1. Read() System Call
#include <unistd.h>
int read(int fd, void *buf, int nbytes);
read() reads nbytes from an opened file descriptor into buf[ ] in user space. The return value is the actual number of bytes read or -1 if read() failed, e.g. due to invalid fd. Note that the buf[ ] area must have enough space to receive nbytes, and the return value may be less than nbytes, e.g. if the file size is less than nbytes or when the file has no more data to read. Note also that the return value is an integer, not any End-Of-File (EOF) symbol since there is no EOF symbol in a file. EOF is a special integer value (-1) returned by I/O library functions when a FILE stream has no more data.
2. Write() System Call
#include <unistd.h>
int write(int fd, void *buf, int nbytes);
write() writes nbytes from buf[ ] in user space to the file descriptor, which must be opened for write, read-write or append mode. The return value is the actual number of bytes written, which usually equal to nbytes, or -1 if write() failed, e.g. due to invalid fd or fd is opened for read-only, etc.
Example: The following code segment uses open(), read(), lseek(), write() and close() syscalls. It copies the first 1KB bytes of a file to byte 2048.
Source: Wang K.C. (2018), Systems Programming in Unix/Linux, Springer; 1st ed. 2018 edition.