Transcript Introduction to Computer Systems

System-Level I/O
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Outline
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•
•
•
I/O redirection
Robust I/O
Standard I/O
Suggested Reading
– 10.4, 10.7~10.9
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I/O Redirection
• Question: How does a shell implement I/O redirection?
unix> ls > foo.txt
• Answer: By calling the dup2(oldfd, newfd) function
– Copies (per-process) descriptor table entry oldfd to entry
newfd
Descriptor table
before dup2(4,1)
fd 0
fd 1
fd 2
fd 3
fd 4
a
b
Descriptor table
after dup2(4,1)
fd 0
fd 1
fd 2
fd 3
fd 4
b
b
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I/O Redirction
unix > ls > foo.txt
• dup2 copies entries in the per-process file descriptor
table.
#include <unistd.h>
int dup2(int oldfd, int newfd);
returns: nonnegative descriptor if OK,
-1 on error
• dup2(oldfd, newfd) overwrites the entry in the
per-process file table for newfd with the entry for
oldfd.
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Redirection
dup2(4,1)
Descriptor table
(one table per
process)
fd0
fd1
fd2
fd3
fd4
fd5
fd6
fd7
open file table
(shared by all
process)
file A
V-node table
(shared by all
processes)
file pos
file A
File access
refcnt = 1
File size
...
File type
file B
…
file pos
file B
File access
refcnt = 1
File size
...
File type
…
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Redirection
dup2(4,1)
Descriptor table
(one table per
process)
fd0
fd1
fd2
fd3
fd4
fd5
fd6
fd7
open file table
(shared by all
process)
file A
V-node table
(shared by all
processes)
file pos
file A
File access
refcnt = 0
File size
...
File type
file B
…
file pos
file B
File access
refcnt = 2
File size
...
File type
…
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Redirection
#include “csapp.h”
foobar.txt
foobar
int main()
{
int fd1, fd2;
char c;
fd1 = open(“foobar.txt”, O_RDONLY, 0) ;
fd2 = open(“foobar.txt”, O_RDONLY, 0) ;
read(fd2, &c, 1) ;
dup2(fd2, fd1) ;
read(fd1, &c, 1) ;
printf(“c = %c\n”, c) ;
exit(0)
}
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Fun with File Descriptors (1)
#include "csapp.h"
int main(int argc, char *argv[])
{
int fd1, fd2, fd3;
char c1, c2, c3;
char *fname = argv[1];
fd1 = Open(fname, O_RDONLY, 0);
fd2 = Open(fname, O_RDONLY, 0);
fd3 = Open(fname, O_RDONLY, 0);
Dup2(fd2, fd3);
Read(fd1, &c1, 1);
Read(fd2, &c2, 1);
Read(fd3, &c3, 1);
printf("c1 = %c, c2 = %c, c3 = %c\n", c1, c2, c3);
return 0;
ffiles1.c
}
• What would this program print for file
containing “abcde”?
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Fun with File Descriptors (2)
#include "csapp.h"
int main(int argc, char *argv[])
{
int fd1;
int s = getpid() & 0x1;
char c1, c2;
char *fname = argv[1];
fd1 = Open(fname, O_RDONLY, 0);
Read(fd1, &c1, 1);
if (fork()) { /* Parent */
sleep(s);
Read(fd1, &c2, 1);
printf("Parent: c1 = %c, c2 = %c\n", c1, c2);
} else { /* Child */
sleep(1-s);
Read(fd1, &c2, 1);
printf("Child: c1 = %c, c2 = %c\n", c1, c2);
}
return 0;
ffiles2.c
}
• What would this program print for file
containing “abcde”?
