Printing the data in multiple structs - c

Question

I'm working with four differest structs, two of which are quite large. I have a function to write each value of each struct to a .txt file, but the code is very long and robust. I'm looking for a way of printing out each value without having to hard code each in, but everything I've found so far in my research indicates that hardcoding is the only way, but I figured I'd check on here before I give up completely. As it stands right now, my code looks like this:
char text[416];
snprintf(text, 416,
"%d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d %d",
epsy.VBUS_voltage_mV,
epsy.temp_internal_degC,
epsy.status,
batty.Z_pos_Camera_Temperature,
batty.Z_neg_Camera_Temperature,
batty.Y_pos_Camera_Temperature,
batty.Y_neg_Camera_Temperature,
batty.X_pos_Camera_Temperature,
batty.FPGA_Temp_1,
batty.FPGA_Temp_2,
batty.Rx_Hinge_Temperature,
batty.Bat_1_Vbat,
batty.Bat_1_Ichg,
batty.Bat_1_Idch,
batty.Bat_1_MCU_Temp,
batty.Bat_1_Temp_Therm,
batty.Bat_1_Status,
batty.Bat_2_Vbat,
batty.Bat_2_Ichg,
batty.Bat_2_Idch,
batty.Bat_2_MCU_Temp,
... and it goes on for a while. (80 values)
Is there a simpler way of doing this? If so, how do I do it?

Answer 1

The following solution defines a union that combines the actual struct with it's integral members with an array of integral values in order to "view" the struct members as array elements accessible through subscription.
To make this save, we need to control the alignment, since the compiler might add padding between the data members. This would then let the "array-view" point to invalid memory and introduce undefined behaviour.
I suggest to use #pragma pack(n) to control the alignment. Note that the code may also work without this alignment thing, yet it might get a problem if data members of other type are introduced before or after the "integral block" (This would also require to offset the "array-view", but that's not shown here).
The surrounding union is required to guarantee that the array and the struct are actually aligned correctly; otherwise, a cast from a struct to an array might introduce undefined behaviour.
I'm aware that #pragma pack is not portable and that it affects the memory layout and probably speed. Yet it is supposed to work on most compilers, and I think it is needed for controlling alignment in order to avoid UB:
#pragma pack(4) /* set alignment to 4 byte boundary */
#define nrOfMyPackedDataElements 3
union MyPackedData {
struct {
int32_t firstInt;
int32_t secondInt;
int32_t thirdInt;
} data;
int32_t array[nrOfMyPackedDataElements];
};
#pragma pack() /* reset alignment to compiler default */
int main() {
union MyPackedData data;
data.data.firstInt = 10;
data.data.secondInt = 20;
data.data.thirdInt = 30;
for (int i=0; i < nrOfMyPackedDataElements; i++) {
printf("%d ",data.array[i]);
}
return 0;
}

Answer 2

In C11 you could rewrite your structs using unnamed structures and union fields.
Something like this:
#include <stdio.h>
struct dir_t
{
int pos;
int neg;
};
struct cam_temp_t
{
dir_t x;
dir_t y;
dir_t z;
};
// it seems that you have only ints in your structure...
#define TOTAL_N_INTS 8
struct batty_t
{
union
{
int data_[TOTAL_N_INTS];
struct
{
struct cam_temp_t camera_temperature;
int fpga; // you get it...
int bat;
}
};
};
int main(void)
{
struct batty_t example = {
.camera_temperature = {
.x = {3, 4},
.y = {5, 6},
.z = {7, 8}
},
.fpga = 1,
.bat = 2
};
for (int i = 0; i < TOTAL_N_INTS; ++i )
{
printf("%4d", example.data_[i]);
}
return 0;
}
Of course if you have different types, you should use different arrays.

Answer 3

Accessing an int array via a struct and visa-versa like this answer might be valid, might not. Yet I am now not so confident for its use with general types. Leaving this as wiki for anyone to add/amend/delete.
OP later commented that the struct members are not all int. Oh well.
If all members are and forever will be int ...
Access each member, one at a time by int offset.
Some untested code to illustrate the idea:
int print_int_struct(char *dest, size_t d_size, const void *st, size_t s_size) {
if (s_size % sizeof(int) != 0) {
return -1; // bad struct size
}
size_t n = s_size / sizeof(int);
const char *delimiter = "";
for (size_t i = 0; i < n; i++) {
int d;
memcpy(&d, st, sizeof d);
st = (char*) st + sizeof(int);
int len = snprintf(dest, d_size, "%s%d", delimiter, d);
if (len < 0 || (size_t) len >= d_size) {
return -1; // out of room
}
dest += len;
d_size -= len;
delimiter = " ";
}
return 0;
}
struct s1 {
int VBUS_voltage_mV;
int temp_internal_degC;
int status;
...
};
struct s1 st = ...;
char buf[1024];
print_int_struct(buf, sizeof buf, &st, sizeof st);
puts(buf);