02-12-2010
Quote:
Originally Posted by
darshan.ghumare
Why contents of a.txt are "ABCDXYZEFGH"?
You write "ABCD" to it. Then you write "XYZ" to it. Then you write "EFGH" to it. "ABCD"+"XYZ"+"EFGH" = "ABCDXYZEFGH".
Quote:
More precisely, why kernel had not created separate entry in File Table for child process?
Because the child process didn't open it -- it inherited it. The behavior you've observed is the whole point. Every time you run a command in shell, you're using this property -- the command just writes to its inherited stdout and reads from its inherited stdin and needn't worry about its parent losing sync. This applies to any stream, not just terminals. If you really want them separate, open them separately.
Last edited by Corona688; 02-12-2010 at 02:53 PM..
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FORK(2) System Calls Manual FORK(2)
NAME
fork - create a new process
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
pid_t fork(void)
DESCRIPTION
Fork causes creation of a new process. The new process (child process) is an exact copy of the calling process except for the following:
The child process has a unique process ID.
The child process has a different parent process ID (i.e., the process ID of the parent process).
The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that,
for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the
child process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish
standard input and output for newly created processes as well as to set up pipes.
The child starts with no pending signals and an inactive alarm timer.
RETURN VALUE
Upon successful completion, fork returns a value of 0 to the child process and returns the process ID of the child process to the parent
process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
indicate the error.
ERRORS
Fork will fail and no child process will be created if one or more of the following are true:
[EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration-
dependent. (The kernel variable NR_PROCS in <minix/config.h> (Minix), or <minix/const.h> (Minix-vmd).)
[ENOMEM] There is insufficient (virtual) memory for the new process.
SEE ALSO
execve(2), wait(2).
3rd Berkeley Distribution May 22, 1986 FORK(2)