CODEWITHSUNDEEP
cmemcpy
MrDuk
MrDuk

Reputation: 18242

segfault during memcpy() of two char*

I have the following:

//MTU = 1472  |  numBytes = 5000
/*char* data = 
#################################################
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
#################################################
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
#################################################
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
*/
char* payload = NULL;
size_t packetSize;
(numBytes > MTU) ? packetSize = MTU : packetSize = numBytes;

memcpy(payload, data, packetSize);

Whenever I try to do memcpy to copy the first 1472 bytes from data to payload, I get a segfault - any hints why?

Upvotes: 0

Views: 115

Answers (3)

M.M
M.M

Reputation: 141574

payload is NULL so you cause undefined behaviour if you try to memcpy to it. Where do you think those bytes are going?

You have to allocate space to write it. For example:

payload = malloc(packetSize);
if ( !payload ) 
    { exit(EXIT_FAILURE); /* error handling */ }

memcpy(payload, data, packetSize);
// ...use payload...
free(payload);

Also you should check that data actually has as much data in it as packetSize. If you write char const data[] = "....."; then you can use sizeof data - 1 to check this.

Upvotes: 3

txtechhelp
txtechhelp

Reputation: 6777

You never initialize payload:

char* payload = NULL; // payload null
size_t packetSize;
(numBytes > MTU) ? packetSize = MTU : packetSize = numBytes;

memcpy(payload, data, packetSize); // copy data to null address ... segfault

Instead you'll need to malloc or new:

char* payload = new char[packetSize];

or

char* payload = (char*)malloc(packetSize);

... don't forget to delete[]/free

Upvotes: 4

C. K. Young
C. K. Young

Reputation: 223013

Because you're trying to copy stuff to a null pointer. That's not the valid location of a buffer.

Upvotes: 2

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