How to solve 'right-hand operand has no effect' error?

Question

I am currently working with Eclipse CDT, the ESP-IDF framework and the ESP32 Wrover DevKit .

I was looking for a C vector implementation and stumbled over rxi/vec on GitHub . I copied vec.h and vec.c to my project and tried to compile. I am getting the following error:

Error: right-hand operand of comma expression has no effect[-Werror=unused-value]

... at this line of code in vec.h :

#define vec_push(v, val)\
  ( vec_expand_(vec_unpack_(v)) ? -1 :\
    ((v)->data[(v)->length++] = (val), 0), 0 )

Any ideas how solve this issue or work around? Any alternative vector implementation for C ?

---

Update A

../main/tools/inc/vec.h:35:42: error: right-hand operand of comma expression has no effect [-Werror=unused-value]
   ( vec_expand_(vec_unpack_(v)) ? -1 :\
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   
     ((v)->data[(v)->length++] = (val), 0), 0 )
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
../main/main.c:44:2: note: in expansion of macro 'vec_push'
  vec_push(&toc.items, item1);
  ^~~~~~~~
../main/tools/inc/vec.h:35:42: error: right-hand operand of comma expression has no effect [-Werror=unused-value]
   ( vec_expand_(vec_unpack_(v)) ? -1 :\
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   
     ((v)->data[(v)->length++] = (val), 0), 0 )
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
../main/main.c:50:2: note: in expansion of macro 'vec_push'
  vec_push(&item1.elements, elem1);
  ^~~~~~~~
../main/tools/inc/vec.h:35:42: error: right-hand operand of comma expression has no effect [-Werror=unused-value]
   ( vec_expand_(vec_unpack_(v)) ? -1 :\
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~   
     ((v)->data[(v)->length++] = (val), 0), 0 )
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
../main/main.c:56:2: note: in expansion of macro 'vec_push'
  vec_push(&item1.elements, elem2);
  ^~~~~~~~
cc1: some warnings being treated as errors

---

Update B

#include <stdio.h>
#include <stdlib.h>
#include "vec.h"

typedef struct toc_element_t {
  uint16_t uid;
  char element[8];
  char type[8];
} toc_element_t;

typedef vec_t(toc_element_t) vec_toc_element_t;

typedef struct toc_item_t {
  char category[8];
  vec_toc_element_t elements;
} toc_item_t;

typedef vec_t(toc_item_t) vec_toc_item_t;

typedef struct toc_t {
    char description[8];
    vec_toc_item_t items;
} toc_t;


void app_main(void) {

    toc_t toc;
    strcpy(toc.description, "toc1");
    vec_init(&toc.items);

    toc_item_t item1;
    strcpy(item1.category, "cat1");
    vec_init(&item1.elements);
    vec_push(&toc.items, item1);

    toc_element_t elem1;
    elem1.uid=0;
    strcpy(elem1.element, "elem1");
    strcpy(elem1.type, "float");
    vec_push(&item1.elements, elem1);

    toc_element_t elem2;
    elem2.uid=1;
    strcpy(elem2.element, "elem2");
    strcpy(elem2.type, "float");
    vec_push(&item1.elements, elem2);


    printf("TOC [%s]:\n", toc.description);

    toc_item_t item; int index1;
    vec_foreach(&toc.items, item, index1) {
        printf(" - item[%d]=%s\n", index1, item.category);
        toc_element_t element; int index2;
        vec_foreach(&item.elements, element, index2) {
            printf("  - element[%d]=%s [%s]\n", index2, element.element, element.type);
        }
    }

    while(1);

}

---

Update C

Perhaps this issue is not related to this question, but since Eric mentioned the last commit with the log message “Fixed vec_insert() and vec_push() for structs” that inserted the 'fishy' ,0 that caused the problem discussed here, it may nonetheless be related.

When executing the code in section "Update B", I get the following output:

TOC [toc1]:
 - item[0]=cat1

But I would expect the following:

TOC [toc1]:
 - item[0]=cat1
  - element[0]=elem1
  - element[1]=elem2

Solved: Why does the inner vector not get printed out?

Answer 1

The compiler is right to warn you. Something is fishy there. The … ? -1 : (…, 0) … ? -1 : (…, 0) part is designed to return a success/failure indication, and then the trailing , 0 throws that away. It is “valid code” in that it does not violate a rule of the C standard, but there is clearly a design or coding error there.

The extra , 0 was introduced in commit dd55e00e17d454f54b905fdcf6718ba0c1ed94b0, with the log message “Fixed vec_insert() and vec_push() for structs”. It and the same change to vec_insert are the only changes in that commit:

--- a/src/vec.h
+++ b/src/vec.h
@@ -33,7 +33,7 @@

 #define vec_push(v, val)\
   ( vec_expand_(vec_unpack_(v)) ? -1 :\
-    ((v)->data[(v)->length++] = (val)), 0 )
+    ((v)->data[(v)->length++] = (val), 0), 0 )


 #define vec_pop(v)\
@@ -52,7 +52,7 @@

 #define vec_insert(v, idx, val)\
   ( vec_insert_(vec_unpack_(v), idx) ? -1 :\
-    ((v)->data[idx] = (val)), (v)->length++, 0 )
+    ((v)->data[idx] = (val), 0), (v)->length++, 0 )

I do not see how those changes comport with the log message. I think it is a mistake. Note that the pre-commit code was wrong; this code:

  ( vec_expand_(vec_unpack_(v)) ? -1 :\
    ((v)->data[(v)->length++] = (val)), 0 )

has the pattern (Test ? -1 : (Operation), 0) . That always produces 0, since the comma operator is the lowest precedence. The commit changes it to (Test ? -1 : (Operation, 0), 0) , which has equivalent behavior—on the : side, the operation is performed, but then the result is discarded, 0 is evaluated and discarded, and then there is another 0. If the commit had changed it to (Test ? -1 : (Operation, 0)) , that would make sense. The commit would be fixing an error, in that the macro previously always produced 0 but would now produce 0 or −1 according to whether the operation was successful or not.

None of the tests in the repository check the “return” value of vec_push .

It should be safe to remove the , 0 . The resulting expansion will continue to evaluate to zero in normal situations. It will change to evaluate to −1 if the push fails (because memory allocation fails), but that would seem to be desirable. Good code should be:

#define vec_push(v, val)\
  ( vec_expand_(vec_unpack_(v)) ? -1 :\
    ((v)->data[(v)->length++] = (val), 0))

Also make the same change to vec_insert and look for any similar code that needs to be fixed.