问题描述
我试图读取UTF文件，并决定以二进制模式读取它，并跳过非ASCII，因为文件基本上由有效的英文文本组成.我被困在 fread 中，返回1而不是请求的字节数.恕我直言，print_hex的第一个输出显示它已读取多个字符.我已经阅读了一些用C读取二进制文件的示例，例如读取和写入二进制文件文件在C中?，请阅读有关 fread 的信息，例如此处 https://en.cppreference.com/w/c/io/fread和此处 fread实际如何工作?，仍然困惑为什么返回1.文件hexdump，并在下面完成C代码和输出.
I'm trying to read UTF file and decided to read it in binary mode and skip non-ASCII as file consists of valid english text basically. I'm stuck at fread returning 1 instead of number of bytes requested. First output of print_hex IMHO shows it has read more than 1 char. I've read some examples of reading binary files in C e.g Read and write to binary files in C?, read about fread e.g. here https://en.cppreference.com/w/c/io/fread and here How does fread really work?, still puzzled why it returns 1. File hexdump, and complete C code and output below.
ADD:由gcc编译，可在Linux上运行.
ADD: compiled by gcc, run on Linux.
文件:
00000000 ff fe 41 00 41 00 42 00 61 00 0d 00 0a 00 41 00 |..A.A.B.a.....A.|
00000010 41 00 45 00 72 00 0d 00 0a 00 66 00 73 00 61 00 |A.E.r.....f.s.a.|
00000020 6a 00 0d 00 0a 00 64 00 73 00 61 00 66 00 64 00 |j.....d.s.a.f.d.|
00000030 73 00 61 00 66 00 64 00 73 00 61 00 0d 00 0a 00 |s.a.f.d.s.a.....|
00000040 64 00 66 00 73 00 61 00 0d 00 0a 00 66 00 64 00 |d.f.s.a.....f.d.|
00000050 73 00 61 00 66 00 73 00 64 00 61 00 66 00 0d 00 |s.a.f.s.d.a.f...|
00000060 0a 00 0d 00 0a 00 0d 00 0a 00 |..........|
代码:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void print_hex(const char *s)
{
while(*s)
printf("%02x ", (unsigned char) *s++);
printf("\n");
}
int main(){
#define files_qty 5
const char* files_array[][2]={{"xx","a"},{"zz","b"},{"xxx","d"},{"d","sd"},{"as","sd"}};
const char* file_postfix = ".txt";
char* file_out_name = "XXX_AD.txt";
FILE* file_out = fopen (file_out_name, "w");
printf ("This app reads txt files with hardcoded names and writes to file %s\n",file_out_name);
/* ssize_t bytes_read = 1; //signed size_t */
size_t n_bytes = 10;
unsigned char* string_in;
char* string_out;
char* file_name;
string_in = (char*) malloc (n_bytes+1);
string_out = (char*) malloc (n_bytes+50);
file_name = (char*) malloc (n_bytes+1); /* more error prone would be to loop through array for max file name length */
int i;
size_t n;
for (i=0;i<files_qty;i++)
{
strcpy (file_name,files_array[i][0]);
FILE* file = fopen (strcat(file_name,file_postfix), "rb");
if (file!= NULL)
{
int k=0;
while (n=fread (string_in, sizeof(char), n_bytes, file)>0)
{
printf("bytes read:%lu\n",(unsigned long) n);
print_hex(string_in);
int j;
for (j=0;j<n;j++)
{
switch (string_in[j])
{
case 0x00:
case 0xff:
case 0xfe:
case 0x0a:
break;
case 0x0d:
string_out[k]=0x00;
fprintf (file_out, "%s;%s;%s\n", files_array[i][0], files_array[i][1], string_out);
k=0;
printf("out:\n");
print_hex(string_out);
break;
default:
string_out[k++]=string_in[j];
}
}
}
fclose (file);
}
else
{
perror (file_name); /* why didn't the file open? */
}
}
free (string_in);
free (string_out);
free (file_name);
return 0;
}
输出:
bytes read:1
ff fe 41
bytes read:1
0d
out:
bytes read:1
72
bytes read:1
61
bytes read:1
73
bytes read:1
61
bytes read:1
0d
out:
72 61 73 61
bytes read:1
61
bytes read:1
73
bytes read:1
61
bytes read:1
0a
推荐答案
您有优先级问题.简单分配的优先级比比较低.所以下面的行:
You have a precedence problem. Simple assignment has lower precedence than comparison. So the following line:
while(n=fread (string_in, sizeof(char), n_bytes, file)>0)
被评估为(附加括号)
while (n=(fread (string_in, sizeof(char), n_bytes, file)>0))
因此 n 被分配为1，因为 fread 返回的值> 0
Therefore n is being assigned as 1 because fread is returning a value > 0
相反，将括号显式添加为:
Instead, explicitly add parenthesis as:
while((n=fread (string_in, sizeof(char), n_bytes, file))>0)
这篇关于fread为什么以二进制模式读取的以ff fe开头的文件返回1的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！