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Diffstat (limited to 'manual/llio.texi')
| -rw-r--r-- | manual/llio.texi | 429 |
1 files changed, 414 insertions, 15 deletions
diff --git a/manual/llio.texi b/manual/llio.texi index 459032e..cf3e1a7 100644 --- a/manual/llio.texi +++ b/manual/llio.texi @@ -41,6 +41,8 @@ directly.) or vice-versa. * Stream/Descriptor Precautions:: Precautions needed if you use both descriptors and streams. +* Scatter-Gather:: Fast I/O to discontinous buffers. +* Memory-mapped I/O:: Using files like memory. * Waiting for I/O:: How to check for input or output on multiple file descriptors. * Synchronizing I/O:: Making sure all I/O actions completed. @@ -58,6 +60,7 @@ directly.) file locking. * Interrupt Input:: Getting an asynchronous signal when input arrives. +* IOCTLs:: Generic I/O Control operations. @end menu @@ -88,7 +91,7 @@ parameters (using the @samp{|} operator in C). @xref{File Status Flags}, for the parameters available. The normal return value from @code{open} is a non-negative integer file -descriptor. In the case of an error, a value of @code{-1} is returned +descriptor. In the case of an error, a value of @math{-1} is returned instead. In addition to the usual file name errors (@pxref{File Name Errors}), the following @code{errno} error conditions are defined for this function: @@ -240,7 +243,7 @@ until the program ends. To avoid this calls to @code{close} should be protected using cancelation handlers. @c ref pthread_cleanup_push / pthread_cleanup_pop -The normal return value from @code{close} is @code{0}; a value of @code{-1} +The normal return value from @code{close} is @math{0}; a value of @math{-1} is returned in case of failure. The following @code{errno} error conditions are defined for this function: @@ -422,7 +425,7 @@ If @code{read} returns at least one character, there is no way you can tell whether end-of-file was reached. But if you did reach the end, the next read will return zero. -In case of an error, @code{read} returns @code{-1}. The following +In case of an error, @code{read} returns @math{-1}. The following @code{errno} error conditions are defined for this function: @table @code @@ -564,7 +567,7 @@ is therefore faster. You can use the @code{O_FSYNC} open mode to make @code{write} always store the data to disk before returning; @pxref{Operating Modes}. -In the case of an error, @code{write} returns @code{-1}. The following +In the case of an error, @code{write} returns @math{-1}. The following @code{errno} error conditions are defined for this function: @table @code @@ -761,7 +764,7 @@ file takes up less space than it appears so; it is then called a @cindex holes in files If the file position cannot be changed, or the operation is in some way -invalid, @code{lseek} returns a value of @code{-1}. The following +invalid, @code{lseek} returns a value of @math{-1}. The following @code{errno} error conditions are defined for this function: @table @code @@ -944,7 +947,7 @@ see @ref{Creating a Pipe}. This function returns the file descriptor associated with the stream @var{stream}. If an error is detected (for example, if the @var{stream} is not valid) or if @var{stream} does not do I/O to a file, -@code{fileno} returns @code{-1}. +@code{fileno} returns @math{-1}. @end deftypefun @cindex standard file descriptors @@ -1122,6 +1125,341 @@ terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. @xref{Terminal Modes}. +@node Scatter-Gather +@section Fast Scatter-Gather I/O +@cindex scatter-gather + +Some applications may need to read or write data to multiple buffers, +which are seperated in memory. Although this can be done easily enough +with multiple calls to @code{read} and @code{write}, it is inefficent +because there is overhead associated with each kernel call. + +Instead, many platforms provide special high-speed primitives to perform +these @dfn{scatter-gather} operations in a single kernel call. The GNU C +library will provide an emulation on any system that lacks these +primitives, so they are not a portability threat. They are defined in +@code{sys/uio.h}. + +These functions are controlled with arrays of @code{iovec} structures, +which describe the location and size of each buffer. + +@deftp {Data Type} {struct iovec} + +The @code{iovec} structure describes a buffer. It contains two fields: + +@table @code + +@item void *iov_base +Contains the address of a buffer. + +@item size_t iov_len +Contains the length of the buffer. + +@end table +@end deftp + +@deftypefun ssize_t readv (int @var{filedes}, const struct iovec *@var{vector}, int @var{count}) + +The @code{readv} function reads data from @var{filedes} and scatters it +into the buffers described in @var{vector}, which is taken to be +@var{count} structures long. As each buffer is filled, data is sent to the +next. + +Note that @code{readv} is not guaranteed to fill all the buffers. +It may stop at any point, for the same reasons @code{read} would. + +The return value is a count of bytes (@emph{not} buffers) read, @math{0} +indicating end-of-file, or @math{-1} indicating an error. The possible +errors are the same as in @code{read}. + +@end deftypefun + +@deftypefun ssize_t writev (int @var{filedes}, const struct iovec *@var{vector}, int @var{count}) + +The @code{writev} function gathers data from the buffers