A special Segmentation fault,I don't know why

Question

#include<stdio.h>
int main(){
    int count[26];
    for(int i=0;i<26;i++){
        count[i]=0;
    }
    printf("xx1");
    FILE *p;
    printf("xx");
    p=fopen("test.txt","r");
    if(p==NULL)
        printf("error");
    int x=fgetc(p);
    while(x!='\0'){
        count[x-97]++;
        x=getc(p);
        }
    for(int i=0;i<26;i++){
        printf("%c:%d\t",i+97,count[i]);
    }
    fclose(p);
    return 0;
}

here are my codes,when I run to getc().Mistake happend...

Program received signal SIGSEGV, Segmentation fault. _IO_getc (fp=0x0) at getc.c:37 37 getc.c: 没有那个文件或目录.

`count[x-97]` doesn't make any sense and is probably the reason for the bug. — Lundin, May 11 '20 at 08:46
What input do you provide? What do you expect to happen when you enter `\n` or any non-alphabet character? — Gerhardh, May 11 '20 at 08:49
Try changing `count[x-97]++;` to `if (x>='a' && x<='z') count[x-'a']++;', and perhaps change `while(x!='\0')` to `while(x!=EOF)`. You're unlikely to find any zero bytes inside a text file. — r3mainer, May 11 '20 at 08:50
@r3mainer Use `islower()` rather than testing character ranges yourself. — Barmar, May 11 '20 at 08:51
OT: your test `if(p==NULL) printf("error");` is bogous. You do not only need to print the error message but you need to prevent any access to the invalid FILE pointer.. — Jabberwocky, May 11 '20 at 08:53
Thanks all your guys.Many mistakes exit in my codes.I will correct them you mentioned and be careful later. — P.rong, May 11 '20 at 09:17

score 0 · Answer 1 · answered May 11 '20 at 08:57

So to summarize the comments:

First of all, '\0' is end of string, not end-of-file. So instead of:

while(x!='\0'){

you should do this:

while (x != EOF) {

Secondly, you should verify that x-97 is in the index range of count before doing:

count[x-97]++;

Third, your function should not continue if p == NULL.

A special Segmentation fault,I don't know why

1 Answers1