C - scanf行为不端

Question

当使用第一个scanf()并回答Y时,第二个scanf()会直接跳到"No option selected.退出......".当密钥文件大于源文件并且最后一个scanf正常工作时,也会出现该消息.所以我在这里不知所措,出了什么问题？(代码编译得很好,所以随意尝试)

编辑:对于downvoters至少发布一个理由会有所帮助.我不是一个非常优秀的程序员,只是想在这里学习.

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>

int main(int argc, char **argv)
{
struct stat statbuf;
struct stat keybuf;

int key;
int data;
int output;
int count;
char ans;
FILE * keyfile;
FILE * sourcefile;
FILE * destfile;

if(argc<4)
{
printf("OTP-Bunny 1.0\n");
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
return (0);
}

/* Check number of arguments. */
if(argc>4)
{
printf("Too many arguments.\n");
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
return(1);
}

/* Check if sourcefile can be opened. */
if((sourcefile = fopen(argv[1], "rb"))== NULL)
{
printf("Can't open source file.\n");
printf("Please enter a valid filename.\n");
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
perror("Error");
return (1);
}

/* Get size of sourcefile */
fstat(fileno(sourcefile), &statbuf); 

/* Check if keyfile can be opened. */
if((keyfile = fopen(argv[3], "rb"))== NULL)
{
printf("Can't open keyfile.\n");
printf("Please enter a valid filename.\n"); 
printf("USAGE: OTP <source file> <output file> <keyfile>\n");
perror("Error");
return(1);
}                               

/* Get size of keyfile */
fstat(fileno(keyfile), &keybuf);

/* Check if keyfile is the same size as, or bigger than the sourcefile */
if((keybuf.st_size) < (statbuf.st_size))
{
printf("Source file is larger than keyfile.\n");
printf("This significantly reduces cryptographic strength.\n");
printf("Do you wish to continue? (Y/N)\n");
scanf("%c", &ans);
if(ans == 'n' || ans == 'N')
{
return (1);
}
if(ans == 'y' || ans == 'Y')
{
    printf("Proceeding with Encryption/Decryption.\n");
    }
else
{
printf("No option selected. Exiting...\n");
return (1);
}
}   

/* Check if destfile can be opened. */
if((keyfile = fopen(argv[2], "wb"))== NULL)
{
printf("Can't open output file.\n");
perror("Error");
return(1);                  
}    

/* Open destfile. */
destfile=fopen(argv[2], "wb");

/* Encrypt/Decrypt and write to output file. */
while(count < (statbuf.st_size))
{
key=fgetc(keyfile);
data=fgetc(sourcefile);

output=(key^data);

fputc(output,destfile);
count++;
}

/* Close files. */
fclose(keyfile);
fclose(sourcefile);
fclose(destfile);

printf("Encryption/Decryption Complete.\n");

/* Delete keyfile option. */
printf("Do you wish to delete the keyfile? (Y/N)\n");
scanf("%c", &ans);
if(ans == 'y' || ans == 'Y')
{
if ( remove(argv[3]) == 0)
    {
    printf("File deleted successfully.\n");
    }
else
    {
    printf("Unable to delete the file.\n");
    perror("Error");
    return(1);
    }
}

if(ans == 'n' || ans == 'N')
{
return(0);
}
else
{
printf("No option selected. Exiting...\n");
}
return(0);
}

Answer 1

实际上,scanf()行为正确,但正确使用它是一个非常棘手的功能,并且您的期望很容易与其记录的行为不同.

您的第一个scanf()电话会读取一个字符,然后留下换行符.

你的第二个scanf()电话会读取换行符,因为那是下一个...导致问题.

您可以使用" %c"; 修复代码; 前导空白跳过(可选)空白区域.

一般来说,更好的解决方案是使用组合fgets()或等效(readline()在POSIX 2008中,也许)和sscanf().使用以下内容将一行数据读入内存fgets(); 分析它sscanf().这样,您也可以更好地报告错误; 您拥有用户键入的所有可用于错误报告的信息.