Problem to allocate memory outside the main - c

Question

As a beginner in C, I learned a fancy way to allocate the memory with struct{} to replace something like ch1Buffer = calloc(u32Size, sizeof *ch1Buffer);and put them outside of the int main{} to boost up the calculation speed.
However, I got an error on my variable x saying: This declaration has no storage class or type specifier, you can see the annotation on the side of the code. Should I declared the variable x or?
Here is my example code:
#include <stdio.h>
// New way for memory allocation
struct {
float ch1Buffer;
double ch2Buffer;
double ch2newBuffer;
} *x;
x = calloc(10, sizeof * x); // The error happened on the left side "x": This declaration has no storage class or type specifier
int main()
{
int i,n;
const int u32Size = 10;
float* ch1Buffer = NULL;
double* ch2Buffer = NULL;
double* ch2newBuffer=NULL;
int pBuffer[] = { 10,2,10,2,10,5,10,5,10,2 };
int* pi16Buffer = pBuffer;
// Old way for memory allocation
//ch1Buffer = (float*)calloc(u32Size, sizeof* ch1Buffer);
//ch2Buffer = (double*)calloc(u32Size, sizeof* ch2Buffer);
//ch2newBuffer = (double*)calloc(u32Size, sizeof * ch2Buffer);
// De-interveal the data to ch1Buffer and ch2Buffer
for (i = 0; i < u32Size/2; i++)
{
ch1Buffer[i] += pi16Buffer[i * 2];
ch2Buffer[i] += pi16Buffer[i * 2 + 1];
}
// Use memcpy to pick out the data we are interested
memcpy(ch2newBuffer, &ch2Buffer[2], 2 * sizeof(ch2Buffer[0]));
for (i = 0; i < 2; i++)
{
printf("a[%d] = %f\n", i, *ch2newBuffer);
ch2newBuffer++;
}
free(ch1Buffer);
free(ch2Buffer);
return 0;
}

Answer 1

You can't do a malloc or calloc outside main or any other function. You can declare it at the beginning of your code if you need it as a global variable but you'll need to allocate the memory inside the main for example.
typedef struct mystruct {
float ch1Buffer;
double ch2Buffer;
double ch2newBuffer;
}mystruct;
mystruct* x;
int main (void) {
x = calloc(10, sizeof(mystruct)); // array of your structs
if (!x) { // always check calloc return value
perror("calloc");
exit(EXIT_FAILURE);
}
/* do stuff */
return 0;
}
Also I would suggest giving self-explanatory names to your structs for a better understanding of what it represents.

Answer 2

This gives you a example to use components of each members. Similar to the above anwers, the pointer is defined as a global, and allocated in main(). This Not a good pratice, just for demo.
#include <stdio.h>
#include <stdlib.h>
#define SIZE 10
struct xxx {
float f1;
double d2;
double d3;
};
typedef struct xxx mytype;
mytype *x;
void print_mytype()
{
int i;
for (i=0; i<SIZE; i++) printf("x[%d] = %f, %lf, %lf\n", i, x[i].f1, x[i].d2, x[i].d3);
return;
}
void setdata()
{
int i;
for (i=0; i<SIZE; i++)
{
x[i].f1 = rand()/(float)RAND_MAX;
x[i].d2 = rand()/(double)RAND_MAX;
x[i].d3 = rand()/(double)RAND_MAX;
}
}
int main()
{
x = calloc(SIZE, sizeof(mytype));
setdata();
print_mytype();
free(x);
return 0;
}

Answer 3

the posted code does not compile!
Here is what the compiler says about the code:
gcc -ggdb3 -Wall -Wextra -Wconversion -pedantic -std=gnu11 -c "untitled1.c" -o "untitled1.o"
untitled1.c:9:1: warning: data definition has no type or storage class
9 | x = calloc(10, sizeof * x); // The error happened on the left side "x": This declaration has no storage class or type specifier
| ^
untitled1.c:9:1: warning: type defaults to ‘int’ in declaration of ‘x’ [-Wimplicit-int]
untitled1.c:9:1: error: conflicting types for ‘x’
untitled1.c:8:4: note: previous declaration of ‘x’ was here
8 | } *x;
| ^
untitled1.c:9:5: warning: implicit declaration of function ‘calloc’ [-Wimplicit-function-declaration]
9 | x = calloc(10, sizeof * x); // The error happened on the left side "x": This declaration has no storage class or type specifier
| ^~~~~~
untitled1.c:9:5: warning: incompatible implicit declaration of built-in function ‘calloc’
untitled1.c:2:1: note: include ‘<stdlib.h>’ or provide a declaration of ‘calloc’
1 | #include <stdio.h>
+++ |+#include <stdlib.h>
2 |
untitled1.c:9:23: error: invalid type argument of unary ‘*’ (have ‘int’)
9 | x = calloc(10, sizeof * x); // The error happened on the left side "x": This declaration has no storage class or type specifier
| ^~~
untitled1.c: In function ‘main’:
untitled1.c:33:22: warning: conversion from ‘int’ to ‘float’ may change value [-Wconversion]
33 | ch1Buffer[i] += pi16Buffer[i * 2];
| ^~
untitled1.c:38:5: warning: implicit declaration of function ‘memcpy’ [-Wimplicit-function-declaration]
38 | memcpy(ch2newBuffer, &ch2Buffer[2], 2 * sizeof(ch2Buffer[0]));
| ^~~~~~
untitled1.c:38:5: warning: incompatible implicit declaration of built-in function ‘memcpy’
untitled1.c:2:1: note: include ‘<string.h>’ or provide a declaration of ‘memcpy’
1 | #include <stdio.h>
+++ |+#include <string.h>
2 |
untitled1.c:47:5: warning: implicit declaration of function ‘free’ [-Wimplicit-function-declaration]
47 | free(ch1Buffer);
| ^~~~
untitled1.c:47:5: warning: incompatible implicit declaration of built-in function ‘free’
untitled1.c:47:5: note: include ‘<stdlib.h>’ or provide a declaration of ‘free’
untitled1.c:13:11: warning: unused variable ‘n’ [-Wunused-variable]
13 | int i,n;
| ^
Compilation failed.
Suggest you fix those problems in the code, that you can, before asking us about any problems that you cannot fix.
As the error messages say:
add the statement:
#include <stdlib.h>
add the statement:
#include <string.h>
We are unlikely to help you when the code you post clearly indicates that you have not put any effort into solving the problem(s) your self.