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Top Forums Programming Some questions regarding old if.c Post 302930427 by orbit on Saturday 3rd of January 2015 08:27:54 PM
Old 01-03-2015
Thank you ongoto and thank you achenle for your great explanation Smilie

I worked really hard on understanding the code and I nearly got everything.
This is the last part I do not understand:

Code:
if(eq(a, "{")) { /* execute a command for exit code */
    if(fork()) /*parent*/ wait(&ccode);
    else { /*child*/
        doex(1);
        goto err;
    }
    while((a=nxtarg()) && (!eq(a,"}")));
    return(ccode? 0 : 1);
}

As described in the man-page (if page from Section 1 of the unix-6th manual), if we put the command in brackets "if expr { command } ", we can obtain his exit code.

So we fork the current process, and then wait for our child process to finish? But where is our child process continuing his work? After the fork, we will go into the while-loop and and just skip some arguments and then return with ccode? Where was ccode changed? What is ccode?

Could you please explain me this the given code snippet? And elaborate on ccode?

The man page of wait: wait page from Section 2 of the unix-6th manual
The source code: [C] code - Pastebin.com

Thank you very much Smilie
 

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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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