Question

All I need this for is strcpy().

I want to see whether the first three bytes of a buffer(byte array) are “JMX” as string.

This is what I did so far:

char * ddj;
strcpy( ddj, buffer ); //buffer is BYTE[]
if ( strcmp( "JMX", ddj ) == 0 ) //check first three chars are "JMX"
{
    buffer += 20;   //increase the index with 20
    size -= 20;     //int
}

I get exception at strcmp() line. What is the problem?

I wish I was writing this in C# 🙁

Answer 1

Tho things go wrong here:

1. ddj does not point to any actual memory. Hence the copy will have undefined behavior
2. The copying is not necessary in the first place.

This is what you can do:

if(strncmp("JMX", buffer, 3) == 0) {
  buffer += 20;
  size -= 20;
}

This uses strncmp instead of strcmp, thus ensuring that no more than three bytes are compared. If buffer can contain less than three bytes, you should do something like:

if(buf_len >= 3 && strncmp("JMX", buffer, 3) == 0) {
  buffer += 20;
  size -= 20;
}