使用memcmp时出现段错误

Question

I want to compare 2 unsigned bytes in hex. This is what I tried:

if (memcmp (msgType ,  0x00003336, 2 ) == 0){}

This is what gcc prints for msgType :

(gdb) x msgType
0x7fffffffbb84: 0x00003336

I'm getting segfault. How can I fix it?

EDIT:

I tried :

const unsigned char broadcast[2] = {0x33, 0x36};

But gdb shows:

(gdb) x broadcast
0x401e28 <broadcast>:   0x62723633

I need: 0x00003336

Answer 1

The first two arguments to memcmp() are pointers to the memory blocks to compare. See the manual page , the prototype is:

int memcmp(const void *s1, const void *s2, size_t n);

You are using the absolute address 0x00003336 as the value of s2 , which seems very wrong; that's not a valid address in the general case.

To fix this, you must create a memory area that holds the value you want to compare against, and pass a pointer to that as the second argument. Try:

const uint8_t data[] = { 0x36, 0x33 };

if(memcmp(msgType, data, sizeof data) == 0)
{
}

Note that the bytes are swapped in the above, on the assumption that you're on a little-endian system.

Answer 2

You need a pointer as second argument, you can't just pass an hex value in there

http://www.cplusplus.com/reference/cstring/memcmp/

Something that might work:

#include <stdio.h>

int main(void) {

    unsigned char msgType[2] = {0x33, 0x36};
    unsigned char your_value[2] = {0x33, 0x36};

    // Make sure your_value is the same size of msgType
    if (memcmp (msgType ,  your_value, sizeof(msgType)/sizeof(unsigned char) ) == 0){
        printf("Yes, memory is equal at that address!");
    }
    return 0;
}

http://ideone.com/EQH6py

Answer 3

If your process does not own the memory at 0x00003336 then you will get undefined behaviour : manifested, in this particular instance, as a segfault.

The normal thing to do here is to pass a pointer to a variable that you have instantiated yourself.

Answer 4

Both of memcmp() 's first two arguments must be pointers to memory. You seem to be passing in a value you want to compare against, rather than a pointer to that value.

Instead, try this:

unsigned short expectedValue = 0x3336;
if (memcmp (msgType, &expectedValue, 2) == 0){}