Another, different open() returns 0 - gcc

Question

In searching the site, I found two other similar (at first glance) questions by users 2253605
and 2135159. I also have tried two different versions of gcc. This started out as a hard to
track problem in an application to keep various forms of data in sync on different media. I
eventually boiled it down to a few lines of code that illustrate the problem.
This one is very defined and puzzling. I have not been able to find a case where my system opens
a file, and returns a non-zero file descriptor. It sometimes does really open the specified file
and allows a subsequent read() to occur without error. But by the third open() the
subsequent read() fails, specifying an invalid argument, which can only be a zero value file
descriptor.
The code below tries to open 5 different files, 4 files exist and one that does not exist.
The first 4 opens all return a file descriptor value of zero (stdin).
stdin is not closed, a read() before the first open() or after any one of these open() calls,
will hang until enter is pressed.
Even if stdin were closed, zero should only be returned for the first open(). The
file descriptors are being set and when the open() for the non-existent file is attempted,
it returns an error.
I can't believe that gcc can't open a file. I think I have some kind of O/S-compiler configuration
issue (lib) or maybe I can't see the forest for the trees.
This is on ubuntu 12.04 LTS 64 bit and 64 bit gcc-4.6 also on gcc-4.7 . The flash drive is
formatted ext4. x86_64 intel processor. The installation commands used for gccc-4.7 on 2/10/16
are also shown below. Both gcc-4.6 and gcc-4.7 give identical results. The makefile
is at the end.
Anybody know what's happening here?
The terminal output is shown below the code.
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h
#include <errno.h>
int main()
{
long ret_val;
char cpy_buf[4096];
char s_ary[20][80] =
{
"/media/FLASH16GB_2/Test_dir/t_dir/dm1", //0 exists
"/media/FLASH16GB_2/Test_dir/t_dir/dm2", //1 exists
"/media/FLASH16GB_2/Test_dir/t_dir/dm3", //2 exists
"/media/FLASH16GB_2/Test_dir/t_dir/dm4", //3 exists
"/media/FLASH16GB_2/Test_dir/t_dir/dm5", //4 does not exist
};
char *s1;
long s_fh_1, s_fh_2, s_fh_3, s_fh_4, s_fh_5;
s_fh_1 = 10000;
s1 = &s_ary[0][0];
if (s_fh_1 = open( s1 , O_RDONLY) < 0) // &s_ary[0][0]
{
printf("Error opening source file, name=%s, line# = %i, errno = %i \n",&s_ary[0][0], __LINE__ , errno);
return -1;
}
if (s_fh_2 = open( &s_ary[1][0], O_RDONLY) < 0)
{
printf("Error opening source file, name=%s, line# = %i, errno = %i \n",&s_ary[1][0], __LINE__ , errno);
return -1;
}
if (s_fh_3 = open( &s_ary[2][0], O_RDONLY,0) < 0)
{
printf("Error opening source file, name=%s, line# = %i, errno = %i \n",&s_ary[2][0], __LINE__ , errno);
return -1;
}
if (s_fh_4 = open( &s_ary[3][0], O_RDONLY,0) < 0)
{
printf("Error opening source file, name=%s, line# = %i, errno = %i \n",&s_ary[3][0], __LINE__ , errno);
return -1;
}
printf("s_fh_1 = %li, s_fh_2 = %li, s_fh_3 = %li, s_fh_4 = %li \n", s_fh_1, s_fh_2, s_fh_3, s_fh_4);
if (s_fh_5 = open( &s_ary[4][0], O_RDONLY,0) < 0)
{
printf("Error opening source file, name=%s, line# = %i, errno = %i \n",&s_ary[4][0], __LINE__ , errno);
return -1;
}
return 0;
}
terminal output:
$ make
gcc -g -c -std=iso9899:1999 -o obj/bug_tst_sync_m.o bug_tst_sync_m.c -I../include
gcc -o bug_tst_sync_m obj/bug_tst_sync_m.o -I../include -L /usr/lib64/X11 -lX11 -lm
$ ./bug_tst_sync_m
s_fh_1 = 0, s_fh_2 = 0, s_fh_3 = 0, s_fh_4 = 0
Error opening source file, name=/media/FLASH16GB_2/Test_dir/t_dir/dm5, line# = 88, errno = 2
$
$
gcc-4.7 installation commands used on 2_10_16.
update-alternatives --display gcc
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-4.7
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.7 60
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.6 40
sudo update-alternatives --config gcc
makefile
### where to look for include files ( locally and globally ? -I /usr/include/X11)
IDIR =../include
### compiler to runand generate debugging info (no -g for production release code)
CC=gcc -g
### list of dependencies
CFLAGS=-I$(IDIR)
### where to put object modules
ODIR=obj
### where to look for local library files locally (or write?)
LDIR = -L /usr/lib64/X11 -lX11
### libraries to include m=-lm includes the math libarary, math lib = -lm
LIBS=-lm
### list of all dependency files (.h files)
_DEPS = queues.h InterlockedExchange.h
DEPS = $(patsubst %,$(IDIR)/%,$(_DEPS))
### list of all object files
_OBJ = bug_tst_sync_m.o
OBJ = $(patsubst %,$(ODIR)/%,$(_OBJ))
### compiles object modules and produces debug info
$(ODIR)/%.o: %.c $(DEPS)
$(CC) -c -std=iso9899:1999 -o $# $< $(CFLAGS)
### left side of colon is executable name
### this line links objects and creates the executable
bug_tst_sync_m: $(OBJ)
gcc -o $# $^ $(CFLAGS) $(LDIR) $(LIBS)
### this gets run if you type "make clean". it deletes source backup and object files.
### run this then next make does everything. Without this you get situations that
.PHONY: clean
clean:
rm -f $(ODIR)/*.o *~ core $(INCDIR)/*~

Answer 1

Your problem is operator precedence, < binds harder than =;
if ( s_fh_1 = open(s1 , O_RDONLY) < 0 )
becomes
if ( s_fh_1 = ( open(s1 , O_RDONLY) < 0 ) )
which means, if open returns a number greater than or equal to zero, s_fh_1 will be 0.