我在程序中使用了系统调用 read() 和 write()，但程序中没有包含“unistd.h”头文件。但是该程序仍然有效并给出了预期的结果。
运行程序后，我想我会阅读 read() 和 write() 的手册页。
在 read() 和 write() 的 man 2 页面中，在 SYNOPSIS 部分中提到我需要包含 unistd.h 头文件才能使用 read() 或 write()。
SYNOPSIS
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
SYNOPSIS
#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count);
所以我很惊讶我的程序是如何工作的，尽管我没有包含 unistd.h ？
下面是我的程序。这是一个使用 read() 和 write() 系统调用将源文件的内容复制到目标文件的程序。
#include<stdio.h>
#include<fcntl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<stdlib.h>
int main()
{
/* Declaring the buffer. */
/* Data read by read() will be stored in this buffer. */
/* Later when write() is used, write() will take the contents of this buffer and write to the file.*/
char buffer[512];
/* Decalring strings to store the source file and target file names. */
char source[128], target[128];
/* Declaring integer variables in which integer returned by open() will be stored. */
/* Note that this program will open a source file, and a target file. So, 2 integers will be needed. */
int inhandle, outhandle;
/* Declaring integer variable which will specify how much bytes to read or write.*/
int bytes;
/* Taking source filename from keyboard.*/
printf("\nSource File name: ");
scanf("%s",source);
/* Open the source file using open().*/
inhandle = open(source, O_RDONLY);
/* If there is error while opening source file.*/
if (inhandle == -1)
{
perror("Error opening source file.\n");
exit(1);
}
/* Taking target filename from keyboard.*/
printf("\nTarget File name: ");
scanf("%s",target);
/* Open the target file using open().*/
outhandle = open(target, O_CREAT | O_WRONLY, 0660);
/* If there is error while opening target file.*/
if (outhandle == -1)
{
perror("Error opening target file.\n");
close(inhandle);
exit(2);
}
/* Below code does following:
1. First reads (at most) 512 bytes from source file
2. Then copies them to buffer
3. If bytes read is greater than 0, write the content stored in buffer to target file.
*/
while((bytes = read(inhandle, buffer, 512)) > 0)
{
write(outhandle, buffer, bytes);
}
/* Close both source and target files. */
close(inhandle);
close(outhandle);
return 0;
}