CODEWITHSUNDEEP
cdebugginggdbarm
Ashton H.
Ashton H.

Reputation: 301

gdb: "Left operand of assignment is not an lvalue."

I am debugging an ARM microcontroller remotely and trying to modify a variable with gdb in the following block of code:

for (int i = 0; i < 100; i++) {
    __asm__("nop");
}

When I execute print i I can see the value of the variable

(gdb) print i
$1 = 0

Executing whatis i returns this

whatis i
~"type = int\n"

But when I try to change the variable I get the following error

(gdb) set variable i=99
Left operand of assignment is not an lvalue.

What am I doing wrong here?

UPDATE: here is the assembler code

!        for (int i = 0; i < 100; i++) {
main+38: subs\tr3, #1
main+40: bne.n\t0x80001d0 <main+36>
main+42: b.n\t0x80001c4 <main+24>
main+44: lsrs\tr0, r0, #16
main+46: ands\tr2, r0
!            __asm__("nop");
main+36: nop

Upvotes: 6

Views: 14644

Answers (4)

user3458845
user3458845

Reputation: 59

There is two issue here change the variable name from i to var_i as there are some set command starting with i so set i=6 will gives the ambiguous command set error.

The "Left operand of assignment is not an lvalue." can be fixed with the code changes as shown below.

volatile int var_i = 1;
  TRACE((2255, 0, NORMAL, "Ravi I am sleeping here........."));
  do
  {
          sleep(5);
          var_i = 1;
  }while(var_i);

(gdb)bt
#1  0x00007f67fd7b9404 in sleep () from /lib64/libc.so.6
#2  0x00000000004cd410 in pgWSNVBUHandleGetUser (warning: Source file is more recent than executable.
ptRequest=<optimized out>, oRequest=<optimized out>,

(gdb) finish
Run till exit from #0  0x00007f67fd7b9550 in __nanosleep_nocancel () from /lib64/libc.so.6
0x00007f67fd7b9404 in sleep () from /lib64/libc.so.6
(gdb) finish
Run till exit from #0  0x00007f67fd7b9404 in sleep () from /lib64/libc.so.6
0x00000000004cd410 in pgWSNVBUHandleGetUser (ptRequest=<optimized out>, oRequest=<optimized out>,
    pptResponse=0x7fff839e8760) at /root/Checkouts/trunk/source/base/webservice/provnvbuuser.c:376
    (gdb)
      │372       volatile int var_i = 1;                                                                                           │
       │373       TRACE((2255, 0, NORMAL, "Ravi I am sleeping here........."));                                            │
       │374       do                                                                                                       │
       │375       {                                                                                                        │
      >│376         sleep(5);                                                                                              │
       │377         var_i = 1;                                                                                             │
       │378       }while(var_i);        

(gdb) set var_i=0
(gdb) n
(gdb) p var_i
$1 = 1
(gdb) set var_i=0
(gdb) p var_i
$2 = 0
(gdb) n
(gdb) n

Upvotes: 0

Ruslan Shaydulin
Ruslan Shaydulin

Reputation: 119

I had the same problem and making the variable volatile helped.

Upvotes: 6

eduardtm
eduardtm

Reputation: 76

Try it this way:

(gdb) print i
$1 = 3
(gdb) set var i=6
(gdb) print i
$2 = 6

Upvotes: 0

Scott Hunter
Scott Hunter

Reputation: 49883

The command would be just set i = 99

Upvotes: 1

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