ENABLE_EXTENDED_FILE_STDIO(3C) Standard C Library Functions ENABLE_EXTENDED_FILE_STDIO(3C)

enable_extended_FILE_stdio - enable extended FILE facility within standard I/O

#include <stdio.h>
#include <stdio_ext.h>
#include <signal.h>
int enable_extended_FILE_stdio(int low_fd, int signal_action);

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.

Upon successful completion, enable_extended_FILE_stdio() returns 0. Otherwise, −1 is returned and errno is set to indicate the error.

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.

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.

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 */
}

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE
Interface Stability Evolving
MT-Level Safe

close(2), fcntl(2), getrlimit(2), fdopen(3C), fopen(3C), popen(3C), stdio(3C), signal.h(3HEAD), attributes(7), extendedFILE(7)

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:

does not directly access any fields in the FILE structure pointed to by the FILE pointer associated with any standard I/O stream,
checks all return values from standard I/O functions for error conditions, and
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 OmniOS