fgets（）多次使用时使C程序崩溃

Question

I have an array declared and populated with 3 strings (Math, Physics, English). I used the fgets() to get new subject to add to the array, and this is working fine. However, whenever I copy the same chunk of code to get another subject from the user, the program crashes.

Why is this happening? How do I get strings from user and add it to the array?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    char *subjectName[1000];
    int numOfSubj = 0;
    int numOfUserAddedSubj = 0;

    subjectName[numOfSubj] = "Math";
    numOfSubj++;
    subjectName[numOfSubj] = "Physics";
    numOfSubj++;
    subjectName[numOfSubj] = "English";
    numOfSubj++;

//int k;
//for(k=0; k<1; k++)
//{
    // add user
    printf("Enter new subject name: ");
    fgets(subjectName[numOfSubj], 50, stdin);
    numOfUserAddedSubj++;
    numOfSubj++;
//}


// add another user
/*printf("Enter new subject name: ");
fgets(subjectName[numOfSubj], 50, stdin);
numOfUserAddedSubj++;
numOfSubj++;*/



// display content of array
int i;
for(i=0; i < ((strlen(subjectName))+numOfUserAddedSubj); i++)
{
    printf("%s\n", subjectName[i]);
}


    return 0;
}

Answer 1

Please note that subjectName is array of type char* . Your first 3 strings defined statically. For the string you want to get via fgets() you need to allocate memory using mallaoc first and only then call fgets() .

Something like:

subjectName[numOfSubj] = (char *) malloc(50*sizeof(char));
fgets(subjectName[numOfSubj], 50, stdin);

Don't forget to free the allocated memory at the end. Please note that you may want to consider different approaches. You may statically define the array: char subjectName[1000][50] and use strcpy() to populate it with "Math", "Physics" and "English".

Answer 2

You have an array of 1000 pointers, but the pointers that are relevant to this problem don't point to anything. You need to malloc some memory for the pointers before calling fgets , eg

printf("Enter new subject name: ");
subjectName[numOfSubj] = malloc( 50 );
fgets(subjectName[numOfSubj], 50, stdin);

Answer 3

It's working in first case because you are initialising the char array with subject name so compiler allocating memory.

In USER case you need to allocate memory before taking input from user.

fgets(subjectName[numOfSubj], 50, stdin); //Here you passed null char array

so to avoid error do

subjectName[numOfSubj] = malloc( 50 ); //memory allocation

 fgets(subjectName[numOfSubj], 50, stdin);

and free the memory (Which is allocated by malloc) whenever you finish the use of it .

free(subjectName[numOfSubj] );