pread, pwrite — read from or write to a file descriptor at a given offset
#define _XOPEN_SOURCE 500 #include <unistd.h>
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); |
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.
On 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.
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.