我的程序中使用了read()和write()系统调用,但没有在程序中包含"unistd.h"头文件。但是程序仍然可以正常工作并给出预期的结果。
运行程序后,我想阅读read()和write()的man手册。
在read()和write()的man 2页面的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);
我很惊讶为什么我的程序没有包含unistd.h头文件却能够正常工作呢?
以下是我的程序。它是一个使用read()和write()系统调用将源文件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;
}
-Wall -Wextra -pedantic -std=c11
- Deduplicator