带有数组的分段错误（核心转储）C 程序

Question

#include <stdio.h>
#include <string.h>
#define SIZE 1000

int main(void)
{
char sent[] = "\0";        
char alpha[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz ";
unsigned int count;
unsigned int k;
unsigned int j;                                                            

printf("Please enter a sentence to analyze\n");
fgets(sent, SIZE, stdin);  


printf("\n     Letter\t ||\tAmount\n");
printf(" ================================\n");

    for(j = 0; alpha[j] != '\0'; j++)
    {
        count = 0;

        for (k = 0; sent[k]!= '\0'; k++)
        {

            if ( alpha[j] == sent[k])
            {                          
                count++;                                   
            }

        }

        printf("\t%c\t ||\t %u\n", alpha[j], count);
        printf(" --------------------------------\n");  
    }  


    return 0;

}

Every time I run this program I get the error "Segmentation Fault (Core Dumped)". However the program seems to run correctly. Why is this happening and what can I do to fix this?

Answer 1

In your code,

 char sent[] = "\0"; 

is allocating the size of the array only equal to the size of the supplied initializer "\\0" (and null-terminator). So, at a later point, by doing

fgets(sent, SIZE, stdin);

you're accessing out of bound memory. This invokes undefined behavior .

To quote C11 standard, chapter §6.7.9

If an array of unknown size is initialized, its size is determined by the largest indexed element with an explicit initializer. [...]

What you need is to provide the array size at definition time like

 char sent[SIZE] = "\0";