How to fill an array of structs using a pointer while reading from a file in C

Question

the following has poped up while studying and I would like whether it does what it is supposed to. Let's say we have the following struct:

typedef struct a{
      int x;
      int y;
}a;

And we have a binary file where we can find info about multiple instances of the above struct, and we want to have an array of these structs and fill them one by one. Can we do the following?

a* aStruct= malloc(sizeof(a)*10); // aStruct[10]
a* temp;
int i = 0;
while(i < 10){
        temp = aStruct+i++;
        fread(&temp->x, sizeof(int), 1, inputFile);
        fread(&temp->y, sizeof(int), 1, inputFile);
}

Does the above means that in the end, the array aStruct will be filled with the contents from the file? If not, how can we do it?

I would use fixed-width integers (<stdint.h>), as you will probably know the size of each integer in the binary file, so that it isn't dependent on a specific implementation of int. — alx - recommends codidact
– alx - recommends codidact, Commented May 7, 2019 at 15:38
While this is unlikely for the example struct, generalizing could lead to the possibility of padding in the struct. Then the read would depend on how the file was written -- the answers given so far (including OP's) would work for sequentially written members or structs with no padding. However, a struct written with padding would have to be read a struct at a time on an architecture with similar padding requirements, or have the padding handled explicitly. — mpez0
– mpez0, Commented May 7, 2019 at 15:45

Barmar · Accepted Answer · 2019-05-07 15:38:54Z

2

Yes, that should work. But there's no need for the temp variable.

for (i = 0; i < 10; i++) {
    fread(&(aStruct[i].x), sizeof aStruct[i].x, 1, inputFile);
    fread(&(aStruct[i].y), sizeof aStruct[i].y, 1, inputFile);
}

It's generally more idiomatic and easier to read if you use array indexing notation when you're using a pointer as an array.

I used sizeof aStruct[i].x rather than sizeof(int) so that it will automatically pick up the type from the structure declaration, rather than requiring you to keep them in sync if the structure changes.

edited May 7, 2019 at 15:38

answered May 7, 2019 at 15:33

Barmar

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3 Comments

tzegian Over a year ago

In the sizeof, can I use int for example instead of aStruct[i].x if I am pretty sure I will find an int there, or not?

Barmar Over a year ago

Using the variable name means that if you change the type of the variable, you don't have to remember to change what's in the sizeof.

tzegian Over a year ago

I see. Thank you.

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