[Linux] [阅读] [APUE] - 第一章 UNIX基础知识

程序清单1-1：列出指定目录中所有文件的名字，即命令ls的简要实现

#include "apue.h" #include <dirent.h> int main(int argc, char *argv[]) { DIR *dp; struct dirent *dirp; if (argc != 2) err_quit("usage: ls directory_name"); if ((dp = opendir(argv[1])) == NULL) err_sys("can't open %s", argv[1]); while ((dirp = readdir(dp)) != NULL) printf("%s\n", dirp->d_name); closedir(dp); exit(0); }

编译生成ls1文件，运行：

# ./ls1 ..   // 列出上一级目录中的所有文件

The opendir function returns a pointer to a DIR structure, and we pass this pointer to the readdir function. We don't care what's in the DIR structure. We then call readdir in a loop, to read each directory entry. The readdir function returns a pointer to a dirent structure or, when it's finished with the directory, a null pointer. All we examine in the dirent structure is the name of each directory entry (d_name). Using this name, we could then call the stat function (Section 4.2) to determine all the attributes of the file.

 

程序清单1-2：将标准输入复制到标准输出

#include "apue.h" #define BUFFSIZE 4096 int main(void) { int n; char buf[BUFFSIZE]; while ((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0) if (write(STDOUT_FILENO, buf, n) != n) err_sys("write error"); if (n < 0) err_sys("read error"); exit(0); }

编译生成a.out文件，并以如下方式运行它：

#./a.out >data  //标准输入时终端，标准输出则重定向至文件data，标准出错也是终端。如果此输出文件不存在，则shell会创建它。该程序将用户键入的各行复制到标准输出，键入文件结束字符时（通常是ctrl+D），则终止该次复制。

#./a.out < infile > outfile //将名为infile文件的内容复制到名为outfile的文件中。

The constants STDIN_FILENO and STDOUT_FILENO are defined in <unistd.h> and specify the file descriptors for standard input and standard output. These values are typically 0 and 1, respectively, but we'll use the new names for portability.

 

程序清单1-3：用标准I/O将标准输入复制到标准输出

#include "apue.h" int main(void) { int c; while ((c = getc(stdin)) != EOF) if (putc(c, stdout) == EOF) err_sys("output error"); if (ferror(stdin)) err_sys("input error"); exit(0); }

The function getc reads one character at a time, and this character is written by putc. After the last byte of input has been read, getc returns the constant EOF (defined in <stdio.h>). The standard I/O constants stdin and stdout are also defined in the <stdio.h> header and refer to the standard input and standard output.

程序清单1-4：打印进程ID

#include "apue.h" int main(void) { printf("hello world from process ID %d\n", getpid()); exit(0); }

程序清单1-5：从标准输入读命令并执行

#include "apue.h" #include <sys/wait.h> int main(void) { char buf[MAXLINE]; /* from apue.h */ pid_t pid; int status; printf("%% "); /* print prompt (printf requires %% to print %) */ while (fgets(buf, MAXLINE, stdin) != NULL) { if (buf[strlen(buf) - 1] == "\n") buf[strlen(buf) - 1] = 0; /* replace newline with null */ if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { /* child */ execlp(buf, buf, (char *)0); err_ret("couldn't execute: %s", buf); exit(127); } /* parent */ if ((pid = waitpid(pid, &status, 0)) < 0) err_sys("waitpid error"); printf("%% "); } exit(0); }

If we run this program, we get the following results. Note that our program has a different promptthe percent signto distinguish it from the shell's prompt. $ ./a.out % date Sun Aug 1 03:04:47 EDT 2004 programmers work late % who sar :0 Jul 26 22:54 sar pts/0 Jul 26 22:54 (:0) sar pts/1 Jul 26 22:54 (:0) sar pts/2 Jul 26 22:54 (:0) % pwd /home/sar/bk/apue/2e % ls Makefile a.out shell1.c % ^D type the end-of-file character $ the regular shell prompt

The most fundamental limitation of this program is that we can't pass arguments to the command that we execute.

程序清单1-6：示例strerror和perror

#include "apue.h" #include <errno.h> int main(int argc, char *argv[]) { fprintf(stderr, "EACCES: %s\n", strerror(EACCES)); errno = ENOENT; perror(argv[0]); exit(0); }

If this program is compiled into the file a.out, we have $ ./a.out EACCES: Permission denied ./a.out: No such file or directory

There are two rules to be aware of with respect to errno. First, its value is never cleared by a routine if an error does not occur. Therefore, we should examine its value only when the return value from a function indicates that an error occurred. Second, the value of errno is never set to 0 by any of the functions, and none of the constants defined in <errno.h> has a value of 0. Two functions are defined by the C standard to help with printing error messages.     #include <string.h> char *strerror(int errnum);     Returns: pointer to message string   This function maps errnum, which is typically the errno value, into an error message string and returns a pointer to the string. The perror function produces an error message on the standard error, based on the current value of errno, and returns.     #include <stdio.h> void perror(const char *msg);       It outputs the string pointed to by msg, followed by a colon and a space, followed by the error message corresponding to the value of errno, followed by a newline.

程序清单1-7：打印用户ID和组ID

#include "apue.h" int main(void) { printf("uid = %d, gid = %d\n", getuid(), getgid()); exit(0); }

We call the functions getuid and getgid to return the user ID and the group ID. Running the program yields $ ./a.out uid = 205, gid = 105

程序清单1-8：从标准输入读命令并执行

// + 为相对于程序清单1-5新增的代码行

#include "apue.h" #include <sys/wait.h> + static void sig_int(int); /* our signal-catching function */ + int main(void) { char buf[MAXLINE]; /* from apue.h */ pid_t pid; int status; + if (signal(SIGINT, sig_int) == SIG_ERR) + err_sys("signal error"); + printf("%% "); /* print prompt (printf requires %% to print %) */ while (fgets(buf, MAXLINE, stdin) != NULL) { if (buf[strlen(buf) - 1] == "\n") buf[strlen(buf) - 1] = 0; /* replace newline with null */ if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { /* child */ execlp(buf, buf, (char *)0); err_ret("couldn't execute: %s", buf); exit(127); } /* parent */ if ((pid = waitpid(pid, &status, 0)) < 0) err_sys("waitpid error"); printf("%% "); } exit(0); } + + void + sig_int(int signo) + { + printf("interrupt\n%% "); + }

To catch this signal, the program needs to call the signal function, specifying the name of the function to call when the SIGINT signal is generated. The function is named sig_int; when it's called, it just prints a message and a new prompt.