Linux ftruncate函数

NAME       truncate, ftruncate - truncate a file to a specified lengthSYNOPSIS       #include <unistd.h>       #include <sys/types.h>       int truncate(const char *path, off_t length);       int ftruncate(int fd, off_t length);   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):       truncate():           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L       ftruncate():           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED           || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112LDESCRIPTION       The  truncate()  and ftruncate() functions cause the regular file named       by path or referenced by fd to be truncated  to  a  size  of  precisely       length bytes.       If  the  file  previously  was larger than this size, the extra data is       lost.  If the file previously was shorter,  it  is  extended,  and  the       extended part reads as null bytes ('\0').       The file offset is not changed.