described in +@var{vector}, which is taken to be @var{count} structures long, and writes +them to @code{filedes}. As each buffer is written, it moves on to the +next. + +Like @code{readv}, @code{writev} may stop midstream under the same +conditions @code{write} would. + +The return value is a count of bytes written, or @math{-1} indicating an +error. The possible errors are the same as in @code{write}. + +@end deftypefun + +@c Note - I haven't read this anywhere. I surmised it from my knowledge +@c of computer science. Thus, there could be subtleties I'm missing. + +Note that if the buffers are small (under about 1kB), high-level streams +may be easier to use than these functions. However, @code{readv} and +@code{writev} are more efficient when the individual buffers themselves +(as opposed to the total output), are large. In that case, a high-level +stream would not be able to cache the data effectively. + +@node Memory-mapped I/O +@section Memory-mapped I/O + +On modern operating systems, it is possible to @dfn{mmap} (pronounced +``em-map'') a file to a region of memory. When this is done, the file can +be accessed just like an array in the program. + +This is more efficent than @code{read} or @code{write}, as only regions +of the file a program actually accesses are loaded. Accesses to +not-yet-loaded parts of the mmapped region are handled in the same way as +swapped out pages. + +Since mmapped pages can be stored back to their file when physical memory +is low, it is possible to mmap files orders of magnitude larger than both +the physical memory @emph{and} swap space. The only limit is address +space. The theoretical limit is 4GB on a 32-bit machine - however, the +actual limit will be smaller since some areas will be reserved for other +purposes. + +Memory mapping only works on entire pages of memory. Thus, addresses +for mapping must be page-aligned, and length values will be rounded up. +To determine the size of a page the machine uses one should use + +@smallexample +size_t page_size = (size_t) sysconf (_SC_PAGESIZE); +@end smallexample + +These functions are declared in @file{sys/mman.h}. + +@deftypefun {void *} mmap (void *@var{address}, size_t @var{length},int @var{protect}, int @var{flags}, int @var{filedes}, off_t @var{offset}) + +The @code{mmap} function creates a new mapping, connected to bytes +(@var{offset}) to (@var{offset} + @var{length}) in the file open on +@var{filedes}. + +@var{address} gives a preferred starting address for the mapping. +@code{NULL} expresses no preference. Any previous mapping at that +address is automatically removed. The address you give may still be +changed, unless you use the @code{MAP_FIXED} flag. + +@vindex PROT_READ +@vindex PROT_WRITE +@vindex PROT_EXEC +@var{protect} contains flags that control what kind of access is +permitted. They include @code{PROT_READ}, @code{PROT_WRITE}, and +@code{PROT_EXEC}, which permit reading, writing, and execution, +respectively. Inappropriate access will cause a segfault (@pxref{Program +Error Signals}). + +Note that most hardware designs cannot support write permission without +read permission, and many do not distinguish read and execute permission. +Thus, you may recieve wider permissions than you ask for, and mappings of +write-only files may be denied even if you do not use @code{PROT_READ}. + +@var{flags} contains flags that control the nature of the map. +One of @code{MAP_SHARED} or @code{MAP_PRIVATE} must be specified. + +They include: + +@vtable @code +@item MAP_PRIVATE +This specifies that writes to the region should never be written back +to the attached file. Instead, a copy is made for the process, and the +region will be swapped normally if memory runs low. No other process will +see the changes. + +Since private mappings effectively revert to ordinary memory +when written to, you must have enough virtual memory for a copy of +the entire mmapped region if you use this mode with @code{PROT_WRITE}. + +@item MAP_SHARED +This specifies that writes to the region will be written back to the +file. Changes made will be shared immediately with other processes +mmaping the same file. + +Note that actual writing may take place at any time. You need to use +@code{msync}, described below, if it is important that other processes +using conventional I/O get a consistent view of the file. + +@item MAP_FIXED +This forces the system to use the exact mapping address specified in +@var{address} and fail if it can't. + +@c One of these is official - the other is obviously an obsolete synonym +@c Which is which? +@item MAP_ANONYMOUS +@itemx MAP_ANON +This flag tells the system to create an anonymous mapping, not connected +to a file. @var{filedes} and @var{off} are ignored, and the region is +initialized with zeros. + +Anonymous maps are used as the basic primitive to extend the heap on some +systems. They are also useful to share data between multiple tasks +without creating a file. + +On some systems using private anonymous mmaps is more efficent than using +@code{malloc} for large blocks. This is not an issue with the GNU C library, +as the included @code{malloc} automatically uses @code{mmap} where appropriate. + +@c Linux has some other MAP_ options, which I have not discussed here. +@c MAP_DENYWRITE, MAP_EXECUTABLE and MAP_GROWSDOWN don't seem