Linux上CP的几个实现

第一个版本，使用的系统调用，没有使用C库的函数。

#include <apue.h>
#include <fcntl.h>
#define BUFFERSIZE 4096
#define COFYMODE 0644

void die(char* string1, char* string2); //print error and quit


void die(char* string1,char* string2){
    fprintf(stderr,"Error: %s ",string1);
    perror(string2);
    exit(1);
    }


int main(int argc, char *argv[])
{
    int source_fd,target_fd,n_chars;
    char buf[BUFFERSIZE];

    if(argc!=3){
        fprintf(stderr,"usage: %s source destination\n",*argv);
        exit(1);
        }

    //try to open files
    if((source_fd=open(argv[1],O_RDONLY))==-1)
        die("cannot open ",argv[1]);
    if((target_fd=creat(argv[2],COFYMODE))==-1)
        die("can not create ",argv[2]);

    //copy from source to target
    while((n_chars=read(source_fd,buf,BUFFERSIZE))>0){
        if(n_chars!=write(target_fd,buf,n_chars))
            die("Write error to ",argv[2]);
            }
    if(-1==n_chars)
        die("read error from ",argv[1]);


    //close both files
    if(close(source_fd)==-1 || close(target_fd)==-1)
        die("Error closing files","");

    return 0;
    }

第二个版本：使用了mmap函数把文件映射入内存，应该是对文件频繁读写最适合的一种解决方式。
1

#include <apue.h>
#include <sys/mman.h>
#include <fcntl.h>
/* cp3.c --a copy program using memory-mapped I/0 */

#define COFYMODE 0666

void die(char* string1,char* string2){
    fprintf(stderr,"Error: %s ",string1);
    perror(string2);
    exit(1);
}


int main(int argc, char *argv[])
{
    int in_fd,out_fd;
    size_t filesize;
    char nullbyte;
    void *source_addr;
    void *dest_addr;

    /* check args */
    if(argc!=3){
        fprintf(stderr,"usage: %s source destination \n", *argv);
        exit(1);
    }

    /* open files */
    if((in_fd=open(argv[1],O_RDONLY))==-1)
        die("Cannot open ",argv[1]);


    #if defined(i386) || defined(__x86_64) || defined(__x86_64__) || defined(i686)
        if((out_fd=open(argv[2],O_RDWR|O_CREAT|O_TRUNC,COFYMODE))==-1)
            die("Can not create ",argv[2]);

    #else
        if((out_fd=open(argv[2],O_WRONLY|O_CREAT|O_TRUNC,COFYMODE))==-1)
            die("can not create ",argv[2]);

    #endif
    /* get the size of the source file by seeking to the end of it :
    lseek ( ) returns the offset location of the file pointer after
    the seek relative to the beginning of the file , so this is a
    good way to get an opened file
    ×/
    if((filesize=lseek(in_fd,0,SEEK_END))==-1)
        die("Could not seek to end of file",argv[1]);


    /* by seeking to filesize in the new file can be grown to that size,
    * its size does not change until a write occurs
    */
    lseek(out_fd,filesize-1,SEEK_SET);




    /* so we write the NULL byte and file size is now set to filesize */
    write(out_fd,&nullbyte,1);


    /* time to setup the memory maps */
    if((source_addr=mmap(NULL,filesize,PROT_READ,MAP_SHARED,in_fd,0))==(void *)-1)
        die("Error mapping file ",argv[1]);


    if((dest_addr=mmap(NULL,filesize,PROT_WRITE,MAP_SHARED,out_fd,0))==(void *)-1)
        die("Error mapping file, ",argv[2]);


    /* copy the input to output by doing a memcpy */
    memcpy(dest_addr,source_addr,filesize);


    /*unmap the fiels */
    munmap(source_addr,filesize);
    munmap(dest_addr,filesize);


    /* close the files */
    close(in_fd);
    close(out_fd);


    return 0;
    }

我还顺便用strace 来看了下：

[root@c653f1u25 system_program]# strace ./cp3 1.txt 4.txt
execve("./cp3", ["./cp3", "1.txt", "4.txt"], [/* 38 vars */]) = 0
brk(0) = 0x11f4000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb194b72000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=150792, ...}) = 0
mmap(NULL, 150792, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fb194b4d000
close(3) = 0
open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\34\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=2107760, ...}) = 0
mmap(NULL, 3932736, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fb194591000
mprotect(0x7fb194747000, 2097152, PROT_NONE) = 0
mmap(0x7fb194947000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b6000) = 0x7fb194947000
mmap(0x7fb19494d000, 16960, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fb19494d000
close(3) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb194b4c000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fb194b4a000
arch_prctl(ARCH_SET_FS, 0x7fb194b4a740) = 0
mprotect(0x7fb194947000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ) = 0
mprotect(0x7fb194b73000, 4096, PROT_READ) = 0
munmap(0x7fb194b4d000, 150792) = 0
open("1.txt", O_RDONLY) = 3
open("4.txt", O_RDWR|O_CREAT|O_TRUNC, 0666) = 4
lseek(3, 0, SEEK_END) = 16
lseek(4, 15, SEEK_SET) = 15
write(4, "\0", 1) = 1
mmap(NULL, 16, PROT_READ, MAP_SHARED, 3, 0) = 0x7fb194b71000
mmap(NULL, 16, PROT_WRITE, MAP_SHARED, 4, 0) = 0x7fb194b70000
munmap(0x7fb194b71000, 16) = 0
munmap(0x7fb194b70000, 16) = 0
close(3) = 0
close(4) = 0
exit_group(0) = ?
+++ exited with 0 +++

看到了很多熟悉的system call.
execve,brk lseek,哈哈。