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PREAD(2)                   Linux Programmer's ManualPREAD(2)NAME       pread, pwrite - read from or write to a file descriptor at a given off-       setSYNOPSIS       #include <unistd.h>       ssize_t pread(int fd, void *buf, size_t count, off_t offset);       ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):       pread(), pwrite():           _XOPEN_SOURCE >= 500           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809LDESCRIPTION       pread() reads up to count bytes from file descriptor fd at offset  off-       set  (from the start of the file) into the buffer starting at buf.  The       file offset is not changed.       pwrite() writes up to count bytes from the buffer starting  at  buf  to       the  file  descriptor  fd  at  offset  offset.   The file offset is not       changed.       The file referenced by fd must be capable of seeking.RETURN VALUE       On success, pread() returns the number of bytes read (a return of  zero       indicates  end  of file) and pwrite() returns the number of bytes writ-       ten.       Note that it is not an error for a successful call  to  transfer  fewer       bytes than requested (seeread(2) andwrite(2)).       On  error, -1 is returned and errno is set to indicate the cause of the       error.ERRORS       pread() can fail and set errno to any error specified  forread(2)  orlseek(2).   pwrite()  can fail and set errno to any error specified forwrite(2) orlseek(2).VERSIONS       The pread() and pwrite() system calls were added to  Linux  in  version       2.1.60; the entries in the i386 system call table were added in 2.1.69.       C library support (including emulation usinglseek(2) on older  kernels       without the system calls) was added in glibc 2.1.CONFORMING TO       POSIX.1-2001, POSIX.1-2008.NOTES       The  pread()  and pwrite() system calls are especially useful in multi-       threaded applications.  They allow multiple threads to perform  I/O  on       the  same file descriptor without being affected by changes to the file       offset by other threads.   C library/kernel differences       On Linux, the underlying system  calls  were  renamed  in  kernel  2.6:       pread()  became  pread64(), and pwrite() became pwrite64().  The system       call numbers remained the same.  The glibc pread() and pwrite() wrapper       functions transparently deal with the change.       On  some  32-bit  architectures, the calling signature for these system       calls differ, for the reasons described insyscall(2).BUGS       POSIX requires that opening a file with the O_APPEND flag  should  have       no  effect  on the location at which pwrite() writes data.  However, on       Linux, if a file is opened with O_APPEND, pwrite() appends data to  the       end of the file, regardless of the value of offset.SEE ALSOlseek(2),read(2),readv(2),write(2)COLOPHON       This  page  is  part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs,  and  the       latest     version     of     this    page,    can    be    found    at       https://www.kernel.org/doc/man-pages/.Linux                             2017-09-15PREAD(2)