A random character showing up at the end of a C array - c

Question

I have the code:
else if ((strtolnum=(strtol(&oldline[b+1],NULL,10)))>0)
{
char* excl = malloc(3*sizeof(char));
excl[0]=oldline[b];
excl[1]=oldline[b+1];
for (int j = 0; j<strtolnum; j++)
{
llist=llist->nextcmd;
}
struct token *p1;
struct token *save1 = llist->cmmd;
char *arra1 = malloc(sizeof(char));
memset(arra1, '\0', 1);
for (p1 = llist->cmmd; p1 != NULL; p1 = p1->next) {
arra = realloc(arra1, strlen(arra1)+strlen(p1->text)+2);
strcat(arra, p1->text);
if (p1->next!=NULL)
{
strcat(arra1, " ");
}// printing token and type
}
printf("%s excl\n", excl); //Line 137
oldline=strreplace(oldline,excl,arra1); //Line 138
llist->cmmd=save1;
for (int j = 0; j<(f-strtolnum); j++)
{
llist=llist->nextcmd;
}
*status=1;
size_t a = sizeof(excl);
memset(excl,'\0', a);
}
What the code should accomplish is get the first integer of a line which is preceded by an exclamation part, such as !3, and puts it in excl (and then do other stuff which is working perfectly fine).
However, when I run the loop more than once, I find that excl often has a random character at the end, such as when it shows up as "!3e" when I try to printf it. Valgrind shows the following errors:
==24878== Conditional jump or move depends on uninitialised value(s)
==24878== at 0x4E7AB5B: vfprintf (in /usr/lib64/libc-2.17.so)
==24878== by 0x4E83CD8: printf (in /usr/lib64/libc-2.17.so)
==24878== by 0x40130F: hExpand (Lex1.c:137)
==24878== by 0x400B6B: main (mainLex.c:27)
==24878==
==24878== Conditional jump or move depends on uninitialised value(s)
==24878== at 0x4C2B308: strlen (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==24878== by 0x4020D7: strreplace (Lex1.c:562)
==24878== by 0x40132C: hExpand (Lex1.c:138)
Apparently, excl is unititialized after the loop goes through once. What is wrong?
For additional information, cmmd is a data structure of type token, and llist is a global static data structure containing tokens.

Answer 1

You haven't null terminated the excl string; malloc() returns random garbage. You might think about adding:
excl[2] = '\0';
You also trample way out of bounds with:
size_t a = sizeof(excl);
memset(excl,'\0', a);
You assign either 4 or 8 (32-bit or 64-bit) to a, and then write over that many bytes, but you only allocated 3 bytes for excl.