c - A legal array assignment. Is it possible? -

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after reading chapter structures in k&r book decided make tests understand them better, wrote piece of code:

#include <stdio.h> #include <string.h>  struct test func(char *c);  struct test {     int ;     int j ;     char x[20]; };  main(void) {     char c[20];     struct  {int ; int j ; char x[20];}  = {5 , 7 , "somestring"} , b;      c = func("another string").x;     printf("%s\n" , c); }  struct test func(char *c) {     struct test temp;     strcpy(temp.x , c);     return temp;     }

my question is: why c = func("another string").x; working (i know it's illegal, why working)? @ first wrote using strcpy() (because seemed logical thing do) kept having error:

structest.c: in function ‘main’: structest.c:16:2: error: invalid use of non-lvalue array 

    char c[20];     ...     c = func("another string").x;

this not valid c code. not in c89, not in c99, not in c11.

apparently compiles latest gcc versions 4.8 in -std=c89 mode without diagnostic assignment (clang issues diagnostic). bug in gcc when used in c89 mode.

relevant quotes c90 standard:

6.2.2.1 "a modifiable lvalue lvalue not have array type, not have incomplete type, not have const-qualified type. , if structure or union. not have member (including. recursively, member of contained structures or unions) const-qualified type."

and

6.3.16 "an assignment operator shall have modifiable lvalue left operand."

6.3.16 constraint , imposes @ least gcc issue diagnostic gcc not, bug.


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