Multithreaded Programming Guide

Shared I/O and New I/O System Calls

When multiple threads are performing I/O operations at the same time with the same file descriptor, you might discover that the traditional UNIX I/O interface is not thread safe. The problem occurs with nonsequential I/O. This uses the lseek(2) system call to set the file offset, which is then used in the next read(2) or write(2) call to indicate where in the file the operation should start. When two or more threads are issuing lseeks() to the same file descriptor, a conflict results.

To avoid this conflict, use the pread(2) and pwrite(2) system calls.

#include <sys/types.h>
#include <unistd.h>

ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);

ssize_t pwrite(int filedes, void *buf, size_t nbyte,
    off_t offset);

These behave just like read(2) and write(2) except that they take an additional argument, the file offset. With this argument, you specify the offset without using lseek(2), so multiple threads can use these routines safely for I/O on the same file descriptor.