C编程文件大小

Question

我试图在创建文件并向其写入数据后获取文件的大小.我得到的值似乎与实际文件大小不符.这是我的计划.请告诉我如何以Bits,Bytes,Kilobytes和Megabytes显示文件大小.据我说,文件大小应该是288位,36字节,0.03515626千字节和0.000034332兆字节.

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h> 
#include <sys/types.h> 
#define PERMS 0777

int main(int argc, char *argv[])
{
    int createDescriptor;
    int openDescriptor;

    char fileName[15]="Filename1.txt";
    umask(0000);

    if ((openDescriptor = creat(fileName, PERMS )) == -1)
    {
        printf("Error creating %s", fileName);
        exit(EXIT_FAILURE);
    }


    if(write(openDescriptor,"This will be output to testfile.txt\n",36 ) != 36)
    {
        write(2,"There was an error writing to testfile.txt\n",43);
        return 1;
    }

    if((close(openDescriptor))==-1)
    {
        write(2, "Error closing file.\n", 19);
    }
    struct stat buf;
    fstat(openDescriptor, &buf);
    int size=buf.st_size;
    printf("%d\n",size);
    printf("%u\n",size);

    return 0;
}

Answer 1

该fstat()函数有一个返回码,检查它.

int r = fstat(openDescriptor, &buf);
if (r) {
    fprintf(stderr, "error: fstat: %s\n", strerror(errno));
    exit(1);
}

这将打印:

error: fstat: Bad file descriptor

是的......你关闭了文件描述符,它不再是文件描述符了.你必须fstat()在打电话之前close().

代码让我担心.

这非常脆弱,在任何情况下都不能推荐:

if (write(openDescriptor,"This will be output to testfile.txt\n",36 ) != 36)

你可以这样做:

const char *str = "This will be output to testfile.txt\n";
if (write(fd, str, strlen(str)) != strlen(str))

它将编译为相同的机器代码,并且它显然是正确的(与原始代码相反,您必须计算字符串中的字符数以确定它是否正确).

更好的是,当您使用时stderr,只需使用标准<stdio.h>功能:

fprintf(stderr, "There was an error writing to %s: %s\n",
        fileName, strerror(errno));

定义时出现相同的错误fileName...

// You should never have to know how to count higher than 4 to figure
// out if code is correct...
char fileName[15]="Filename1.txt";

// Do this instead...
static const char fileName[] = "Filename1.txt";

你实际上错误计算了这个时间,[15]应该是[14],但最好把它留给编译器.使编译器的工作更容易没有任何好处,因为编译器可能没有更好的事情要做.

关于机器代码:

$ cat teststr.c
#include <unistd.h>
void func(int openDescriptor) {
    write(openDescriptor,"This will be output to testfile.txt\n",36 );
}
$ cat teststr2.c
#include <string.h>
#include <unistd.h>
void func(int openDescriptor) {
    const char *str = "This will be output to testfile.txt\n";
    write(openDescriptor, str, strlen(str));
}
$ cc -S -O2 teststr.c
$ cc -S -O2 teststr2.c
$ diff teststr.s teststr2.s
1c1
<       .file   "teststr.c"
---
>       .file   "teststr2.c"

是的.如图所示,调用strlen()实际上不会导致不同的机器代码.