applicable to +@c user programs (and I don't understand the last two). MAP_LOCKED does +@c not appear to be implemented. + +@end vtable + +@code{mmap} returns the address of the new mapping, or @math{-1} for an +error. + +Possible errors include: + +@table @code + +@item EINVAL + +Either @var{address} was unusable, or inconsistent @var{flags} were +given. + +@item EACCES + +@var{filedes} was not open for the type of access specified in @var{protect}. + +@item ENOMEM + +Either there is not enough memory for the operation, or the process is +out of address space. + +@item ENODEV + +This file is of a type that doesn't support mapping. + +@item ENOEXEC + +The file is on a filesystem that doesn't support mapping. + +@c On Linux, EAGAIN will appear if the file has a conflicting mandatory lock. +@c However mandatory locks are not discussed in this manual. +@c +@c Similarly, ETXTBSY will occur if the MAP_DENYWRITE flag (not documented +@c here) is used and the file is already open for writing. + +@end table + +@end deftypefun + +@deftypefun int munmap (void *@var{addr}, size_t @var{length}) + +@code{munmap} removes any memory maps from (@var{addr}) to (@var{addr} + +@var{length}). @var{length} should be the length of the mapping. + +It is safe to un-map multiple mappings in one command, or include unmapped +space in the range. It is also possible to unmap only part of an existing +mapping, however only entire pages can be removed. If @var{length} is not +an even number of pages, it will be rounded up. + +It returns @math{0} for success and @math{-1} for an error. + +One error is possible: + +@table @code + +@item EINVAL +The memory range given was outside the user mmap range, or wasn't page +aligned. + +@end table + +@end deftypefun + +@deftypefun int msync (void *@var{address}, size_t @var{length}, int @var{flags}) + +When using shared mappings, the kernel can write the file at any time +before the mapping is removed. To be certain data has actually been +written to the file and will be accessable to non-memory-mapped I/O, it +is neccessary to use this function. + +It operates on the region @var{address} to (@var{address} + @var{length}). +It may be used on part of a mapping or multiple mappings, however the +region given should not contain any unmapped space. + +@var{flags} can contain some options: + +@vtable @code + +@item MS_SYNC + +This flag makes sure the data is actually written @emph{to disk}. +Normally @code{msync} only makes sure that accesses to a file with +conventional I/O reflect the recent changes. + +@item MS_ASYNC + +This tells @code{msync} to begin the synchronization, but not to wait for +it to complete. + +@c Linux also has MS_INVALIDATE, which I don't understand. + +@end vtable + +@code{msync} returns @math{0} for success and @math{-1} for +error. Errors include: + +@table @code + +@item EINVAL +An invalid region was given, or the @var{flags} were invalid. + +@item EFAULT +There is no existing mapping in at least part of the given region. + +@end table + +@end deftypefun + +@deftypefun {void *} mremap (void *@var{address}, size_t @var{length}, size_t @var{new_length}, int @var{flag}) + +This function can be used to change the size of an existing memory +area. @var{address} and @var{length} must cover a region entirely mapped +in the same @code{mmap} statement. A new mapping with the same +characteristics will be returned, but a with the length @var{new_length} +instead. + +One option is possible, @code{MREMAP_MAYMOVE}. If it is given in +@var{flags}, the system may remove the existing mapping and create a new +one of the desired length in another location. + +The address of the resulting mapping is returned, or @math{-1}. Possible +error codes include: + +This function is only available on a few systems. Except for performing +optional optimizations one should not rely on this function. +@table @code + +@item EFAULT +There is no existing mapping in at least part of the original region, or +the region covers two or more distinct mappings. + +@item EINVAL +The address given is misaligned or inappropriate. + +@item EAGAIN +The region has pages locked, and if extended it would exceed the +process's resource limit for locked pages. @xref{Limits on Resources}. + +@item ENOMEM +The region is private writable, and insufficent virtual memory is +available to extend it. Also, this error will occur if +@code{MREMAP_MAYMOVE} is not given and the extension would collide with +another mapped region. + +@end table +@end deftypefun + +Not all file descriptors may be mapped. Sockets, pipes, and most devices +only allow sequential access and do not fit into the mapping abstraction. +In addition, some regular files may not be mmapable, and older kernels may +not support mapping at all. Thus, programs using @code{mmap} should +have a fallback method to use should it fail. @xref{Mmap,,,standards,GNU +Coding Standards}. + +@c XXX madvice documentation missing + @node Waiting for I/O @section Waiting for Input or Output @cindex waiting for input or output @@ -2336,7 +2674,7 @@ the file descriptor returned should be the next available one greater than or equal to this value. The return value from @code{fcntl} with this command is normally the value -of the new file descriptor. A return value of @code{-1} indicates an +of the new file