一个简单的alarm实例
errors.h头文件
1 #ifndef __ERRORS_H 2 #define __ERORRS_H 3 4 #include<stdio.h> 5 #include<unistd.h> 6 #include<errno.h> 7 #include<stdlib.h> 8 #include<string.h> 9 10 #ifdef DEBUG 11 #define DPRINTF(arg) printf arg 12 #else 13 #define DPRINTF(arg) 14 #endif 15 16 #define err_abort(code, text) do { 17 fprintf(stderr, "%s at "%s":%d: %s ", 18 text, __FILE__, __LINE__, strerror(code)); 19 abort(); 20 } while(0) 21 22 #define errno_abort(text) do { 23 fprintf(stderr, "%s at "%s":%d: %s ", 24 text, __FILE__, __LINE__, strerror(errno)); 25 abort(); 26 }while(0) 27 28 #endif
普通实现：alarm.c
1 #include"errors.h" 2 3 /* alarm的普通实现 */ 4 int main(int argc, char* argv[]) 5 { 6 int seconds; 7 char line[128]; 8 char message[64]; 9 10 while(1) 11 { 12 printf("Alarm> "); 13 if(fgets(line, sizeof(line), stdin) == NULL ) exit(0); 14 if(strlen(line) <= 1 ) continue; 15 if(sscanf(line, "%d %64[^ ]", &seconds, message) < 2 ) 16 fprintf(stderr, "Bad command "); 17 else 18 { 19 sleep(seconds); 20 printf("(%d) %s ", seconds, message); 21 } 22 23 } 24 }
多进程实现：alarm_fork.c
1 #include "errors.h" 2 #include<sys/types.h> 3 #include<wait.h> 4 5 /* alarm的多进程实现 */ 6 int main(int argc, char* argv[]) 7 { 8 int status; 9 char line[128]; 10 int seconds; 11 pid_t pid; 12 char message[64]; 13 14 while(1) 15 { 16 printf("Alarm> "); 17 if(fgets(line, sizeof(line), stdin) == NULL ) exit(0); 18 if(strlen(line) <= 1) continue; 19 if(sscanf(line, "%d %64[^ ]", 20 &seconds, message) < 2) 21 { 22 fprintf(stderr, "Bad command "); 23 } 24 else 25 { 26 pid = fork(); 27 if(pid == (pid_t)-1) 28 errno_abort("fork"); 29 if(pid == (pid_t)0) 30 { 31 sleep(seconds); 32 printf("(%d) %s ", seconds, message); 33 exit(0); 34 } 35 else 36 { 37 do 38 { 39 pid = waitpid((pid_t)-1, NULL, WNOHANG); 40 if(pid == (pid_t)-1) 41 errno_abort("wait child"); 42 }while(pid != (pid_t)0); 43 } 44 } 45 } 46 }
多线程实现：alarm_thread.c
1 #include<pthread.h> 2 #include "errors.h" 3 4 /* alarm的多线程实现 */ 5 6 typedef struct alarm_tag { 7 int seconds; 8 char message[64]; 9 } alarm_t; 10 11 void * alarm_thread(void *arg) 12 { 13 alarm_t *alarm = (alarm_t*)arg; 14 int status; 15 16 status = pthread_detach(pthread_self()); 17 if( status != 0 ) 18 err_abort(status, "pthread_detach"); 19 sleep(alarm->seconds); 20 printf("(%d) %s ", alarm->seconds, alarm->message); 21 free(alarm); 22 return NULL; 23 } 24 25 int main(int argc, char* argv[]) 26 { 27 int status; 28 char line[128]; 29 alarm_t *alarm; 30 pthread_t thread; 31 32 while(1) 33 { 34 printf("Alarm> "); 35 if(fgets(line, sizeof(line), stdin ) == NULL ) exit(0); 36 if(strlen(line) <= 1) continue; 37 alarm = (alarm_t*)malloc(sizeof(alarm_t)); 38 if( alarm == NULL ) 39 errno_abort("malloc"); 40 if( sscanf(line, "%d %64[^ ]", 41 &alarm->seconds, alarm->message) < 2) 42 { 43 fprintf(stderr, "Bad command "); 44 free(alarm); 45 } 46 else 47 { 48 status = pthread_create(&thread, NULL, alarm_thread, alarm); 49 if( status != 0) 50 err_abort(status, "pthread_creat"); 51 } 52 } 53 }
在以上alarm_thread.c代码执行时候，发现如果输入正好是128-1的倍数，将会出现Bad Command的提示。
追究下原因，原来是fgets导致：
fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or
a newline. If a newline is read, it is stored into the buffer. A terminating null byte (' ') is stored after the last character in the buffer.
1 #include<stdio.h> 2 #include<unistd.h> 3 #include<error.h> 4 #include<stdlib.h> 5 6 void main(void) 7 { 8 char line[4]; 9 char* message; 10 11 while(1) 12 { 13 printf("INPUT> "); 14 if(fgets(line, sizeof(line), stdin ) == NULL ) exit(0); 15 message = (char*)malloc(8); 16 if( sscanf(line, "%2[^ ]", 17 message) < 1) 18 { 19 fprintf(stderr, "sscanf error. "); 20 } 21 else 22 { 23 printf("message: %s ", message); 24 } 25 free(message); 26 } 27 } 28 29 /* 30 *root@jdu-virtual-machine:~# ./a.out 31 *INPUT> aaa 32 *message: aa 33 *sscanf error. 34 *INPUT> INPUT> ^C 35 */
Breakpoint 1 at 0x4007ca: file test.c, line 15. (gdb) r Starting program: /root/a.out INPUT> aaa Breakpoint 1, main () at test.c:15 15 message = (char*)malloc(8); (gdb) p line $1 = "aaa" (gdb) n 16 if( sscanf(line, "%2[^ ]", (gdb) 23 printf("message: %s ", message); (gdb) p message $2 = 0x602010 "aa" (gdb) n message: aa 25 free(message); (gdb) 26 } (gdb) 13 printf("INPUT> "); (gdb) 14 if(fgets(line, sizeof(line), stdin ) == NULL ) exit(0); (gdb) Breakpoint 1, main () at test.c:15 15 message = (char*)malloc(8); (gdb) p line $3 = " 00a" (gdb) n 16 if( sscanf(line, "%2[^ ]", (gdb) 19 fprintf(stderr, "sscanf error. "); (gdb) sscanf error. 25 free(message); (gdb)