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Programming
Is this a legal close-on-exec-move?
Post 302125865 by Perderabo on Sunday 8th of July 2007 01:30:30 PM
07-08-2007
Well, but if a process is running out of fd's, it will never be due to the use of fork(). fork() creates a new process and the new process has its own set of newly created fd's. But the number of fd's in the original process does not grow. And while exec() can indeed close a fd if the close-on-exec bit was set, exec() does not have an "open-on-exec" concept. On the other hand, those dup2() do not look right. And dup2() can indeed add an fd to a process. We do not see how STDIN_FILENO and STDOUT_FILENO are defined so we can't be sure of what is happening. The return code from dup2 is being discarded so we don't know if the calls worked or not. I'm worried that this code might effectively be:
dup2(0,0);
dup2(1,1);
And it's not real clear what that would do. In a call like dup2(first, second), the man page says that since the second fd is open, it must be closed first.
Or despite being all-caps, STDIN_FILENO and STDOUT_FILENO might actually be variables that get incremented somewhere. Also nowhere do I see anything about close-on-exec in the code... I don't understand why close-on-exec is even being mentioned here.
dup2(0,0);
dup2(1,1);
And it's not real clear what that would do. In a call like dup2(first, second), the man page says that since the second fd is open, it must be closed first.
Or despite being all-caps, STDIN_FILENO and STDOUT_FILENO might actually be variables that get incremented somewhere. Also nowhere do I see anything about close-on-exec in the code... I don't understand why close-on-exec is even being mentioned here.|
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LEARN ABOUT REDHAT
dup2
DUP(2) Linux Programmer's Manual DUP(2) NAME
dup, dup2 - duplicate a file descriptor SYNOPSIS
#include <unistd.h> int dup(int oldfd); int dup2(int oldfd, int newfd); DESCRIPTION
dup and dup2 create a copy of the file descriptor oldfd. After successful return of dup or dup2, the old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other. The two descriptors do not share the close-on-exec flag, however. dup uses the lowest-numbered unused descriptor for the new descriptor. dup2 makes newfd be the copy of oldfd, closing newfd first if necessary. RETURN VALUE
dup and dup2 return the new descriptor, or -1 if an error occurred (in which case, errno is set appropriately). ERRORS
EBADF oldfd isn't an open file descriptor, or newfd is out of the allowed range for file descriptors. EMFILE The process already has the maximum number of file descriptors open and tried to open a new one. WARNING
The error returned by dup2 is different to that returned by fcntl(..., F_DUPFD, ...) when newfd is out of range. On some systems dup2 also sometimes returns EINVAL like F_DUPFD. CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents additional EINTR and ENOLINK error conditions. POSIX.1 adds EINTR. SEE ALSO
fcntl(2), open(2), close(2) Linux 1.1.46 1994-08-21 DUP(2)