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| |
| <h4 class="subsection">14.9.10 File Size</h4> |
| |
| <p>Normally file sizes are maintained automatically. A file begins with a |
| size of 0 and is automatically extended when data is written past |
| its end. It is also possible to empty a file completely by an |
| <code>open</code> or <code>fopen</code> call. |
| |
| <p>However, sometimes it is necessary to <em>reduce</em> the size of a file. |
| This can be done with the <code>truncate</code> and <code>ftruncate</code> functions. |
| They were introduced in BSD Unix. <code>ftruncate</code> was later added to |
| POSIX.1. |
| |
| <p>Some systems allow you to extend a file (creating holes) with these |
| functions. This is useful when using memory-mapped I/O |
| (see <a href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO">Memory-mapped I/O</a>), where files are not automatically extended. |
| However, it is not portable but must be implemented if <code>mmap</code> |
| allows mapping of files (i.e., <code>_POSIX_MAPPED_FILES</code> is defined). |
| |
| <p>Using these functions on anything other than a regular file gives |
| <em>undefined</em> results. On many systems, such a call will appear to |
| succeed, without actually accomplishing anything. |
| |
| <!-- unistd.h --> |
| <!-- X/Open --> |
| <div class="defun"> |
| — Function: int <b>truncate</b> (<var>const char *filename, off_t length</var>)<var><a name="index-truncate-1592"></a></var><br> |
| <blockquote> |
| <p>The <code>truncate</code> function changes the size of <var>filename</var> to |
| <var>length</var>. If <var>length</var> is shorter than the previous length, data |
| at the end will be lost. The file must be writable by the user to |
| perform this operation. |
| |
| <p>If <var>length</var> is longer, holes will be added to the end. However, some |
| systems do not support this feature and will leave the file unchanged. |
| |
| <p>When the source file is compiled with <code>_FILE_OFFSET_BITS == 64</code> the |
| <code>truncate</code> function is in fact <code>truncate64</code> and the type |
| <code>off_t</code> has 64 bits which makes it possible to handle files up to |
| 2^63 bytes in length. |
| |
| <p>The return value is 0 for success, or -1 for an error. In |
| addition to the usual file name errors, the following errors may occur: |
| |
| <dl> |
| <dt><code>EACCES</code><dd>The file is a directory or not writable. |
| |
| <br><dt><code>EINVAL</code><dd><var>length</var> is negative. |
| |
| <br><dt><code>EFBIG</code><dd>The operation would extend the file beyond the limits of the operating system. |
| |
| <br><dt><code>EIO</code><dd>A hardware I/O error occurred. |
| |
| <br><dt><code>EPERM</code><dd>The file is "append-only" or "immutable". |
| |
| <br><dt><code>EINTR</code><dd>The operation was interrupted by a signal. |
| |
| </dl> |
| |
| </blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Function: int <b>truncate64</b> (<var>const char *name, off64_t length</var>)<var><a name="index-truncate64-1593"></a></var><br> |
| <blockquote><p>This function is similar to the <code>truncate</code> function. The |
| difference is that the <var>length</var> argument is 64 bits wide even on 32 |
| bits machines, which allows the handling of files with sizes up to |
| 2^63 bytes. |
| |
| <p>When the source file is compiled with <code>_FILE_OFFSET_BITS == 64</code> on a |
| 32 bits machine this function is actually available under the name |
| <code>truncate</code> and so transparently replaces the 32 bits interface. |
| </p></blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- POSIX --> |
| <div class="defun"> |
| — Function: int <b>ftruncate</b> (<var>int fd, off_t length</var>)<var><a name="index-ftruncate-1594"></a></var><br> |
| <blockquote> |
| <p>This is like <code>truncate</code>, but it works on a file descriptor <var>fd</var> |
| for an opened file instead of a file name to identify the object. The |
| file must be opened for writing to successfully carry out the operation. |
| |
| <p>The POSIX standard leaves it implementation defined what happens if the |
