12-14-2008
pipe-fork-execve
Hi everyone , after a pipe() system call i've forked and entred into the child process to execve a shell script .the problem here is that when the execve sys call fail , i want to send the error code (eg errno) to the parent process using the pipe writer side p[1] , there is nothing received in the reader-side of the parent process , checkout:
int p[2];
pid_t pid;
pipe(p);
pid = fork();
if( pid < 1 ){
/* bla bla */
close(p[0]);
close(p[1]);
return ;
}
if( pid == 0 ){
close(p[0]);
fcntl(p[1],F_SETFD,0) ; /* do not close on exec */
execve("/bin/sh",arg,env);
int err = errno;
write(p[1],&err,sizeof(int)); /* why there is nothing received in my parent process */
_exit(127);
}
/* parent process */
close(p[1]);
if( read(p[0],&code,sizeof(int) ) < 0){
perror("nothing received");
/* this is true even if the execve() fail and there is something sent.. why */
}
Any one can help..thanks
Last edited by xtremejames183; 12-14-2008 at 07:51 PM..
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CLOSE(2) System Calls Manual CLOSE(2)
NAME
close - delete a descriptor
SYNOPSIS
#include <unistd.h>
int close(int d)
DESCRIPTION
The close call deletes a descriptor from the per-process object reference table. If this is the last reference to the underlying object,
then it will be deactivated. For example, on the last close of a file the current seek pointer associated with the file is lost; on the
last close of a TCP/IP descriptor associated naming information and queued data are discarded; on the last close of a file holding an advi-
sory lock the lock is released (see further fcntl(2)).
A close of all of a process's descriptors is automatic on exit, but since there is a limit on the number of active descriptors per process,
close is necessary for programs that deal with many descriptors.
When a process forks (see fork(2)), all descriptors for the new child process reference the same objects as they did in the parent before
the fork. If a new process is then to be run using execve(2), the process would normally inherit these descriptors. Most of the descrip-
tors can be rearranged with dup2(2) or deleted with close before the execve is attempted, but if some of these descriptors will still be
needed if the execve fails, it is necessary to arrange for them to be closed if the execve succeeds. For this reason, the call ``fcntl(d,
F_SETFD, flags)'' is provided, that can be used to mark a descriptor "close on exec" by setting the FD_CLOEXEC flag:
fcntl(d, F_SETFD, fcntl(d, F_GETFD) | FD_CLOEXEC);
RETURN VALUE
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and the global integer variable errno is set to
indicate the error.
ERRORS
Close will fail if:
[EBADF] D is not an active descriptor.
SEE ALSO
open(2), pipe(2), execve(2), fcntl(2).
4th Berkeley Distribution May 22, 1986 CLOSE(2)