1 .\" Copyright (C) 1999 Joseph Samuel Myers.
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25 .TH PREAD 2 2021-03-22 "Linux" "Linux Programmer's Manual"
27 pread, pwrite \- read from or write to a file descriptor at a given offset
30 .B #include <unistd.h>
32 .BI "ssize_t pread(int " fd ", void *" buf ", size_t " count \
34 .BI "ssize_t pwrite(int " fd ", const void *" buf ", size_t " count \
39 Feature Test Macro Requirements for glibc (see
40 .BR feature_test_macros (7)):
47 || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
53 bytes from file descriptor
57 (from the start of the file) into the buffer starting at
59 The file offset is not changed.
64 bytes from the buffer starting at
66 to the file descriptor
70 The file offset is not changed.
72 The file referenced by
74 must be capable of seeking.
78 returns the number of bytes read
79 (a return of zero indicates end of file)
82 returns the number of bytes written.
84 Note that it is not an error for a successful call to transfer fewer bytes
90 On error, \-1 is returned and
92 is set to indicate the error.
97 to any error specified for
104 to any error specified for
113 system calls were added to Linux in
114 version 2.1.60; the entries in the i386 system call table were added
116 C library support (including emulation using
118 on older kernels without the system calls) was added in glibc 2.1.
120 POSIX.1-2001, POSIX.1-2008.
126 system calls are especially useful in multithreaded applications.
127 They allow multiple threads to perform I/O on the same file descriptor
128 without being affected by changes to the file offset by other threads.
130 .SS C library/kernel differences
131 On Linux, the underlying system calls were renamed
140 The system call numbers remained the same.
145 wrapper functions transparently deal with the change.
147 On some 32-bit architectures,
148 the calling signature for these system calls differ,
149 for the reasons described in
152 POSIX requires that opening a file with the
154 flag should have no effect on the location at which
157 However, on Linux, if a file is opened with
158 .\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=43178
161 appends data to the end of the file, regardless of the value of