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Fun with File Descriptors (3)
#include "csapp.h"
int main(int argc, char *argv[])
{
int fd1, fd2, fd3;
char *fname = argv[1];
fd1 = Open(fname, O_CREAT|O_TRUNC|O_RDWR, S_IRUSR|S_IWUSR);
Write(fd1, "pqrs", 4);
fd3 = Open(fname, O_APPEND|O_WRONLY, 0);
Write(fd3, "jklmn", 5);
fd2 = dup(fd1); /* Allocates descriptor */
Write(fd2, "wxyz", 4);
Write(fd3, "ef", 2);
return 0;
ffiles3.c
}
• What would be the contents of the resulting
file?
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The RIO Package
• RIO is a set of wrappers that provide efficient
and robust I/O in apps, such as network
programs that are subject to short counts
• RIO provides two different kinds of functions
– Unbuffered input and output of binary data
• rio_readn and rio_writen
– Buffered input of binary data and text lines
• rio_readlineb and rio_readnb
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Unbuffered I/O
• Transfer data directly between memory and a file, with
no application-level buffering
#include "csapp.h"
ssize_t rio_readn(int fd, void *usrbuf, size_t count);
ssize_t rio_writen(int fd, void *usrbuf, size_t count);
return: number of bytes read (0 if EOF) or written,
-1 on error
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rio_readn returns short count only if it encounters EOF
Only use it when you know how many bytes to read
rio_writen never returns a short count
Calls to rio_readn and rio_writen can be interleaved arbitrarily on
the same descriptor
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1 ssize_t rio_readn(int fd, void *buf, size_t count)
2 {
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size_t nleft = count;
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ssize_t nread;
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char *ptr = buf;
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while (nleft > 0) {
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if ((nread = read(fd, ptr, nleft)) < 0) {
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if (errno == EINTR)
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nread = 0; /* and call read() again */
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else
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return -1; /* errno set by read() */
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}
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else if (nread == 0)
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break; /* EOF */
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nleft -= nread;
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ptr += nread;
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}
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return (count - nleft); /* return >= 0 */
20 }
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1 ssize_t rio_writen(int fd, const void *buf, size_t count)
2 {
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size_t nleft = count;
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ssize_t nwritten;
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const char *ptr = buf;
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while (nleft > 0) {
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if ((nwritten = write(fd, ptr, nleft)) <= 0) {
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if (errno == EINTR)
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nwritten = 0; /* and call write() again */
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else
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return -1; /* errorno set by write() */
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}
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nleft -= nwritten;
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ptr += nwritten;
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}
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return count;
18 }
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Buffered I/O: Motivation
• Applications often read/write one character
at a time
– getc, putc, ungetc
– gets, fgets
• Read line of text on character at a time, stopping at
newline
• Implementing as Unix I/O calls expensive
– read and write require Unix kernel calls
• > 10,000 clock cycles
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Buffered I/O: Motivation
• Solution: Buffered read
– Use Unix read to grab block of bytes
– User input functions take one byte at a time from
buffer
• Refill buffer when empty
Buffer already read
unread
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Buffered I/O: Implementation
• For reading from file
• File has associated buffer to hold bytes that
have been read from file but not yet read by
user code
rio_cnt
Buffer already read
rio_buf
unread
rio_bufptr
typedef struct {
int rio_fd;
int rio_cnt;
char *rio_bufptr;
char rio_buf[RIO_BUFSIZE];
} rio_t;
/*
/*
/*
/*
descriptor for this internal buf */
unread bytes in internal buf */
next unread byte in internal buf */
internal buffer */
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Buffered I/O: Implementation
• Layered on Unix file:
Buffered Portion
not in buffer
already read
unread
unseen
Current File Position
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Buffered RIO Input Function
• Efficiently read text lines and binary data from a file
whose contents are cached in an application-level buffer
#include "csapp.h"
void rio_readinitb(rio_t *rp, int fd) ;
ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size t maxlen);
returns: number of bytes read (0 if EOF), -1 on error
• rio_readlineb reads a text line of up to maxlen
bytes from file fd and stores the line in usrbuf
 Especially useful for reading text lines from network sockets
• Stopping conditions
 maxlen bytes read
 EOF encountered
 Newline (‘\n’) encountered
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Buffered RIO Input Functions (cont)
#include "csapp.h"
void rio_readinitb(rio_t *rp, int fd);
ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);
ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n);
Return: num. bytes read if OK, 0 on EOF, -1 on error
• rio_readnb reads up to n bytes from file fd
• Stopping conditions
 maxlen bytes read
 EOF encountered
• Calls to rio_readlineb and rio_readnb can be
interleaved arbitrarily on the same descriptor
 Warning: Don’t interleave with calls to rio_readn
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Robust I/O
#define RIO_BUFSIZE 8192
typedef struct {
int rio_fd;
int rio_cnt;
char *rio_bufptr;
char rio_buf[RIO_BUFSIZE];
} rio_t ;
void rio_readinitb(rio_t *rp, int fd)
{
rp->rio_fd = fd ;
rp->rio_cnt = 0 ;
rp->rio_bufptr = rio_buf ;
}
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Robust I/O
#include “csapp.h”
int main(int argc, char **argv)
{
int n;
rio_t rio;
char buf[MAXLINE];
Rio_readinitb(&rio, STDIN_FILENO);
while ((n = Rio_readlineb(
&rio, buf, MAXLINE) ) != 0 )
Rio_writen(STDOUT_FILENO, buf, n);
}
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1 static ssize_t rio_read(rio_t *rp, char *usrbuf, size_t n)
2 {
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int cnt = 0;
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while (rp->rio_cnt <= 0) { /* refill if buf is empty */
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rp->rio_cnt = read(rp->rio_fd, rp->rio_buf,
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sizeof(rp->rio_buf));
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if ( rp->rio_cnt < 0) {
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if (errno != EINTR)
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return –1 ;
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}
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else if (rp->rio_cnt == 0) /* EOF */
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return 0;
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else
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rp->rio_bufptr = rp->rio_buf;/* reset buffer ptr */
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}
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/* Copy min(n, rp->rio_cnt) bytes
from internal buf to user buf */
cnt = n ;
if ( rp->rio_cnt < n)
cnt = rp->rio_cnt ;
memcpy(usrbuf, rp->rio_bufptr, cnt) ;
rp->rio_buffer += cnt ;
rp->rio_cnt -= cnt ;
return cnt ;
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1 ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n)
2 {
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size_t nleft = n;
ssize_t nread ;
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char *bufp = usrbuf;
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while (nleft > 0) {
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if ((nread = rio_read(rp, bufp, nleft)) < 0) {
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if ( errno = EINTR)
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/* interrupted by sig handler return */
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nread = 0;
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else
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return –1;
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}
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else if (nread == 0)
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break;
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nleft -= nread;
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bufp += nread;
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}
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return (n – nleft);
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ssize_t rio_readlineb (rio_t *rp,
void *usrbuf, size_t maxlen)
{
int n, rc;
char c, *bufp = usrbuf;
for (n=1; n < maxlen; n++) {
if ((rc = rio_read(rp, &c, 1)) == 1) {
*bufp++ = c;
if (c == ‘\n’)
break;
} else if (rc == 0) {
if (n== 1)
return 0; /* EOF, no data read */
else
break;
} else
return –1; /* error */
}
*bufp = 0 ;
return n ;
}
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Standard I/O
• The C standard library (libc.so) contains
a collection of higher-level standard I/O
functions
• Examples of standard I/O functions:
–
–
–
–
Opening and closing files (fopen and fclose)
Reading and writing bytes (fread and fwrite)
Reading and writing text lines (fgets and fputs)
Formatted reading and writing (fscanf and fprintf)
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Standard I/O
• Standard I/O models open files as streams
– Abstraction for a file descriptor and a buffer in
memory.
– Similar to buffered RIO
• C programs begin life with three open streams
(defined in stdio.h)
– stdin (standard input fd=0)
– stdout (standard output fd=1)
– stderr (standard error fd=2)
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Buffering in Standard I/O
• Standard I/O functions use buffered I/O
buf
printf("h");
printf("e");
printf("l");
printf("l");
printf("o");
printf("\n");
h
e
l
l
o
\n
.
