Advanced Linux Programming 2

Writing Good GNU/Linux Software

1, Interaction With the Parameters
getopt_long.c

#include <getopt.h>
#include <stdio.h>
#include <stdlib.h>

void print_usage(FILE *stream, int exit_code)
{
    fprintf(stream, "Usage:..\n");
    exit(exit_code);
}

int main(int argc, char* argv[])
{
    int next_option;
    /** A string listing valid short options letters. */
    const char* const short_options = "ho:v";
    /** A array describing valid long options. */
    const struct option long_options[] = {
        { "help",       0, NULL, 'h' },
        { "output",     1, NULL, 'o' },
        { "verbose",    0, NULL, 'v' },
        {  NULL,        0, NULL,  0  }
    };

    const char* output_filename = NULL;

    do {
        next_option = getopt_long(argc, argv, short_options,
                long_options, NULL);
        switch(next_option)
        {
            case 'h':  /* -h or --help */
                print_usage(stdout, 0);
            case 'o':  /* -o or --output */
                output_filename = optarg;
                printf("output file=%s\n", output_filename);
                break;
            case 'v':  /* -v or --verbose */
                /* do something */
                break;
            case '?':  /* Invalid option */
                print_usage(stderr, 1);
            case -1:   /* Done with options */
                break;
            default:
                abort();
        }
    } while(next_option != -1);

    return 0;
}

tom@debian:~/project/linuxc/ch02$ ./getopt_long
tom@debian:~/project/linuxc/ch02$ ./getopt_long -h
Usage:..
tom@debian:~/project/linuxc/ch02$ ./getopt_long -o log.txt
output file=log.txt
tom@debian:~/project/linuxc/ch02$ ./getopt_long --verbose
tom@debian:~/project/linuxc/ch02$ ./getopt_long -?
./getopt_long: invalid option -- ?
Usage:..
tom@debian:~/project/linuxc/ch02$

2, Printing the Execution Environment

#include <stdio.h>

/** The ENVIRON variable contains the environment. */
extern char** environ;

int main()
{
 char** var;
 for(var=environ; *var!=NULL; ++var)
    {
  printf("%s\n", *var);
 }

 return 0;
}

3, Interact with environ variables

#include <stdio.h>
#include <stdlib.h>

int main()
{
 char* server_name = getenv("SERVER_NAME");
 if(server_name == NULL)
    {
  server_name = "localhost";
 }

 printf("SERVER_NAME: %s\n", server_name);

 return 0;
}

tom@debian:~/project/linuxc/ch02$ ./client
SERVER_NAME: localhost
tom@debian:~/project/linuxc/ch02$ export SERVER_NAME="sina.com"
tom@debian:~/project/linuxc/ch02$ ./client
SERVER_NAME: sina.com
tom@debian:~/project/linuxc/ch02$

4, Do as the kernel does

1. Allocate the buffer.

2. Open the file.

3. Read from the file into the buffer.

4. Close the file.

5. Return the buffer.

5, Writing and Using libraries

tom@debian:~/project/linuxc/ch02$ echo "int f(){return 3;}" > test.c
tom@debian:~/project/linuxc/ch02$ echo "int main() {return f();}" > app.c
tom@debian:~/project/linuxc/ch02$ gcc -c test.c
tom@debian:~/project/linuxc/ch02$ ar cr libtest.a test.o
tom@debian:~/project/linuxc/ch02$ gcc -c app.c
tom@debian:~/project/linuxc/ch02$ gcc -o app app.o -L. -ltest
tom@debian:~/project/linuxc/ch02$ ./app
tom@debian:~/project/linuxc/ch02$ echo $?
3

tom@debian:~/project/linuxc/ch02$ gcc -c -fPIC test.c
tom@debian:~/project/linuxc/ch02$ gcc -shared -fPIC -o libtest.so test.o
tom@debian:~/project/linuxc/ch02$ gcc -o app app.c -L. -ltest
tom@debian:~/project/linuxc/ch02$ sudo cp libtest.so /usr/lib/
tom@debian:~/project/linuxc/ch02$ ldd app
 linux-gate.so.1 =>  (0xb77c3000)
 libtest.so => /usr/lib/libtest.so (0xb77a5000)
 libc.so.6 => /lib/i686/cmov/libc.so.6 (0xb764a000)
 /lib/ld-linux.so.2 (0xb77c4000)
tom@debian:~/project/linuxc/ch02$ ./app
tom@debian:~/project/linuxc/ch02$ echo $?
3