to zero bytes if the file already exists; file permissions read+write for owner, nothing for all others. So essentially if you do "touch bleh" and bleh already exists it will zero it out. Granted, touch doesn't behave like this (it simply won't let you touch a file if it already exists), but you can change the open flags for that functionality if you wish - just use O_EXCL and O_CREAT exclusively and it will error if the file exists.

This is applicable for the Linux kernel. I highly recommend the following book: http://www.amazon.com/Linux-Programming-Interface-System-Handbook/dp/1593272200

This is the actual call touch uses:

fd = open (file, O_WRONLY | O_CREAT | O_NONBLOCK | O_NOCTTY,
		 S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);

Enjoy.

Tillotson

Use strace to show system calls:

$ strace touch test.txt 2>&1 | grep ^open | tail -n 1
open("test.txt", O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = 3
$

I'm guessing "S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH" is equlivant to 0666 in octal.