array of structs in c-user input

Question

I am new to programming in general and to C in particular. I am trying to write a program that uses an array of structs, but I am experiencing problems if that struct contains strings. Somehow the compiler crashes after the user has given the last input.

The struct below is just a simplified version containing only one item, because the problem seems to be reading strings into the array. Any help is much appreciated, thanks in advance.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct
{
    char* name;
}student;

int main()
{
    int size;
    printf("enter number of entries\n");
    scanf("%d" , &size);
    student* all=malloc(size*sizeof(student));

    int i;
    for(i=0;i<size;i++)
    {
        printf("enter name\n");
        scanf("%s" , all[i].name);
    }

    return 0;
}

and don't produce any other bugs while you'll be adding this — 4pie0
– 4pie0, Commented Jan 8, 2016 at 13:52
Very good that you include a minimal version of your problem and not your original code!!! That makes this a much better question than just dumping whatever you have on us. Remember to do the same for future questions. — Mattias Lindberg
– Mattias Lindberg, Commented Jan 8, 2016 at 14:02
We still badly need a canonical duplicate for "program crashes when I try to dump stuff into the address of an uninitialized pointer" :( — Lundin
– Lundin, Commented Jan 8, 2016 at 14:26

ameyCU · Accepted Answer · 2016-01-08 14:00:00Z

6

Before taking input scanf("%s" , all[i].name); , you need to allocate memory to all[i].name .

An example-

for(i=0;i<size;i++)
{
    all[i].name=malloc(20*sizeof(*(all[i].name)));
    if(all[i].name!=NULL){
       printf("enter name\n");
       scanf("%19s" , all[i].name);
    }
}
//use these strings
for(i=0;i<size;i++){
       free(all[i].name);                  //free the allocated memory 
}
free(all);

Or in your structure instead of char * ,declare name as a char array (if you don't want to use dynamic allocation)-

typedef struct{
  char name[20];                     //give any desired size
 }student;
/*           no need to free in this case   */

edited Jan 8, 2016 at 14:00

answered Jan 8, 2016 at 13:41

ameyCU

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1 Comment

ameyCU Over a year ago

@iharob Mentioned that .

Danny_ds · Accepted Answer · 2016-01-08 15:55:34Z

0

No memory is allocated for the students names (char* name), so when trying to scanf to that pointer, invalid memory is accessed and the program crashes.

The easiest way is to declare name as an array: char name[28];

The return value of malloc() needs to be checked too, in case there was problem allocating the memory for the students, which would return a NULL pointer. At the end, the allocated memory needs to be freed with free().

For example:

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

typedef struct {
    char name[28];
    unsigned int age;
} student;

int main()
{
    size_t size = 0;
    printf("\nEnter number of entries: ");
    scanf("%zu", &size);
    // add some check for size

    student* students = (student*)malloc(size * sizeof(student));

    if (students == NULL) {
        printf("\nProblem with allocating memory:\n"
               " - size: %zu\n"
               " - total size needed: %zu\n",
               size, size * sizeof(student));
        return 0;
    }

    for (size_t i = 0; i < size; ++i) {
        printf("Enter name: ");
        scanf("%27s", students[i].name);
        printf(" Enter age: ");
        scanf("%u", &students[i].age);
    }

    printf("\nList of students:\n");

    for (size_t i = 0; i < size; ++i) {
        printf("%s (%u)\n", students[i].name, students[i].age);
    }

    free(students); // free the allocated memory

    return 0;
}

edited Jan 8, 2016 at 15:55

answered Jan 8, 2016 at 14:57

Danny_ds

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3 Comments

Marco Over a year ago

Don't forget to clear the input buffer after using scanf: while(getchar() != '\n');

Marco Over a year ago

I edited your answer: the format specifiers for scanf were also wrong.

Danny_ds Over a year ago

@d3l - Thanks, I updated the format specifiers (although "%zu" might not be supported on all (older) compilers). I don't think clearing \n is needed here since we're not scanning for chars.

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