为什么只给字符分配一个字符指针，而给它分配一个字符串呢? [英] Why character pointer is getting assigned just a character, while a string is supposed to get assigned to it?
问题描述
一个字符串包含三个由空格分隔的部分.前两部分是字符串，第三部分是整数.
A string contains three parts separated by space. First two parts are strings and the third part is an integer.
以下程序的输出令我惊讶.
The output of the below program is surprising me.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void split(char *ch,char **part1,char **part2,int *num){
int length=strlen(ch), *lc1,*lc2;
int i=0,j=0;
printf("value of length is %d\n",length);
printf("this function recieved %s for splitting into pieces\n",ch);
lc1=(char *)malloc(length*sizeof(char));
lc2=(char *)malloc(length*sizeof(char));
while(ch[i]!=' '){
lc1[i]=ch[i];
printf("\nin loop with lc1[i] = %c and ch[i] = %c",lc1[i],ch[i]);
i++;
}
lc1[i]='\0';
i++;
while(ch[i]!=' '){
lc2[j]=ch[i];
printf("\nin loop with lc2[j] = %c and ch[i] = %c",lc2[j],ch[i]);
j++;
i++;
}
lc2[j]='\0';
i++;
*num=atoi(&ch[i]);
*part1=lc1;
*part2=lc2;
printf("\nsplit results are:\n");
printf("part1=%s and part2=%s and num=%d and lc1=%s lc2=%s //this is surprising me",*part1,*part2,*num,lc1,lc2);
}
int main()
{
int N,i,j,n,*numArray,count=0;
char **arr,*part1,*part2,*token;
scanf("%d",&N);
arr=malloc(N*sizeof **arr);
numArray=malloc(N*sizeof *numArray);
for(i=0;i<N;i++){
arr[i]=(char *)malloc(50*sizeof(char));
}
for(i=0;i<N;i++){
printf("plz enter %d th :",i);
scanf(" ");
gets(&arr[i][0]);
}
for(i=0;i<N;i++){
printf("%s",arr[i]);
}
for(i=0;i<N;i++){
/*token=strtok(arr[i]," ");
part1=token;
token=strtok(NULL," ");
part2=token;
token=strtok(NULL," ");
n=atoi(token);*/
split(arr[i],&part1,&part2,&n);
//some logic to use part1 and part2 of the sentence
}
return 0;
}
我提供的输入如下:
1
abcd efgh 2
输入的第一行包含我要继续进行的句子数. 我得到的输出如下:
The first line of the input contains number of sentences I want to proceed. Output I got is as below:
plz enter 0 th :abcd efgh 2value of length is 11
this function recieved abcd efgh 2 for splitting into pieces
in loop with lc1[i] = a and ch[i] = a
in loop with lc1[i] = b and ch[i] = b
in loop with lc1[i] = c and ch[i] = c
in loop with lc1[i] = d and ch[i] = d
in loop with lc2[j] = e and ch[i] = e
in loop with lc2[j] = f and ch[i] = f
in loop with lc2[j] = g and ch[i] = g
in loop with lc2[j] = h and ch[i] = h
split results are:
part1=a and part2=e and num=2 and lc1=a lc2=e //this is surprising me
Success time: 0 memory: 2296 signal:0
plz enter 0 th :abcd efgh 2value of length is 11
this function recieved abcd efgh 2 for splitting into pieces
in loop with lc1[i] = a and ch[i] = a
in loop with lc1[i] = b and ch[i] = b
in loop with lc1[i] = c and ch[i] = c
in loop with lc1[i] = d and ch[i] = d
in loop with lc2[j] = e and ch[i] = e
in loop with lc2[j] = f and ch[i] = f
in loop with lc2[j] = g and ch[i] = g
in loop with lc2[j] = h and ch[i] = h
split results are:
part1=a and part2=e and num=2 and lc1=a lc2=e //this is surprising me
尽管split()中的循环函数已针对字符串中的所有字符成功执行，但我在part1(也在lc1中)和part2(也在lc2中)中仅得到了一个字符.此输出的原因是什么?
Although the loop function inside the split() is getting executed successfully for all the characters in the string, yet I am getting just a character in part1 (also in lc1) and part2 (also in lc2). What is the reason for this output?
推荐答案
此行是错误的:
arr=malloc(N*sizeof **arr);
应为:
arr=malloc(N*sizeof *arr);
**arr是一个char，因此它仅为N个字节分配空间，但是您需要为N个指针(4个字节)分配空间.因此，您没有分配足够的空间，而是在数组范围之外进行写入，从而导致行为未定义.
**arr is a char, so it's only allocating space for N bytes, but you need space for N pointers, which are 4 bytes. So you're not allocating enough space, and then you're writing outside the array bounds, resulting in undefined behavior.
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