|PREAD(2)||Linux Programmer's Manual||PREAD(2)|
NAMEpread, pwrite - read from or write to a file descriptor at a given offset
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE >= 500|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
DESCRIPTIONpread() 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 VALUEOn success, the number of bytes read or written is returned (zero indicates that nothing was written, in the case of pwrite(), or end of file, in the case of pread()), or -1 on error, in which case errno is set to indicate the error.
ERRORSpread() 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).
VERSIONSThe 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.
NOTESThe 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.
BUGSPOSIX requires that opening a file with the O_APPEND flag should have no affect 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)
COLOPHONThis page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.