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Very simple Unix Prog in C, could use advice/help
Post 17210 by AtleRamsli on Tuesday 12th of March 2002 07:55:31 AM
03-12-2002
Excellent comment about the race condition!
One thing puzzles me, this code looks C++-ish?
Are // accepted as C comments, now?
Anyway, what I wanted to add, was that even though there is no _syntactical_ distinction between 'Procedure' and 'Function' in C, the _semantic_ one remains! So, unless a function returns void (the Function is not a "Function" but a "Procedure"
) its return value must always be checked.
This is why Perderabo tells you that 'it's ok to check first, but you must check after'
I have a 'standard' way for dealing with opening/creation.
/*
Try to open, if that fails, _try_to_ create.
If create was successful, close.
Now, we have done what we can to make sure
we can open the file, so now try to open
and return the result.
*/
myopen(...)
{
if (fd = open(...) == -1) {
if (fd = creat(...) != -1)
close(fd);
fd = open(...);
}
return(fd);
}
It all depends on what you define to be an 'error'.
One thing puzzles me, this code looks C++-ish?
Are // accepted as C comments, now?
Anyway, what I wanted to add, was that even though there is no _syntactical_ distinction between 'Procedure' and 'Function' in C, the _semantic_ one remains! So, unless a function returns void (the Function is not a "Function" but a "Procedure"
) its return value must always be checked.This is why Perderabo tells you that 'it's ok to check first, but you must check after'
I have a 'standard' way for dealing with opening/creation.
/*
Try to open, if that fails, _try_to_ create.
If create was successful, close.
Now, we have done what we can to make sure
we can open the file, so now try to open
and return the result.
*/
myopen(...)
{
if (fd = open(...) == -1) {
if (fd = creat(...) != -1)
close(fd);
fd = open(...);
}
return(fd);
}
It all depends on what you define to be an 'error'.
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LEARN ABOUT HPUX
dup2
dup2(2) System Calls Manual dup2(2) NAME
dup2 - duplicate an open file descriptor to a specific slot SYNOPSIS
DESCRIPTION
fildes is a file descriptor obtained from a or system call. fildes2 is a non-negative integer less than the maximum value allowed for file descriptors. causes fildes2 to refer to the same file as fildes. If fildes2 refers to an already open file, the open file is closed first. The file descriptor returned by has the following in common with fildes: o Same open file (or pipe). o Same file pointer (that is, both file descriptors share one file pointer.) o Same access mode (read, write or read/write). o Same file status flags (see fcntl(2), The new file descriptor is set to remain open across system calls. See fcntl(2). This routine is found in the C library. Programs using but not using other routines from the Berkeley importability library (such as the routines described in bsdproc(3C)) should not give the option to ld(1). RETURN VALUE
Upon successful completion, returns the new file descriptor as a non-negative integer, fildes2. Otherwise, it returns -1 and sets to indi- cate the error. ERRORS
fails if the following is true: fildes is not a valid open file descriptor or fildes2 is not in the range of legal file descriptors. An attempt to close fildes2 was interrupted by a signal. The file is still open. WARNINGS
A multithreaded application or an application with a signal handler can exhibit a race between on one thread and another kernel call that assigns a new file descriptor while running on a second thread or signal handler. If fildes2 is free before the call was made, the other thread or signal handler may win the race and acquire that descriptor (e.g., in an call). The thread calling can then close this file and reuse the descriptor. This results in multiple functions improperly referring to the same file. This race can be avoided either by ensuring that fildes2 references an open file before calling or by providing user-level synchronization (or signal disabling) which makes sure that the thread and another thread or signal handler don't make competing calls into the kernel at the same time. SEE ALSO
close(2), creat(2), dup(2), exec(2), fcntl(2), open(2), pipe(2), privileges(5). STANDARDS CONFORMANCE
dup2(2)