system calls and library functions under Linux

Source: Internet
Author: User

Library functions are based on system calls, eg:printf (library functions), and write (System calls). After redirection, the contents of fwrite were printed two times in the file because: 1. Buffer mode changed (from row buffer to full buffer) 2. The library function has a buffer, and the system call has no buffer.

Write prototype: ssize_t write (int fd, const void *buf, size_t count);

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys /types.h>int main () {        pid_t id;         printf ("Hello world");         const  char* msg= "I love you";         write (1,msg,strlen (msg));         id=fork ();         if ( id<0)         {                 perror ("fork");                 exit (1);         }        else if (id==0)          {                printf ("pid: %d, Ppid: %d ", Getpid (), Getppid ());        }         else        {                  printf ("pid: %d,ppid: %d", Getpid (), Getppid ());         }          return 0;}

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REDIRECT output to file

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Modify the above code as: int main () {pid_t id;       printf ("Hello World");       Const char* msg= "I Love You";       Close (1);       file* fp=fopen ("File", "w+");       Write (1,msg,strlen (msg));       Id=fork (); ...}

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Close file descriptor: Standard output, the next file descriptor will start from the front empty small integer subscript, at which time file occupies the descriptor 1.

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system calls and library functions under Linux

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