read()返回文件中的额外字符 [英] read() return extra characters from file

问题描述

我正在尝试从文件读取文本以进行打印..在尝试提供char缓冲区大小时，它返回了一些额外的字符..

I am trying to read text from file to print.. While trying if I give the char buffer size it returns some extra character ..

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

int main(){

    int fd = open("text.txt",O_RDONLY);

    char cbuffer[100];
    int a =0;
    if(fd>0){
        puts("File open");
        ssize_t len = read(fd,cbuffer,sizeof(cbuffer));
        a =printf("%s",&cbuffer);
        printf("\n return data count  %d",a);
    }

    return 0;

}

如果不是

ssize_t len = read(fd,cbuffer,sizeof(cbuffer));

到

ssize_t len = read(fd,cbuffer,10);

精确返回10个字符. 谁能解释为什么会这样?

returns 10 chars exactly . Can anyone explain why this is happening?

推荐答案

之所以会发生这种情况，是因为read()不会对输出进行空终止.在将其用作 string 之前，您需要对目标缓冲区进行空终止.

This happens because, read() does not null-terminate the output. You need to null-terminate the destination buffer before using it as a string.

基本上将非空终止的char数组传递给printf()作为%s的参数，从而创建未定义的行为，因为可能会有超出范围的内存访问.

Basically passing a non-null terminated char array to printf() as the argument to %s creates undefined behavior as there can be out of bound memory access.

实现此目标的一种方法是0-初始化目标.这样，在read()读取并存储有效值之后，它将被视为以空值终止.像

One way of achieving this is to 0-initialize the destination. That way, it will be considered null-terminated after the valid values read and stored by read(). Something like

  char cbuffer[100] = {0};

可以提供帮助.

也就是说，改变

 printf("%s",&cbuffer);

到

printf("%s",cbuffer);

as %s需要一个指向以char s为空终止的数组的指针.将数组名称传递给函数时，它会衰减到指向第一个元素的指针，因此应该可以.

as %s expects a pointer to a null-terminated array of chars. When you pass the array name to a function, it decays to the pointer to the first element, so you should be ok.

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