Memory fault in memcpy call

Question

Im facing with a "memory fault" when calling to:

extern void *memcpy (void *__restrict __dest,
         __const void *__restrict __src, size_t __n)
 __THROW __nonnull ((1, 2));

This is the piece of code:

    fprintf(stderr, "sysconfig line 440 \n");
fprintf(stderr, "Value size: %d ; Pointer mymsg: %p ; Pointer value: %p ; mymsg->mtext: %s ; value: %s ; size: %d ;   \n", strlen(value), mymsg, value, mymsg->mtext, value, size);
memcpy(mymsg->mtext, value, size);
fprintf(stderr, "sysconfig line 442 \n");

mymsg is a pointer to struct:

MSG_T *mymsg;

MSG_T:

typedef struct msgInfo {
int cmd;
int arg1;
int arg2;
char    mtext[MAX_SEND_SIZE];
} MSG_T;

MAX_SEND_SIZE = 4096, value is a (void *)char *string containing "" (\0 or empty chain) and size = 4096, so i dont understand why im getting a memory fault. Im checking there is not memory overlapping. This is the output of execution:

sysconfig line 440 
Value size: 0 ; Pointer mymsg: 0x7fd49ac4 ; Pointer value: 0x7fd4ab4c ; mymsg->mtext:      ; value:  ; size: 4096 ;   
Memory fault

I have not found What kind of condition here can result in segmentation since all memory has been well allocated, value is a empty string and there is no memory overlapping.

I only get memory fault sometimes (it seems randomly) and other times the process follow executing and exit with no errors.

Answer 1

memcpy(mymsg->mtext, value, size);

value is a string of length 0 and you are trying to copy 4096 bytes from this string. You are therefore accessing 4095 unallocated bytes which means you are invoking undefined behavior.