pread64

pread(2)                       System Calls Manual                      pread(2)

NAME
       pread, pwrite - read from or write to a file descriptor at a given offset

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <unistd.h>

       ssize_t pread(size_t count;
                     int fd, void buf[count], size_t count,
                     off_t offset);
       ssize_t pwrite(size_t count;
                     int fd, const void buf[count], size_t count,
                     off_t offset);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       pread(), pwrite():
           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

DESCRIPTION
       pread() reads up to count bytes from file descriptor fd at offset offset
       (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 written.

       Note that it is not an error for a successful call to transfer fewer
       bytes than requested (see read(2) and write(2)).

       On error, -1 is returned and errno is set to indicate the error.

ERRORS
       pread() can fail and set errno to any error specified for read(2) or
       lseek(2).  pwrite() can fail and set errno to any error specified for
       write(2) or lseek(2).

STANDARDS
       POSIX.1-2024.

HISTORY
       POSIX.1-2001.

       Added in Linux 2.1.60; the entries in the i386 system call table were
       added in Linux 2.1.69.  C library support (including emulation using
       lseek(2) on older kernels without the system calls) was added in glibc
       2.1.

   C library/kernel differences
       On Linux, the underlying system calls were renamed in Linux 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 in syscall(2).

NOTES
       The pread() and pwrite() system calls are especially useful in
       multithreaded 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.

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 ALSO
       lseek(2), read(2), readv(2), write(2)

Linux man-pages 6.16               2025-10-29                           pread(2)