illumos: manual page: enable_extended_FILE

ENABLE_EXTENDED_FILE_STDIO(3C) Standard C Library Functions

NAME

enable_extended_FILE_stdio - enable extended FILE facility within
standard 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
unallocatable 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 signal_action
is -1, the default signal (SIGABRT) will be sent. If signal_action is 0,
no signal will be sent. Otherwise, the signal specified 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 standard 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(7) 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 character 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(7) for binary relief.

EXAMPLES

Example 1: Increase the file limit and enable the extended FILE facility.

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

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
application 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 system 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
applications. Although the extended FILE facility reports errors,
applications that directly reference FILE->_file should be updated to use
public 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
applications 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 behavior
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 perform
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.

October 22, 2014 ENABLE_EXTENDED_FILE_STDIO(3C)