Comparing struct char array with char pointer in assembly

Question

I'm trying to compare the first character of the first struct in list with the first character of token in assembly. However, the two characters are never equal for some reason. I know the first char in the first student is the same as the first char in token.

struct student
{
  long    ID;          /* 8 bytes in 64-bit */
  char    name[24];
};

/*
* list - the starting address of the list of structures to be searched
* count - total number of names in list
* token - name to be searched in the list
*/
long search_by_name (char* list, long count, char* token)
{
    long index = 0;

asm
(
    "movq %1, %%rax;" // list to rax       
    "movq %2, %%rbx;" // token to rbx
    "movq %3, %%rcx;" // count to rcx 

    "movq 8(%%rax), %%rsi;" // Offset rax by 8 to get to first char in struct
    "movq (%%rbx), %%rdi;" // Move memory address rbx into rdi
    "cmpq %%rdi, %%rsi;" // Compare the two chars
    "je equals;"         // Two chars are never equal for some reason

    "jmp finish;"

"equals:"
    "movq 0(%%rax), %%rax;"
    "jmp finish;"

"finish:"
      : "=a" (index)
     : "r" (list), "r" (token), "r" (count)
     : 
     );
     return index;
}

What it's supposed to do in C:

struct student *clist = (struct student*)list;
for (index = 0; index < count; index++)  
    if ( strcasecmp( clist[index].name, token ) == 0)
        return clist[index].ID;
return 0;

The C code is what the end result is supposed to be, but I'm still trying to figure out how to compare clist[index].name and token. How would I compare the struct char array with the token char*?

Edit: Comparison worked after changing it to:

movq 8(%%rax), %%r10; 
movq (%%rbx), %%r11; 
cmpb %%r10b, %%r11b;

Why is `list` a `char*` and not a `struct student*`? And where's the code where you call `search_by_name`? — Joseph Sible-Reinstate Monica, Apr 25 '20 at 00:32
`list` is a `char*` that points to an array of structures and `search_by_name` is called in a main method too long to post here. You can assume that the main method works as intended. The only problem is comparing the first character of the first `student` in `char* list` with `char* token`. — Arkitek, Apr 25 '20 at 00:40
Your asm statement doesn't declare clobbers on any of the hard-coded registers you picked (other than the `"=a"` output), and no `"memory"` clobber to tell the compiler that memory pointed-to by input operands is also an input. [How can I indicate that the memory \*pointed\* to by an inline ASM argument may be used?](https://stackoverflow.com/q/56432259). — Peter Cordes, Apr 25 '20 at 00:55
Also the actual asm looks buggy: the comments say "compare two chars" but you've actually loaded 8 bytes of char data from each. This would be obvious if you single-stepped your code with a debugger and looked at registers. Also, you could simplify `je equals` to just use `jne finish` instead of jumping over a `jmp`. — Peter Cordes, Apr 25 '20 at 00:56
Can you write `search_by_name` in pure C, and edit that into your question? That might make it more clear what you're going for. — Joseph Sible-Reinstate Monica, Apr 25 '20 at 01:19

Joseph Sible-Reinstate Monica · Accepted Answer · 2020-04-25T01:07:36.907

1

Two main problems: First, cmpq compares the first 8 characters, not just one. Second, when the comparison fails, you return the pointer list, rather than any sort of sentinel.

Also, you're clobbering a bunch of registers but not letting GCC know. After your last :, add "rbx", "rcx", "rsi", "rdi", "cc", "memory".

edited Apr 25 '20 at 01:07

answered Apr 25 '20 at 00:57

Joseph Sible-Reinstate Monica

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I return pointer `list` because it points to the first struct in the `list`. It's just a placeholder until I figure out how to compare the `char[]` in the first struct with the `char* token`. – Arkitek Apr 25 '20 at 01:19
I took what you said about how cmpq compares the first 8 characters and after looking at my notes, I changed it so that it compares byte 0 instead. It works after changing it to: `movq 8(%%rax), %%r10; movq (%%rbx), %%r11; cmpb %%r10b, %%r11b;` – Arkitek Apr 25 '20 at 02:14

Comparing struct char array with char pointer in assembly

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