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truncate, ftruncate — truncate or extend a file to a specified length
Standard C Library (libc, -lc)
#include <unistd.h> int truncate(const char *path, off_t length); int ftruncate(int fd, off_t length);
The truncate() system call causes the file named by path or referenced by fd to be truncated or extended to length bytes in size. If the file was larger than this size, the extra data is lost. If the file was smaller than this size, it will be extended as if by writing bytes with the value zero. The ftruncate() system call causes the file or shared memory object backing the file descriptor fd to be truncated or extended to length bytes in size. The file descriptor must be a valid file descriptor open for writing. The file position pointer associated with the file descriptor fd will not be modified.
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error. If the file to be modified is not a directory or a regular file, the truncate() call has no effect and returns the value 0.
The truncate() system call succeeds unless: [ENOTDIR] A component of the path prefix is not a directory. [ENAMETOOLONG] A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters. [ENOENT] The named file does not exist. [EACCES] Search permission is denied for a component of the path prefix. [EACCES] The named file is not writable by the user. [ELOOP] Too many symbolic links were encountered in translating the pathname. [EPERM] The named file has its immutable or append-only flag set, see the chflags(2) manual page for more information. [EISDIR] The named file is a directory. [EROFS] The named file resides on a read-only file system. [ETXTBSY] The file is a pure procedure (shared text) file that is being executed. [EFBIG] The length argument was greater than the maximum file size. [EINVAL] The length argument was less than 0. [EIO] An I/O error occurred updating the inode. [EFAULT] The path argument points outside the process's allocated address space. The ftruncate() system call succeeds unless: [EBADF] The fd argument is not a valid descriptor. [EINVAL] The fd argument references a file descriptor that is not a regular file or shared memory object. [EINVAL] The fd descriptor is not open for writing.
The truncate() and ftruncate() system calls appeared in 4.2BSD.
These calls should be generalized to allow ranges of bytes in a file to be discarded. Use of truncate() to extend a file is not portable.