GNU C Library (libc) Programming Guide

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14.9.10 File Size

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 open or fopen call.

However, sometimes it is necessary to reduce the size of a file. This can be done with the truncate and ftruncate functions. They were introduced in BSD Unix. ftruncate was later added to POSIX.1.

Some systems allow you to extend a file (creating holes) with these functions. This is useful when using memory-mapped I/O (see Memory-mapped I/O), where files are not automatically extended. However, it is not portable but must be implemented if mmap allows mapping of files (i.e., _POSIX_MAPPED_FILES is defined).

Using these functions on anything other than a regular file gives undefined results. On many systems, such a call will appear to succeed, without actually accomplishing anything.

— Function: int truncate (const char *filename, off_t length)

The truncate function changes the size of filename to length. If length 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.
If length is longer, holes will be added to the end. However, some systems do not support this feature and will leave the file unchanged.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the truncate function is in fact truncate64 and the type off_t has 64 bits which makes it possible to handle files up to 2^63 bytes in length.
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:

EACCES
The file is a directory or not writable.
EINVAL
length is negative.
EFBIG
The operation would extend the file beyond the limits of the operating system.
EIO
A hardware I/O error occurred.
EPERM
The file is "append-only" or "immutable".
EINTR
The operation was interrupted by a signal.

— Function: int truncate64 (const char *name, off64_t length)

This function is similar to the truncate function. The difference is that the length argument is 64 bits wide even on 32 bits machines, which allows the handling of files with sizes up to 2^63 bytes.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is actually available under the name truncate and so transparently replaces the 32 bits interface.

— Function: int ftruncate (int fd, off_t length)

This is like truncate, but it works on a file descriptor fd 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.
The POSIX standard leaves it implementation defined what happens if the specified new length of the file is bigger than the original size. The ftruncate 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 ftruncate 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.
ftruncate is especially useful in combination with mmap. 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.
When the source file is compiled with _FILE_OFFSET_BITS == 64 the ftruncate function is in fact ftruncate64 and the type off_t has 64 bits which makes it possible to handle files up to 2^63 bytes in length.
The return value is 0 for success, or -1 for an error. The following errors may occur:

EBADF
fd does not correspond to an open file.
EACCES
fd is a directory or not open for writing.
EINVAL
length is negative.
EFBIG
The operation would extend the file beyond the limits of the operating system.
EIO
A hardware I/O error occurred.
EPERM
The file is "append-only" or "immutable".
EINTR
The operation was interrupted by a signal.

— Function: int ftruncate64 (int id, off64_t length)

This function is similar to the ftruncate function. The difference is that the length argument is 64 bits wide even on 32 bits machines which allows the handling of files with sizes up to 2^63 bytes.
When the source file is compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is actually available under the name ftruncate and so transparently replaces the 32 bits interface.

As announced here is a little example of how to use ftruncate in combination with mmap:

     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;
     }

The function add 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 mmap. The program has to keep track of the real size, and when it has finished a final ftruncate call should set the real size of the file.