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Full Discussion: FORK/EXEC technique
Top Forums UNIX for Dummies Questions & Answers FORK/EXEC technique Post 302103464 by Perderabo on Thursday 18th of January 2007 05:36:53 PM
Old 01-18-2007
I don't really understand this question. But let's say you have a process running that is a shell. A user has entered the command "date". So no wthe shell would like to run the separate program called "date" and display the results to the user. So the shell will fork and exec the date program. Why? Because that is the only game in town. There is no other way for it to run another program. Either it can fork/exec and thus run the program or it can tell the user "sorry I don't know how to run programs".
 

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FORK(2) 							System Calls Manual							   FORK(2)

NAME
fork - spawn new process SYNOPSIS
fork( ) DESCRIPTION
Fork is the only way new processes are created. The new process's core image is a copy of that of the caller of fork. The only distinc- tion is the fact that the value returned in the old (parent) process contains the process ID of the new (child) process, while the value returned in the child is 0. Process ID's range from 1 to 30,000. This process ID is used by wait(2). Files open before the fork are shared, and have a common read-write pointer. In particular, this is the way that standard input and output files are passed and also how pipes are set up. SEE ALSO
wait(2), exec(2) DIAGNOSTICS
Returns -1 and fails to create a process if: there is inadequate swap space, the user is not super-user and has too many processes, or the system's process table is full. Only the super-user can take the last process-table slot. ASSEMBLER
(fork = 2.) sys fork (new process return) (old process return, new process ID in r0) The return locations in the old and new process differ by one word. The C-bit is set in the old process if a new process could not be cre- ated. FORK(2)
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