| specified new <var>length</var> of the file is bigger than the original size. |
| The <code>ftruncate</code> function might simply leave the file alone and do |
| nothing or it can increase the size to the desired size. In this later |
| case the extended area should be zero-filled. So using <code>ftruncate</code> |
| is no reliable way to increase the file size but if it is possible it is |
| probably the fastest way. The function also operates on POSIX shared |
| memory segments if these are implemented by the system. |
| |
| <p><code>ftruncate</code> is especially useful in combination with <code>mmap</code>. |
| Since the mapped region must have a fixed size one cannot enlarge the |
| file by writing something beyond the last mapped page. Instead one has |
| to enlarge the file itself and then remap the file with the new size. |
| The example below shows how this works. |
| |
| <p>When the source file is compiled with <code>_FILE_OFFSET_BITS == 64</code> the |
| <code>ftruncate</code> function is in fact <code>ftruncate64</code> and the type |
| <code>off_t</code> has 64 bits which makes it possible to handle files up to |
| 2^63 bytes in length. |
| |
| <p>The return value is 0 for success, or -1 for an error. The |
| following errors may occur: |
| |
| <dl> |
| <dt><code>EBADF</code><dd><var>fd</var> does not correspond to an open file. |
| |
| <br><dt><code>EACCES</code><dd><var>fd</var> is a directory or not open for writing. |
| |
| <br><dt><code>EINVAL</code><dd><var>length</var> is negative. |
| |
| <br><dt><code>EFBIG</code><dd>The operation would extend the file beyond the limits of the operating system. |
| <!-- or the open() call - with the not-yet-discussed feature of opening --> |
| <!-- files with extra-large offsets. --> |
| |
| <br><dt><code>EIO</code><dd>A hardware I/O error occurred. |
| |
| <br><dt><code>EPERM</code><dd>The file is "append-only" or "immutable". |
| |
| <br><dt><code>EINTR</code><dd>The operation was interrupted by a signal. |
| |
| <!-- ENOENT is also possible on Linux - however it only occurs if the file --> |
| <!-- descriptor has a `file' structure but no `inode' structure. I'm not --> |
| <!-- sure how such an fd could be created. Perhaps it's a bug. --> |
| </dl> |
| |
| </blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- Unix98 --> |
| <div class="defun"> |
| — Function: int <b>ftruncate64</b> (<var>int id, off64_t length</var>)<var><a name="index-ftruncate64-1595"></a></var><br> |
| <blockquote><p>This function is similar to the <code>ftruncate</code> function. The |
| difference is that the <var>length</var> argument is 64 bits wide even on 32 |
| bits machines which allows the handling of files with sizes up to |
| 2^63 bytes. |
| |
| <p>When the source file is compiled with <code>_FILE_OFFSET_BITS == 64</code> on a |
| 32 bits machine this function is actually available under the name |
| <code>ftruncate</code> and so transparently replaces the 32 bits interface. |
| </p></blockquote></div> |
| |
| <p>As announced here is a little example of how to use <code>ftruncate</code> in |
| combination with <code>mmap</code>: |
| |
| <pre class="smallexample"> int fd; |
| void *start; |
| size_t len; |
| |
| int |
| add (off_t at, void *block, size_t size) |
| { |
| if (at + size > len) |
| { |
| /* Resize the file and remap. */ |
| size_t ps = sysconf (_SC_PAGESIZE); |
| size_t ns = (at + size + ps - 1) & ~(ps - 1); |
| void *np; |
| if (ftruncate (fd, ns) < 0) |
| return -1; |
| np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); |
| if (np == MAP_FAILED) |
| return -1; |
| start = np; |
| len = ns; |
| } |
| memcpy ((char *) start + at, block, size); |
| return 0; |
| } |
| </pre> |
| <p>The function <code>add</code> writes a block of memory at an arbitrary |
| position in the file. If the current size of the file is too small it |
| is extended. Note the it is extended by a round number of pages. This |
| is a requirement of <code>mmap</code>. The program has to keep track of the |
| real size, and when it has finished a final <code>ftruncate</code> call should |
| set the real size of the file. |
| |
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