descriptor. A return value of @math{-1} indicates an error. The following @code{errno} error conditions are defined for this command: @@ -2420,7 +2758,7 @@ The normal return value from @code{fcntl} with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags (except that currently there is only one flag to use). -In case of an error, @code{fcntl} returns @code{-1}. The following +In case of an error, @code{fcntl} returns @math{-1}. The following @code{errno} error conditions are defined for this command: @table @code @@ -2443,7 +2781,7 @@ fcntl (@var{filedes}, F_SETFD, @var{new-flags}) @end smallexample The normal return value from @code{fcntl} with this command is an -unspecified value other than @code{-1}, which indicates an error. +unspecified value other than @math{-1}, which indicates an error. The flags and error conditions are the same as for the @code{F_GETFD} command. @end deftypevr @@ -2848,7 +3186,7 @@ individual flags. Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with @code{O_ACCMODE} to compare them. -In case of an error, @code{fcntl} returns @code{-1}. The following +In case of an error, @code{fcntl} returns @math{-1}. The following @code{errno} error conditions are defined for this command: @table @code @@ -2873,7 +3211,7 @@ You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing. The normal return value from @code{fcntl} with this command is an -unspecified value other than @code{-1}, which indicates an error. The +unspecified value other than @math{-1}, which indicates an error. The error conditions are the same as for the @code{F_GETFL} command. @end deftypevr @@ -3012,7 +3350,7 @@ If no lock applies, the only change to the @var{lockp} structure is to update the @code{l_type} to a value of @code{F_UNLCK}. The normal return value from @code{fcntl} with this command is an -unspecified value other than @code{-1}, which is reserved to indicate an +unspecified value other than @math{-1}, which is reserved to indicate an error. The following @code{errno} error conditions are defined for this command: @@ -3043,9 +3381,9 @@ on that part is replaced with the new lock. You can remove a lock by specifying a lock type of @code{F_UNLCK}. If the lock cannot be set, @code{fcntl} returns immediately with a value -of @code{-1}. This function does not block waiting for other processes +of @math{-1}. This function does not block waiting for other processes to release locks. If @code{fcntl} succeeds, it return a value other -than @code{-1}. +than @math{-1}. The following @code{errno} error conditions are defined for this function: @@ -3213,7 +3551,7 @@ fcntl (@var{filedes}, F_SETOWN, @var{pid}) The @var{pid} argument should be a process ID. You can also pass a negative number whose absolute value is a process group ID. -The return value from @code{fcntl} with this command is @code{-1} +The return value from @code{fcntl} with this command is @math{-1} in case of error and some other value if successful. The following @code{errno} error conditions are defined for this command: @@ -3227,3 +3565,64 @@ There is no process or process group corresponding to @var{pid}. @end deftypevr @c ??? This section could use an example program. + +@node IOCTLs +@section Generic I/O Control operations +@cindex generic i/o control operations +@cindex IOCTLs + +The GNU system can handle most input/output operations on many different +devices and objects in terms of a few file primitives - @code{read}, +@code{write} and @code{lseek}. However, most devices also have a few +peculiar operations which do not fit into this model. Such as: + +@itemize @bullet + +@item +Changing the character font used on a terminal. + +@item +Telling a magnetic tape system to rewind or fast forward. (Since they +cannot move in byte increments, @code{lseek} is inapplicable). + +@item +Ejecting a disk from a drive. + +@item +Playing an audio track from a CD-ROM drive. + +@item +Maintaining routing tables for a network. + +@end itemize + +Although some such objects such as sockets and terminals +@footnote{Actually, the terminal-specific functions are implemented with +IOCTLs on many platforms.} have special functions of their own, it would +not be practical to create functions for all these cases. + +Instead these minor operations, known as @dfn{IOCTL}s, are assigned code +numbers and multiplexed through the @code{ioctl} function, defined in +@code{sys/ioctl.h}. The code numbers themselves are defined in many +different headers. + +@deftypefun int ioctl (int @var{filedes}, int @var{command}, @dots{}) + +The @code{ioctl} function performs the generic I/O operation +@var{command} on @var{filedes}. + +A third argument is usually present, either a single number or a pointer +to a structure. The meaning of this argument, the returned value, and +any error codes depends upon the command used. Often @math{-1} is +returned for a failure. + +@end deftypefun + +On some systems, IOCTLs used by different devices share the same numbers. +Thus, although use of an inappropriate IOCTL @emph{usually} only produces +an error, you should not attempt to use device-specific IOCTLs on an +unknown device. + +Most IOCTLs are OS-specific and/or only used in special system utilities, +and are thus beyond the scope of this document. For an example of the use +of an IOCTL, @xref{Out-of-Band Data}. |