Writing an array of structs to a binary file in C

Question

I have an array of structs I would like to write to a binary file. I have a write.c program and a read.c program. The write.c program seems to be working properly but when I run the read.c program I get a segmentation fault. I'm new to C so It would be great if someone could look over my code for any obvious errors. I promise it's not too long :)

write.c:

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

struct Person 
{
    char f_name[256];
    char l_name[256];
    int age;
};

int main(int argc, char* argv[])
{
    struct Person* people;
    int people_count;

    printf("How many people would you like to create: ");
    scanf("%i", &people_count);
    people = malloc(sizeof(struct Person) * people_count);  

    int n;
    for (n = 0; n < people_count; n++)
    {
        printf("Person %i's First Name: ", n);
        scanf("%s", people[n].f_name);

        printf("Person %i's Last Name: ", n);
        scanf("%s", people[n].l_name);

        printf("Person %i's Age: ", n);
        scanf("%i", &people[n].age);
    }

    FILE* data;
    if ( (data = fopen("data.bin", "wb")) == NULL )
    {
        printf("Error opening file\n");
        return 1;   
    }

    fwrite(people, sizeof(struct Person) * people_count, 1, data);
    fclose(data);

    return 0;
}

read.c:

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

struct Person 
{
    char f_name[256];
    char l_name[256];
    int age;
};

int main(int argc, char* argv[])
{
    FILE* data;
    if ((data = fopen("data.bin", "rb")) == NULL)
    {
        printf("Error opening file\n");
        return 1;
    }

    struct Person* people;

    fread(people, sizeof(struct Person) * 1/* Just read one person */, 1, data);
    printf("%s\n", people[0].f_name);

    fclose(data);

    return 0;
}

Thanks for the help!

How many times a day does this question get asked? There should be a 'C' FAQ that will allow us to point to the answer rather than spending the time writing all this down time after time after time...... — KevinDTimm
– KevinDTimm, Commented Dec 6, 2010 at 16:57

aschepler · Accepted Answer · 2010-12-06 16:37:51Z

6

struct Person* people;

This allocates just a pointer to struct, but you don't have any allocated space for actual struct contents. Either use malloc similarly to your write program, or try something like:

struct Person people;
fread(&people, sizeof(people), 1, data);

answered Dec 6, 2010 at 16:37

aschepler

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Comments

Przemek Kryger · Accepted Answer · 2010-12-08 13:52:01Z

4

You need to allocate memory for the person first. Change: struct Person* people; into struct Person* people = malloc(sizeof(struct Person));. And don't forget to free your memory at the end: free(people);.

edited Dec 8, 2010 at 13:52

answered Dec 6, 2010 at 16:39

Przemek Kryger

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4 Comments

John Jacquay Over a year ago

Can you explain when the (struct Person*) means before the malloc(sizeof(struct Person))? Is that a cast?

Przemek Kryger Over a year ago

@jjacquay712 Yes it is. I just like to not relay too much on default compiler options.

Karl Knechtel Over a year ago

Please do not cast the result from malloc. It is standard and well-defined C behaviour, and idiomatic C to leave it out. You are guaranteed to get an implicit cast. If you do cast, the compiler is no longer able to diagnose the error if you forget to #include <stdlib.h>.

Przemek Kryger Over a year ago

@Karl Knechtel Thanks, for pointing this out. Edited and removed casting.

Steve Townsend · Accepted Answer · 2010-12-06 16:37:54Z

2

You either need to malloc memory into the pointer variable people before you do the fread, or (easier) just read directly into a local variable:

struct Person people;

fread(&people, sizeof(struct Person) * 1/* Just read one person */, 1, data);

answered Dec 6, 2010 at 16:37

Steve Townsend

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Comments

Marcelo Cantos · Accepted Answer · 2010-12-06 16:37:59Z

2

You need to allocate space for the data you are reading in:

people = malloc(sizeof(*people)*numPeople);

answered Dec 6, 2010 at 16:37

Marcelo Cantos

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1 Comment

Steve Townsend Over a year ago

This seems more extensible once OP decides to read > 1 element. Could loop on a single local variable instead.

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