GCC: warning: useless storage class specifier in empty declaration

Applications Programming
1

I am looking to create histograms in C, each of which is identified by a symbol and each bar is defined by a x-value and y-value. For this I have the following declarations:

#define RESOLUTION 200

typedef struct sCohort {
  float x;  // location of cohort
  int y;    // count
} chrt;

typedef struct sHistogram {
  char *symbol;
  struct sCohort cohort[RESOLUTION];  // there can never be more than RES cohorts
} histogram;

Without the variable name chrt a warning appears in gcc-9.4: **warning**: useless storage class specifier in empty declaration
This tells me there is a more efficient way to declare my histogram. Can someone indicate a better approach I could have taken?

For what it is worth, a histogram in the current setup would look something like the following:

A -0.030009   61
A -0.025005   99
A -0.020003  189
A -0.015001  312
A -0.010000  679
A -0.005000 1329
A  0.000000 1999
A  0.005000 1500
A  0.010000  833
A  0.015001  387
A  0.020003  165
A  0.025005   88
A  0.030009   56
2

Hi, c or c++ ? As (am sure you know this) c doesn't have classes

What is the full compiler command line used?

https://stackoverflow.com/questions/2743949/useless-class-storage-specifier-in-empty-declaration#2743984

Am not at a computer atm so limited as to what help I provide right now.

3

gcc histogram.c -g -Wall -o histogram

And yes, the language is C.

4

The canonical way to do this is to define the struct, and then declare the type separately.

struct sA { int mA; };
typedef struct sA tA;

You can condense that it two ways:

(a) Omit the declaration of the struct name (so all references are via the typedef).

typedef struct { int mB; } tB;

(b) Declare both the struct name and the typedef name together.

typedef struct sC ( int mC; } tC;

What you cannot do is specify you are declaring a typedef, and then not supply a name for it, because without a name you cannot instantiate it.

typedef struct sD ( int mD; } /* tD needed here */;

In your code, you never reference chrt, so you do not need to typedef that struct at all.

1 Like
5

as @Paul_Pedant mentions, if you are not using the typedef 'chrt' / 'histogram' you can remove them and just use 'struct sCohort and struct sHistogram'

here's a simple example using both

cat histogram.c 
#include <stdio.h>

#define RESOLUTION 200

typedef struct sCohort {
  float x;
  int y;
} chrt;          // chrt is effectively a synonym for 'struct sCohort' 

typedef struct sHistogram {
  char *symbol;
  chrt cohort[RESOLUTION];                   // an array of chrt's 
  struct sCohort anotherCohort[RESOLUTION];  // an array of 'struct sCohort' not using the synonym
} histogram; // histogram is a synonym for struct sHistogram

int main( int argc, char **argv )
{
	histogram h;
	
	h.symbol = "a random string";
	h.cohort[0].x = 3.143;
	h.cohort[0].y = 143;

	printf(" h.symbol      ==  %s\n" , h.symbol ); 
	printf(" h.cohort[0].x ==  %f\n" ,h.cohort[0].x ); 
	printf(" h.cohort[0].y ==  %d\n" ,h.cohort[0].y ); 
	
	h.anotherCohort[0].x = 5432.1;
	h.anotherCohort[0].y = 666;

	printf(" h.anotherCohort[0].x ==  %f\n" ,h.anotherCohort[0].x ); 
	printf(" h.anotherCohort[0].y ==  %d\n" ,h.anotherCohort[0].y ); 
}

gcc -std=c99 -Wall -o hst histogram.c 

./hst
 h.symbol      ==  a random string
 h.cohort[0].x ==  3.143000
 h.cohort[0].y ==  143
 h.anotherCohort[0].x ==  5432.100098
 h.anotherCohort[0].y ==  666
3 Likes
6

Excellent stuff! That really helps.

1 Like