Open-source Software Project Development: Writing Realistic Server Software (1)

Open-Source Software Project Development Instructor: Dr. T.Y. Wong

Week 10, Part 2 Writing Realistic Server Software (1)

Servers? • We sometimes call them daemons. – Apache; MySQL; Samba; etc. – Many of them are open source software. – You may have a chance to write them.

Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

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Realistic Server? • Properties?

No STDOUT nor STDERR?

I/O Redirection?

Spring semester 2008-2009

(Usually) Run in background

(Usually) Only one execution instance is allowed. How?

CSC4140 – Open-Source Software Project Development

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Background process revisited… • Using fork(). Filename: bg.c

int pid = fork(); if(pid == 0) { while(1) This is the most easy part sleep(1); } else { printf(“Created a bg process: %d\n”, pid); exit(0); }

Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

Page 4

Background process revisited… • How about STDOUT and STDERR? Filename: bg_with_print.c int pid = fork(); if(pid == 0) { while(1) { fprintf(stdout, “STDOUT: I’m alive\n”); fprintf(stderr, “STDERR: I’m alive\n”); sleep(1); } } else { printf(“Created a bg process: %d\n”, pid); exit(0); $ ./bg_with_print } Created a bg process: 1234 $ STDOUT: I’m alive Very irritating…you’ll STDERR: I’m alive STDOUT: I’m alive never find this kind of STDERR: I’m alive servers running… ......

Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

Page 5

Background process revisited… • How about STDOUT and STDERR? Filename: bg_with_print.c int pid = fork(); if(pid == 0) { while(1) { fprintf(stdout, “STDOUT: I’m alive\n”); fprintf(stderr, “STDERR: I’m alive\n”); sleep(1); } } else { printf(“Created a bg process: %d\n”, pid); exit(0); $ ./bg_with_print } Created a bg process: 1234 $ STDOUT: I’m alive Very irritating…you’ll STDERR: I’m alive STDOUT: I’m alive never find this kind of STDERR: I’m alive servers running… ......

Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

Page 6

I/O redirection revisited… • How about STDOUT and STDERR? Filename: bg_with_redirect.c int pid = fork(); char buf[1024]; if(pid == 0) { sprintf(buf, “log/%d.stdout”, getpid()); YEAH! I can have a process redirect(stdout, buf); sprintf(buf, “log/%d.stderr”, getpid()); redirect(stderr, buf);

running silently, giving outputs at the same time.

while(1) { fprintf(stdout, “STDOUT: I’m alive\n”);*in_fp, char *buf) { void redirect(FILE fprintf(stderr, “STDERR:int I’mfd alive\n”); = fileno(in_fp); sleep(1); FILE *fp = fopen(buf, “w”); } if(fp == NULL) { ...... }; // kill yourself } close(fd); else { dup2(fileno(fp), fd); printf(“Created a bg process: %d\n”, pid); fclose(fp); exit(0); } } Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

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One-instance only? • Using a technique called pidfile. NO…

1st instance

Step (1) check if exist?

Apache

Step (2) create and write

1234

t?

2nd instance

ki

p Ste

c he c )

(1

f

is ex

/var/run/apache2.pid

s…

Ye

Apache Step (2) Kill himself!

Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

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One-instance only? • Using a technique called pidfile. 1st instance Step (1) remove file.

Apache I wanna quit…

1234 5678 Step (2) Kill himself! t?

ki

3rd instance

p Ste

(1

c he c )

f

is ex

/var/run/apache2.pid

… No

d write

Apache

reate an c ) 2 ( p e St

Filename: bg_with_pidfile.c Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

The story goes on… Page 9

To be continued… Spring semester 2008-2009

CSC4140 – Open-Source Software Project Development

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