c - 系统调用 open()

Question

我试图通过系统调用获取 txt 文件的内容，但是当我的程序再次调用 open() 函数时，缓冲区得到了奇怪的数据。

void delFunction(){
int BUF_SIZE=8192;
int line=0,cont=0,x=0,y=0;
char buf[BUF_SIZE];
char buf2[BUF_SIZE];
char buf3[BUF_SIZE];
fflush(stdin);
int filedesc = open("testfile.txt", O_RDONLY);
read(filedesc, &buf, BUF_SIZE);
close(filedesc);
printf("BUFFER: \n");
puts(buf);
printf("Choose the line you want delete\n");
scanf("%d",&line);

for(x=0;x<strlen(buf);x++){
    if(line==cont){
    }else{
        buf2[y]=buf[x];
        y++;`
    }
    if(buf[x]=='\n'){
        puts(buf2);
        cont++;
    }
}
int filedesc2 = open("testfile.txt", O_WRONLY | O_TRUNC);
write(filedesc2,buf2, strlen(buf2));
close(filedesc2);
buf2[0]='\0';

第一次程序运行良好，但第二次缓冲区获取.txt内容加上错误的数据

Answer 1

根据 the read manual :

Upon successful completion, read() [XSI] [Option Start] and pread() [Option End] shall return a non-negative integer indicating the number of bytes actually read.

...您需要使用返回值来确定实际读取了多少字节(您可能不应该在此处的 buf 之前使用＆符号)。

例如。 ssize_t how_many_bytes = read(filedesc, buf, BUF_SIZE);

puts 不允许您指定 how_many_bytes，所以也许您最好使用 fwrite。

例如。 fwrite(buf, 1, how_many_bytes, stdout);

同样，the scanf manual告诉你如何使用scanf的返回值:

Upon successful completion, these functions shall return the number of successfully matched and assigned input items; this number can be zero in the event of an early matching failure. If the input ends before the first matching failure or conversion, EOF shall be returned. If a read error occurs, the error indicator for the stream is set, EOF shall be returned, [CX] [Option Start] and errno shall be set to indicate the error. [Option End]

例如。 if (scanf("%d", &line) != 1) { 退出(0); }

Don't flush stdin ...