Array of struct within struct (C)

Question

I am trying to set up an array of structs within a struct in C. The size of the array is not known until runtime, so I have tried to define the array as dynamic. I have reduced my problem to the code shown below (there are a few more members in each struct, but don't pose a problem).

//struct definitions
struct intStream {
int test;
};

struct io {
struct inStream **inputStream;
};

// then proceed to main
int numDevices = 4; //(derived from number of devices found at runtime)
struct io *devices;
devices->inputStream = malloc(sizeof(struct inStream) * numDevices)

// Fails on
devices->inputStream[0]->test = 1;

I am still relatively new to C style programming, so my understanding of memory is not complete, but through some searching I have not been able to find alternate solutions to my problem besides the struct hack which aesthetically I do not like, as well as wanting the option to include more arrays in future.

You have an unitialised variable devices. What do you think it is pointing at? — n. m. could be an AI
– n. m. could be an AI, Commented Nov 22, 2016 at 10:56
@n.m. I have been shown the error of my ways ;) not too sure what I expected looking back! — Rampartisan
– Rampartisan, Commented Nov 22, 2016 at 11:27
@SouravGhosh Sure, I just wanted to include a "fuller" version of the answer, thanks. — Rampartisan
– Rampartisan, Commented Nov 22, 2016 at 11:27
There is no array of structs in your code. Nor is there an array of pointers to struct in your struct. If you need a flexible array, use a flexible array member. — too honest for this site
– too honest for this site, Commented Nov 22, 2016 at 11:31

Sourav Ghosh · Accepted Answer · 2016-11-22 11:30:07Z

5

In this code, devices is uninitialized. Using uninitialized memory invokes undefined behavior.

You need to make devices point to some valid memory before you can dererefence that pointer.

That said,

 devices->inputStream = malloc(sizeof(struct inStream) * numDevices)

also looks wrong. What you may want is

 devices->inputStream = malloc(sizeof(struct inStream *) * numDevices);

and then, you need to allocate memory to each devices->inputStream[i], also.

[Modified code by OP, removed from question and added into the answer, just for reference]

struct io *devices;
devices = malloc(sizeof(*devices));  

devices->inputStream = malloc(sizeof(struct inStream *) * numDevices);

for(int i = 0; i < 4; i++) {  
    devices->inputStream[i] = malloc(sizeof(struct inStream));  
}

edited Nov 22, 2016 at 11:30

answered Nov 22, 2016 at 10:55

Sourav Ghosh

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2 Comments

Rampartisan Over a year ago

Thanks for the solution, makes perfect sense in the way you explained. I tried to put in a full form answer in this comment but it didnt work, so I have updated the question with your advice/solution too.

Sourav Ghosh Over a year ago

@user2142136 I have corrected and added your code in my answer itself, hope that's ok. :)

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