ENABLE_EXTENDED_FILE_STDIO(3C)ENABLE_EXTENDED_FILE_STDIO(3C)NAMEenable_extended_FILE_stdio - enable extended FILE facility within stan‐
dard I/O
SYNOPSIS
#include <stdio.h>
#include <stdio_ext.h>
#include <signal.h>
int enable_extended_FILE_stdio(int low_fd, int signal_action);
DESCRIPTION
The enable_extended_FILE_stdio() function enables the use of the
extended FILE facility (see NOTES) and determines which, if any, signal
will be sent when an application uses FILE->_file inappropriately.
The low_fd argument specifies the lowest file descriptor in the range 3
through 255 that the application wants to be selected as the unallocat‐
able file descriptor. File descriptors 0, 1, and 2 cannot be used
because they are reserved for use as the default file descriptors
underlying the stdin, stdout, and stderr standard I/O streams. The
low_fd argument can also be set to −1 to request that
enable_extended_FILE_stdio() select a "reasonable" unallocatable file
descriptor. In this case, enable_extended_FILE_stdio() will first
attempt to reserve a relatively large file descriptor, but will keep
trying to find an unallocatable file descriptor until it is known that
no file descriptor can be reserved.
The signal_action argument specifies the signal that will be sent to
the process when the unallocatable file descriptor is used as a file
descriptor argument to any system call except close(2). If sig‐
nal_action is −1, the default signal (SIGABRT) will be sent. If sig‐
nal_action is 0, no signal will be sent. Otherwise, the signal speci‐
fied by signal_action will be sent.
The enable_extended_FILE_stdio() function calls
unallocatablefd = fcntl(low_fd, F_BADFD, action);
to reserve the unallocatable file descriptor and set the signal to be
sent if the unallocatable file descriptor is used in a system call. If
the fcntl(2) call succeeds, the extended FILE facility is enabled and
the unallocatable file descriptor is saved for later use by the stan‐
dard I/O functions. When an attempt is made to open a standard I/O
stream (see fdopen(3C), fopen(3C), and popen(3C)) with an underlying
file descriptor greater than 255, the file descriptor is stored in an
auxiliary location and the field formerly known as FILE->_file is set
to the unallocatable file descriptor.
If the file descriptor limit for the process is less than or equal to
256 (the system default), the application needs to raise the limit (see
getrlimit(2)) for the extended FILE facility to be useful. The
enable_extended_FILE_stdio() function does not attempt to change the
file descriptor limit.
This function is used by the extendedFILE(5) preloadable library to
enable the extended FILE facility.
RETURN VALUES
Upon successful completion, enable_extended_FILE_stdio() returns 0.
Otherwise, −1 is returned and errno is set to indicate the error.
ERRORS
The enable_extended_FILE_stdio() function will fail if:
EAGAIN
All file descriptors in the inclusive range 3 through 255
refer to files that are currently open in the process.
EBADF
The low_fd argument is greater than 255, or is less than 3
and not equal to -1.
EEXIST
A file descriptor has already been marked by an earlier call
to fcntl().
EINVAL
The signal_action argument is not −1, is not 0, and is not a
valid signal number.
USAGE
The enable_extended_FILE_stdio() function is available only in the
32-bit compilation environment.
The fdopen(3C), fopen(3C), and popen(3C) functions all enable the use
of the extended FILE facility. For source changes, a trailing F charac‐
ter in the mode argument can be used with any of these functions if the
FILE *fptr is used only within the context of a single function or
group of functions and not meant to be returned to a caller. All of
the source code to the application must then be recompiled, thereby
exposing any improper usage of the FILE structure fields.
The F character must not be used if the FILE *fptr is to be returned to
a caller. The calling application might not understand how to process
it. Alternatively, the enable_extended_FILE_stdio() function can be
used at a higher level in the code.
Use extendedFILE(5) for binary relief.
EXAMPLES
Example 1 Increase the file limit and enable the extended FILE facil‐
ity.
The following example demonstrates how to programmatically increase the
file limit and enable extended FILE facility.
(void) getrlimit(RLIMIT_NOFILE, &rlp);
rlp.rlim_cur = 1000; /* set the desired number of file descriptors */
retval = setrlimit(RLIMIT_NOFILE, &lrp);
if (retval == -1) {
/* error */
}
/* enable extended FILE facility */
retval = enable_extended_FILE_stdio(-1, SIGABRT);
if (retval == -1) {
/* error */
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Evolving │
├────────────────────┼─────────────────┤
│MT-Level │ Safe │
└────────────────────┴─────────────────┘
SEE ALSOclose(2), fcntl(2), getrlimit(2), fdopen(3C), fopen(3C), popen(3C),
signal.h(3HEAD), stdio(3C), attributes(5), extendedFILE(5)NOTES
Historically, 32-bit Solaris applications have been limited to using
only the file descriptors 0 through 255 with the standard I/O functions
(see stdio(3C)) in the C library. The extended FILE facility allows
well-behaved 32-bit applications to use any valid file descriptor with
the standard I/O functions.
For the purposes of the extended FILE facility, a well-behaved applica‐
tion is one that:
o does not directly access any fields in the FILE structure
pointed to by the FILE pointer associated with any standard
I/O stream,
o checks all return values from standard I/O functions for
error conditions, and
o behaves appropriately when an error condition is reported.
The extended FILE facility generates EBADF error returns and optionally
delivers a signal to the calling process on most attempts to use the
file descriptor formerly stored in FILE->_file as an argument to a sys‐
tem call when a file descriptor value greater than 255 is being used to
access the file underlying the corresponding FILE pointer. The only
exception is that calls to the close() system call will return an EBADF
error in this case, but will not deliver the signal. The FILE->_file
has been renamed to help applications quickly detect code that needs to
be updated.
The extended FILE facility should only be used by well-behaved applica‐
tions. Although the extended FILE facility reports errors, applica‐
tions that directly reference FILE->_file should be updated to use pub‐
lic interfaces rather than rely on implementation details that no
longer work as the application expects (see __fbufsize(3C) and
fileno(3C).
This facility takes great care to avoid problems in well-behaved appli‐
cations while maintaining maximum compatibility. It also attempts to
catch dangerous behavior in applications that are not well-behaved as
soon as possible and to notify those applications as soon as bad behav‐
ior is detected.
There are, however, limitations. For example, if an application
enables this facility and is linked with an object file that had a
standard I/O stream using an extended FILE pointer, and then used the
sequence
(void) close(FILE->_file);
FILE->_file = myfd;
to attempt to change the file descriptor associated with the stream,
undesired results can occur. The close() function will fail, but since
this usage ignores the return status, the application proceeds to per‐
form low level I/O on FILE->_file while calls to standard I/O functions
would continue to use the original, extended FILE pointer. If the
application continues using standard I/O functions after changing
FILE->_file, silent data corruption could occur because the application
thinks it has changed file descriptors with the above assignment but
the actual standard I/O file descriptor is stored in the auxiliary
location. The chances for corruption are even higher if myfd has a
value greater than 255 and is truncated by the assignment to the 8-bit
_file field.
Since the_file field has been renamed, attempts to recompile this code
will fail. The application should be changed not to use this field in
the FILE structure.
The application should not use this facility if it uses _file directly,
including using the fileno() macro that was provided in stdio.h(3HEAD)
in Solaris 2.0 through 2.7.
Apr 18, 2006 ENABLE_EXTENDED_FILE_STDIO(3C)