1PREAD(2) Linux Programmer's Manual PREAD(2)
2
6 pread, pwrite - read from or write to a file descriptor at a given off‐
7 set
8
10 #include <unistd.h>
11 ssize_t pread(int fd, void *buf, size_t count, off_t offset);
13 ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);
15 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
17 pread(), pwrite():
19 _XOPEN_SOURCE >= 500
20 || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
21
23 pread() reads up to count bytes from file descriptor fd at offset off‐
24 set (from the start of the file) into the buffer starting at buf. The
25 file offset is not changed.
26 pwrite() writes up to count bytes from the buffer starting at buf to
28 the file descriptor fd at offset offset. The file offset is not
29 changed.
30 The file referenced by fd must be capable of seeking.
32
34 On success, the number of bytes read or written is returned (zero indi‐
35 cates that nothing was written, in the case of pwrite(), or end of
36 file, in the case of pread()), or -1 on error, in which case errno is
37 set to indicate the error.
38
40 pread() can fail and set errno to any error specified for read(2) or
41 lseek(2). pwrite() can fail and set errno to any error specified for
42 write(2) or lseek(2).
43
45 The pread() and pwrite() system calls were added to Linux in version
46 2.1.60; the entries in the i386 system call table were added in 2.1.69.
47 C library support (including emulation using lseek(2) on older kernels
48 without the system calls) was added in glibc 2.1.
49
51 POSIX.1-2001.
52
54 The pread() and pwrite() system calls are especially useful in multi‐
55 threaded applications. They allow multiple threads to perform I/O on
56 the same file descriptor without being affected by changes to the file
57 offset by other threads.
58 On Linux, the underlying system calls were renamed in kernel 2.6:
60 pread() became pread64(), and pwrite() became pwrite64(). The system
61 call numbers remained the same. The glibc pread() and pwrite() wrapper
62 functions transparently deal with the change.
63 On some 32-bit architectures, the calling signature for these system
65 calls differ, for the reasons described in syscall(2).
66
68 POSIX requires that opening a file with the O_APPEND flag should have
69 no affect on the location at which pwrite() writes data. However, on
70 Linux, if a file is opened with O_APPEND, pwrite() appends data to the
71 end of the file, regardless of the value of offset.
72
74 lseek(2), read(2), readv(2), write(2)
75
77 This page is part of release 3.53 of the Linux man-pages project. A
78 description of the project, and information about reporting bugs, can
79 be found at http://www.kernel.org/doc/man-pages/.
80Linux 2013-06-21 PREAD(2)