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I am trying to use pread and pwrite so that I can lseek to the beginning of the file and start reading or writing in one atomic operation. Both of these functions will do that for me however, the issue I am having is that the compiler is giving me warning: implicit declaration of function ‘pread’ even after I added #define _XOPEN_SOURCE 500 and #include<unistd.h> like the man pages said. Am I missing something? The two statements with the function calls are below.
Thanks!
#include<unistd.h>
#define _XOPEN_SOURCE 500
int main (int argc, char *argv[])
{
while ( (read_in += pread(source_fd, in_buf, in_buf_size,read_in) ) )
{
if (write_out += pwrite(dest_fd, in_buf, read_in, write_out) == -1)
{
342
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 pread() function performs the same action as read(), except that it reads from a given position in the file without changing the file pointer. The first three arguments to pread() are the same as read(), with the addition of a fourth argument offset for the desired position inside the file.
You need to define _XOPEN_SOURCE 500 before your includes:
#define _XOPEN_SOURCE 500
#include<unistd.h>
Otherwise, the unistd.h header won't see the macro definition.
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