PREAD(2) Linux Programmer's Manual PREAD(2)
pread, pwrite - read from or write to a file descriptor at a given off-
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 (see feature_test_macros(7)):
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
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
The file referenced by fd must be capable of seeking.
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-
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 cause of the
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).
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 using lseek(2) on older kernels
without the system calls) was added in glibc 2.1.
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 in syscall(2).
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.
lseek(2), read(2), readv(2), write(2)
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Linux 2017-09-15 PREAD(2)