advanced c programming over linux day1

transcript

Mohamed Saad 1

Advanced Topics in C ProgrammingProgramming under Linux

By: Mohamed Ahmed Saad

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Day1 1/2

1- System and Process Information.

– Your First Linux C Program.– Command-Line Arguments.– Environment variables.– User Information.– Host Information.

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Day1 2/2

2- Process Control.

– Process Identifiers.– Duplicating a Process Image.– Starting New Processes.– Replacing a Process Image.

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● Your First Linux C Program (1/4).

In file hello.c

#include <stdio.h> //1

int main(int argc, char *argv[]) //2

printf("hello, world\n"); //3

exit(0); //4

System & Process Information (01/12)

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● Compile and Run Program:

$gcc -o hello.exe hello.c

$./hello.exe

hello, world

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#include <stdio.h> //1Included file where functions like (printf) is defined.

int main(int argc, char *argv[]) //2int : returned value of the program (represents program status, 0: succeed, otherwise: failed.

To print return result of your program $echo $?

Argc : argument count ( number of arguments passed to the program).

Argv : an array of character pointers.

First entry is a pointer to program name.

Subsequent entries point to subsequent strings on the command line.

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printf("hello, world\n"); //3Prints “hello, world” on the screen

exit(0); //4Indicates that the program executed successfully.

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● Command-Line Arguments (1/2):in file “echoarg.c”

#include <stdio.h>

#include <stdlib.h>

int main(int argc, char *argv[])

int i;

for (i = 0; i < argc; i++)

printf("argv[%d]: %s\n", i, argv[i]);

exit(0);

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● Command-Line Arguments (2/2):$ ./echoarg arg1 TEST foo

argv[0]: ./echoarg

argv[1]: arg1

argv[2]: TEST

argv[3]: foo

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● Environment variables (1/4):● Environment List:

Each program is also passed an environment list. Like the argument list, the environment list is an array of character pointers, with each pointer containing the address of a null-terminated C string.

The address of the array of pointers is contained in the global variable environ:

extern char **environ;

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● Environment List:

environ:

environmentpointer

environment List

HOME=/home/msaad\0

PATH=:/usr/lib/mit/sbin:\0

SHELL=/bin/bash\0

USER=msaad\0

LOGNAME=msaad\0

environment String

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● Environment Variables (3/4):The environment strings are usually of the form

name=value

● Get Environment value:

#include <stdlib.h>

char *getenv(const char *name);

Returns: pointer to value associated with name, NULL if not found

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● Environment Variables (4/4):● Set Environment value:

#include <stdlib.h>

int putenv(char *str);

Returns: 0 if OK, nonzero on error.

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● User Information (1/1):

Password File:● passwd File fields are ontained in a passwd structure

that is defined in <pwd.h>

#include <pwd.h>

struct passwd *getpwuid

(uid_t uid);

struct passwd *getpwnam

(const char *name);

Both return: pointer if OK,

NULL on error

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Host Information (1/1):#include <sys/utsname.h>

int uname(struct utsname *name);

Returns: 0 value if OK, -1 on error

struct utsname {

char sysname[]; /* name of the operating system */

char nodename[]; /* name of this node */

char release[]; /* current release of operating system */

char version[]; /* current version of this release */

char machine[]; /* name of hardware type */

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2- Process Control.

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Process Control

● Process Identifiers (1/1):#include <unistd.h>

pid_t getpid(void);

Returns: process ID of calling process

pid_t getppid(void);

Returns: parent process ID of calling process

uid_t getuid(void);

Returns: real user ID of calling process

gid_t getgid(void);

Returns: real group ID of calling process

Process ControlProcess ControlProcess ControlProcess ControlProcess ControlProcess Control

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2- Process Control.

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● Duplicating a Process Image (1/5):● An existing process can create a new one by calling the

fork function.

#include <unistd.h>

pid_t fork(void);

Returns: 0 in child, process ID of child in parent, -1 on error

Process Control

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● Duplicating a Process Image (2/5):

Process Control

Initial Process

fork()

Returns a childProcess Id

Origin process continue

Returns Zero

Child Process

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● Duplicating a Process Image (3/5):● An existing process can create a new one by calling the

fork function.

#include <unistd.h>

pid_t fork(void);

Returns: 0 in child, process ID of child in parent, -1 on error

Process Control

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pid_t new_pid;

new_pid = fork();

switch(new_pid){

case -1: /*Error*/ break;

case 0: /*Child*/ break;

default : /*Parent*/

Process Control

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● Duplicating a Process Image (5/5):#include <sys/wait.h>

pid_t wait(int *statloc);

● Returns: process ID if OK, 0 (see later), or 1 on error.

● If stat_loc is not a null pointer, the exit status of

the child process will be written to the location to which it points

Process Control

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2- Process Control.

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● Starting New Processes (1/1):You can cause a program to run from

inside another program.

#include <stdlib.h>

int system (const char *cmdstring);

Process Control

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2- Process Control.

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● Replacing a Process Image (1/2): When a process calls one of the exec functions, that process is completely replaced by the new program, and

the new program starts executing at its main function. The process ID does not change across an

exec, because a new process is not created; exec merely replaces the current process its text, data,

heap, and stack segments with a brand new program from disk.

Process Control

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● Replacing a Process Image (2/2):#include <unistd.h>

● int execl(const char *pathname, const char *arg0,

... /* (char *)0 */ );

● int execv(const char *pathname, char *const argv []);

Process Control

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