用FREAD（）和fseek的读取文件的一部分，C（） [英] Reading part of a file in C using fread() and fseek()

问题描述

我想文件读入缓冲区的大小 BLOCK_SIZE （目前相等块 1000无符号字符）。我的code最初是通过一个发现的块数，将必须按顺序读取整个文件（通常2-4）读取，然后遍历for循环读取文件（忽略了 + 17 + filenamesize 的东西，这是所有需要在节目后。

然而，仅在第一次，当 J = 1 ，它实际上把数据放到 BUF 数组。在其他情况下，当 J 1 = 1 ，的strlen（BUF）收益 0 。

我认为这个问题是不是与使用 fseek的（）阅读它，或内存分配问题之前寻求到一个文件中的第二部分。

任何帮助将是AP preciated得到它来读取文件中的 1000-1999 ^个字符到 BUF 阵列。

附件是code的相关部分：

 无符号字符* BUF;
来源= FOPEN（localPath来，R）;
TEMP =文件大小/ BLOCK_SIZE + 1;为（J = 1; J＆LT; =温度; J ++）{
  如果（J == 1）{
     BUF =（无符号字符*）malloc的（（sizeof的（无符号字符））*（BLOCK_SIZE + 17 + filenamesize））;
     FREAD（BUF + 17 + filenamesize，sizeof的（无符号字符），BLOCK_SIZE，源）;
   }否则如果（J ==临时）{
     BUF =（无符号字符*）malloc的（（sizeof的（无符号字符））*（文件大小+ 5  -  BLOCK_SIZE *（J-1）））;
     fseek的（源，BLOCK_SIZE *（J-1），SEEK_SET）;一个警告//关闭
     FREAD（BUF + 5的sizeof（无符号字符），文件大小 -  BLOCK_SIZE *（J-1），源）;
   }其他{
     BUF =（无符号字符*）malloc的（（sizeof的（无符号字符））*（5 + BLOCK_SIZE *（J-1）））;
     fseek的（源，BLOCK_SIZE *（J-1），SEEK_SET）;一个警告//关闭
     FREAD（BUF + 5的sizeof（无符号字符），BLOCK_SIZE，源）;
   }
   //做的东西跟这里BUF   BUF =;
   免费（BUF）;
}
 

解决方案

我会建议您检查的 fseek的和 FREAD 。特别是，要确保fseek的是返回0 - 如果不是，这可能是整个问题

只要fseek的正在取得成功，FREAD应该告诉你读取的字节总数。

此外，strlen的是不是一定要使用有效的东西，因为它会假设这是一个空结尾的字符串。如果你读了第一个字符是0字节，strlen的将返回0。你不是这当作一个空结尾的字符串（你是不是对德空终止分配足够的空间 - 恰好是相反的，以满足您的二进制数据需要些什么） ，所以strlen的可能是不合适的。

I'm trying to read a file into a buffer in blocks of size BLOCK_SIZE (currently equal to 1000 unsigned chars). My code initially finds the number of blocks it will have to read in order to read the entire file (usually 2-4), then iterates through a for loop reading the file (ignore the "+17+filenamesize" stuff, that is all needed for later in the program.

However, only on the first time, when j=1, does it actually put data into the buf array. In other cases, when j != 1, strlen(buf) returns 0.

I think the problem is either with the use of fseek() to seek to the second part of a file before reading it or a memory allocation issue.

Any help would be appreciated for getting it to read the 1000-1999^th chars of the file into the buf array.

Attached is the relevant part of the code:

unsigned char *buf;
source = fopen(localpath,"r");
temp = filesize / BLOCK_SIZE + 1;

for (j=1; j <= temp; j++) {
  if (j == 1) {
     buf = (unsigned char *) malloc((sizeof(unsigned char)) * (BLOCK_SIZE + 17 + filenamesize));
     fread(buf+17+filenamesize, sizeof(unsigned char), BLOCK_SIZE, source);
   } else if (j == temp) {
     buf = (unsigned char *) malloc((sizeof(unsigned char)) * (filesize + 5 - BLOCK_SIZE*(j-1)));
     fseek(source, BLOCK_SIZE*(j-1), SEEK_SET); // off by one warning
     fread(buf+5, sizeof(unsigned char), filesize - BLOCK_SIZE*(j-1), source);
   } else {
     buf = (unsigned char *) malloc((sizeof(unsigned char)) * (5+BLOCK_SIZE*(j-1)));
     fseek(source, BLOCK_SIZE*(j-1), SEEK_SET); // off by one warning
     fread(buf+5, sizeof(unsigned char), BLOCK_SIZE, source);
   }
   // do stuff with buf here

   buf = "";
   free(buf);
}

解决方案

I would recommend checking the results of fseek and fread. In particular, make sure fseek is returning 0 - if it's not, this may be the entire problem.

Provided that fseek is succeeding, fread should tell you the total number of bytes read.

Also, strlen is not necessarily a valid thing to use, since it's going to assume that this is a null terminated string. If the first character you read is a 0 byte, strlen will return 0. You're not treating this as a null terminated string (you aren't allocating enough space for teh null terminator - just exactly what's needed to fit your binary data), so strlen is probably inappropriate.

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