Ｕnix环境高级编程学习笔记（一）

代码

#include <sys/wait.h>
#include <unistd.h>
#include <stdio.h>
 
#define MAXLINE 3

int 
main(void)
{
char buf[MAXLINE];
pid_t pid;
int status;

printf("%% ");
while(fgets(buf, MAXLINE, stdin) != NULL)
{
int i;
for(i = 0; i<strlen(buf); i++)
printf("%c",buf[i]);
printf("Done");
if(buf[strlen(buf) -1] == '\n')
{
buf[strlen(buf)-1] = 0;
printf("Test %c",buf[strlen(buf)-1]);
}
if((pid = fork()) < 0)
{
printf("fork error\n");
}
else if(pid == 0)
{
printf("The current buf is %s", buf);
execlp(buf, buf, (char *)0);
}

if((pid = waitpid(pid, &status, 0)) < 0)
printf("waitpid error");
printf("%% ");
}

exit(0);

}

分析过程：

(gdb) b main
Breakpoint 1 at 0x80485be: file 1-5.c, line 14.
(gdb) r 1-5
Starting program: /home/helei/UnixProgram/1-5 1-5

Breakpoint 1, main () at 1-5.c:14
14    printf("%% ");
(gdb) n
15    while(fgets(buf, MAXLINE, stdin) != NULL)
(gdb) n
% pwd
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
19    printf("%c",buf[i]);
(gdb) watch buf[i]
Hardware watchpoint 2: buf[i]
(gdb) watch i
Hardware watchpoint 3: i
(gdb) n
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
Hardware watchpoint 2: buf[i]

Old value = 112 'p'
New value = 119 'w'
Hardware watchpoint 3: i

Old value = 0
New value = 1
0x080485f3 in main () at 1-5.c:18
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
19    printf("%c",buf[i]);
(gdb) n
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
Hardware watchpoint 2: buf[i]

Old value = 119 'w'
New value = 0 '\000'
Hardware watchpoint 3: i

Old value = 1
New value = 2
0x080485f3 in main () at 1-5.c:18
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
20    printf("Done");
(gdb) n
21    if(buf[strlen(buf) -1] == '\n')
(gdb) n
26    if((pid = fork()) < 0)
(gdb) n
30    else if(pid == 0)
(gdb) n
36    if((pid = waitpid(pid, &status, 0)) < 0)
(gdb) n
38    printf("%% ");
(gdb) n
15    while(fgets(buf, MAXLINE, stdin) != NULL)
(gdb) n
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
Hardware watchpoint 2: buf[i]

Old value = 0 '\000'
New value = 100 'd'
Hardware watchpoint 3: i

Old value = 2
New value = 0
0x080485d8 in main () at 1-5.c:18
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
19    printf("%c",buf[i]);
(gdb) n
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
Hardware watchpoint 2: buf[i]

Old value = 100 'd'
New value = 10 '\n'
Hardware watchpoint 3: i

Old value = 0
New value = 1
0x080485f3 in main () at 1-5.c:18
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
19    printf("%c",buf[i]);
(gdb) n
pwDone% d
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
Hardware watchpoint 2: buf[i]

Old value = 10 '\n'
New value = 0 '\000'
Hardware watchpoint 3: i

Old value = 1
New value = 2
0x080485f3 in main () at 1-5.c:18
18    for(i = 0; i<strlen(buf); i++)
(gdb) n
20    printf("Done");
(gdb) n
21    if(buf[strlen(buf) -1] == '\n')
(gdb) n
23    buf[strlen(buf)-1] = 0;
(gdb) n
24    printf("Test %c",buf[strlen(buf)-1]);
(gdb) n
26    if((pid = fork()) < 0)
(gdb) n
DoneTest dThe current buf is dwaitpid error% 30    else if(pid == 0)

结论：

（１）fget函数在已达到文件结尾或出错时返回ＮＵＬＬ；

（２）若缓冲区不能足够容纳读入的一行，则缓冲区存储的是不完整的行，且以null字符结尾；

（３）输入的行未读完，剩余部分再从头写缓冲区，将覆盖原来的内容；

（4）读完最后一个字符\n后，再填null字符。

还未想明白的地方：

（１）在第一次写缓冲区和第二次写的间隔，子进程部分为何不执行（如调试部分加粗标记），求看官指点。