C fread退货<;从二进制文件读取记录时为1
我试图从二进制文件中读取学生记录(SREC)。该文件确保包含至少1条完整记录(上一次运行写入该文件)。在我的输出中,我得到了两条记录的“ERROR:cannotreadrecordfromfile”语句。顶部的printf(readingrecord,numR)显示了3次,因此我知道while循环在达到eof时被输入了3次 有什么想法吗? 提前谢谢C fread退货<;从二进制文件读取记录时为1,c,file-io,binaryfiles,fread,feof,C,File Io,Binaryfiles,Fread,Feof,我试图从二进制文件中读取学生记录(SREC)。该文件确保包含至少1条完整记录(上一次运行写入该文件)。在我的输出中,我得到了两条记录的“ERROR:cannotreadrecordfromfile”语句。顶部的printf(readingrecord,numR)显示了3次,因此我知道while循环在达到eof时被输入了3次 有什么想法吗? 提前谢谢 /*Create and populate all 4 lists if lname list exists from previous usage
/*Create and populate all 4 lists if lname list exists from previous usage*/
if((readL = fopen("last", "rb")) == NULL)
{
printf("\nNew file will be created after server terminates.\n");
created = 1;
}
else
{
created = 0;
/*Read in record from binary file*/
while(!feof(readL))
{
numR++;
printf("Reading record %d\n", numR);
/*Create new temporary SREC*/
newSREC = (SREC*)malloc(sizeof(SREC));
newSREC2 = (SREC*)malloc(sizeof(SREC));
newSREC3 = (SREC*)malloc(sizeof(SREC));
newSREC4 = (SREC*)malloc(sizeof(SREC));
/*Read in one record from file*/
if(fread(newSREC, sizeof(SREC), 1, readL) < 1)
{
newSREC2 = newSREC;
newSREC3 = newSREC;
newSREC4 = newSREC;
firstL = insert(newSREC, 1, firstL);
firstF = insert(newSREC2, 2, firstF);
firstS = insert(newSREC3, 3, firstS);
firstG = insert(newSREC4, 4, firstG);
}
else
{
printf("ERROR: Could not read record from file.\n");
}
}
fclose(readL);
}
/*如果以前使用过lname列表,则创建并填充所有4个列表*/
if((readL=fopen(“last”,“rb”))==NULL)
{
printf(“\n服务器终止后将创建新文件。\n”);
创建=1;
}
其他的
{
已创建=0;
/*从二进制文件读入记录*/
而(!feof(readL))
{
numR++;
printf(“读取记录%d\n”,numR);
/*创建新的临时SREC*/
newSREC=(SREC*)malloc(sizeof(SREC));
newSREC2=(SREC*)malloc(sizeof(SREC));
newSREC3=(SREC*)malloc(sizeof(SREC));
newSREC4=(SREC*)malloc(sizeof(SREC));
/*从文件中读入一条记录*/
if(fread(新记录,sizeof(SREC),1,readL)<1)
{
newSREC2=newSREC;
newSREC3=newSREC;
newSREC4=newSREC;
firstL=插入(newSREC,1,firstL);
firstF=插入(newSREC2,2,firstF);
firstS=插入(newSREC3,3,firstS);
firstG=插入(newSREC4,4,firstG);
}
其他的
{
printf(“错误:无法从文件中读取记录。\n”);
}
}
fclose(readL);
}
EOF的错误指示器仅在尝试读取超出范围时设置
在您阅读的地方测试是否成功阅读
在您的示例中,测试是否准确读取1条记录。来自
fread
手册页:
fread()不区分文件结尾和错误,调用方必须使用feof(3)和feror(3)来确定发生了什么
将测试更改为以下内容:
while (fread(...) > 0)
// ...
if (ferror(file))
// an error happened
表面问题
从表面上看,该代码的错误检测是相反的:
if (fread(newSREC, sizeof(SREC), 1, readL) < 1)
{
newSREC2 = newSREC;
newSREC3 = newSREC;
newSREC4 = newSREC;
firstL = insert(newSREC, 1, firstL);
firstF = insert(newSREC2, 2, firstF);
firstS = insert(newSREC3, 3, firstS);
firstG = insert(newSREC4, 4, firstG);
}
else
{
printf("ERROR: Could not read record from file.\n");
}
深入研究 循环不应使用
feof()
-请参阅。您应该使用以下内容:
SREC srec;
while ((fread(&srec, sizeof(srec), 1, readL) == 1)
{
SREC *newSREC1 = malloc(sizeof(*newSREC1));
SREC *newSREC2 = malloc(sizeof(*newSREC2));
SREC *newSREC3 = malloc(sizeof(*newSREC3));
SREC *newSREC4 = malloc(sizeof(*newSREC4));
if (newSREC1 == 0 || newSREC2 == 0 || newSREC3 == 0 || newSREC4 == 0)
{
free(newSREC1);
free(newSREC2);
free(newSREC3);
free(newSREC4);
…report error…
break;
}
*newSREC1 = srec;
*newSREC2 = srec;
*newSREC3 = srec;
*newSREC4 = srec;
firstL = insert(newSREC1, 1, firstL);
firstF = insert(newSREC2, 2, firstF);
firstS = insert(newSREC3, 3, firstS);
firstG = insert(newSREC4, 4, firstG);
}
// Optional analysis of why the loop ended — feof() vs ferror() vs short read
代码会泄漏内存,就像从来没有内存不足一样。分配了newSREC2
等之后,它会用指向newSREC1
的指针覆盖指针。作业
应该是:
*newSREC2 = *newSREC; // Original code — see above for revision
我注意到您不能在一个数组中分配所有4条记录,因为您几乎肯定需要能够独立地释放这4条记录。这在代码中留下了大量重复。相关:另外,.问题的可能重复不是直接的
,而(!feof(readL))
,尽管那里的注释肯定适用。
*newSREC2 = *newSREC; // Original code — see above for revision