.
fflush(stdout);
write(1, buf, 6);
• Buffer flushed to output fd on “\n” or fflush() call
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Standard I/O Buffering in Action
• You can see this buffering in action for
yourself, using the always fascinating Unix
strace program:
#include <stdio.h>
int main()
{
printf("h");
printf("e");
printf("l");
printf("l");
printf("o");
printf("\n");
fflush(stdout);
exit(0);
}
linux> strace ./hello
execve("./hello", ["hello"], [/* ... */]).
...
write(1, "hello\n", 6)
= 6
...
exit_group(0)
= ?
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Unix I/O, Standard I/O, and Robust I/O
• Standard I/O and RIO are implemented using lowlevel Unix I/O
fopen
fread
fscanf
sscanf
fgets
fflush
fclose
fdopen
fwrite
fprintf
sprintf
fputs
fseek
open
write
stat
read
lseek
close
C application program
Standard I/O
functions
RIO
functions
Unix I/O functions
(accessed via system calls)
• Which ones should you use in your programs?
rio_readn
rio_writen
rio_readinitb
rio_readlineb
rio_readnb
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Pros and Cons of Unix I/O
• Pros
– Unix I/O is the most general and lowest overhead
form of I/O.
• All other I/O packages are implemented using Unix I/O
functions.
– Unix I/O provides functions for accessing file
metadata.
– Unix I/O functions are async-signal-safe and can
be used safely in signal handlers.
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Pros and Cons of Unix I/O
• Cons
– Dealing with short counts is tricky and error prone.
– Efficient reading of text lines requires some form of
buffering, also tricky and error prone.
– Both of these issues are addressed by the standard
I/O and RIO packages.
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Pros and Cons of Standard I/O
• Pros:
– Buffering increases efficiency by decreasing the
number of read and write system calls
– Short counts are handled automatically
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Pros and Cons of Standard I/O
• Cons:
– Provides no function for accessing file metadata
– Standard I/O functions are not async-signal-safe,
and not appropriate for signal handlers.
– Standard I/O is not appropriate for input and
output on network sockets
• There are poorly documented restrictions on streams that
interact badly with restrictions on sockets (Sec 10.9)
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Choosing I/O Functions
• General rule: use the highest-level I/O
functions you can
– Many C programmers are able to do all of their work
using the standard I/O functions
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Choosing I/O Functions
• When to use standard I/O
– When working with disk or terminal files
• When to use raw Unix I/O
– Inside signal handlers, because Unix I/O is asyncsignal-safe.
– In rare cases when you need absolute highest
performance.
• When to use RIO
– When you are reading and writing network sockets.
– Avoid using standard I/O on sockets.
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Aside: Working with Binary Files
• Binary File Examples
– Object code, Images (JPEG, GIF),
• Functions you shouldn’t use on binary files
– Line-oriented I/O such as fgets, scanf,
printf, rio_readlineb
• Different systems interpret 0x0A (‘\n’) (newline)
differently:
– Linux and Mac OS X: LF(0x0a) [‘\n’]
– HTTP servers & Windoes: CR+LF(0x0d 0x0a)
[‘\r\n’]
• Use rio_readn or rio_readnb instead
– String functions
• strlen, strcpy
• Interprets byte value 0 (end of string) as special
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For Further Information
• The Unix bible:
– W. Richard Stevens & Stephen A. Rago, Advanced Programming
in the Unix Environment, 2nd Edition, Addison Wesley, 2005
• Updated from Stevens’s 1993 classic text.
• Stevens is arguably the best technical writer ever.
– Produced authoritative works in:
•
•
•
•
Unix programming
TCP/IP (the protocol that makes the Internet work)
Unix network programming
Unix IPC programming
• Tragically, Stevens died Sept. 1, 1999
– But others have taken up his legacy
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Next
• Network Programming
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