0901假设存在一个可以从标准输入读入字母并且将其从小写转换为大写输出的可执行程序upcase

/*

• 假设存在一个可以从标准输入读入字母并且将其从小写转换为大写输出的可执行程序upcase,
• 使用pipe函数编写一个应用程序，实现将一个指定文本文件中的字母都转换为大写。
  */

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

#define BUFSIZE PIPE_BUF

int main(int argc, char argv[])
{
int n,fd[2];
pid_t pid;
char buffer[BUFSIZE+1];
FILE fp;
if(argc<=1)
{
printf("usage:%s <pathname>\n",argv[0]);
exit(1);
}
//open txt file
if((fp=fopen(argv[1],"r"))==NULL)
{
printf("Can't open %s\n",argv[1]);
exit(1);
}
//create pipe
if(pipe(fd)<0)
{
printf("pipe failed!\n");
exit(1);
}
//create son process
if((pid=fork())<0)
{
printf("fork failed!\n");
exit(1);
}
else if(pid>0)// father process
{
close(fd[0]);
while(fgets(buffer,BUFSIZE,fp)!=NULL)
{
n=strlen(buffer);
//write data to pipe
if(write(fd[1],buffer,n)!=n)
{
printf("write error to pipe.\n");
exit(1);
}
}
if(ferror(fp))
{
printf("fgets error.\n");
exit(1);
}
close(fd[1]);
if(waitpid(pid,NULL,0)<0)
{
printf("waitpid error!\n");
exit(1);
}
exit(0);
}
else
{
close(fd[1]);
if(fd[0]!=STDIN_FILENO)
{
//copy pipe to stdin
if(dup2(fd[0],STDIN_FILENO)!=STDIN_FILENO)
{
printf("dup2 error to stdin!\n");
exit(1);
}
close(fd[0]);
}
//run user program
if(execl("upcase","upcase",(char *)0)<0)
{
printf("execl error for upcase.\n");
exit(1);
}
exit(0);
}
//return 0;
}