strcat 사용시 segment fault 발생합니다.

char *procId;
char *buffer, tmp[128] = {'\0'};
char ch='\0';
int fd;
int i=0, size=0;

DIR *dp;
struct dirent *dirp;

procId = (char *)malloc(sizeof(char)*128);
buffer = (char *)malloc(sizeof(char)*128);
memset(procId, '\0', sizeof(char)*128);
memset(buffer, '\0', sizeof(char)*128;

procId = "/proc/";

if((dp = opendir(procId)) == NULL)
continue;
while((dirp = readdir(dp)) != NULL)
{
if(strcmp(dirp->d_name, ".") == 0 || strcmp(dirp->d_name, "..") == 0)
continue;

else
{
if(isdigit(*dirp->d_name) == 0)
continue;

size = strlen(dirp->d_name);
strncpy(tmp, dirp->d_name, size);

size = strlen(procId);
strcat(procId, tmp); <<------------ 여기에서 에러 발생

코어파일 보니 signal 11번 즉 invalid address reference라고 나옵니다.

Reading symbols from shared object read from target memory...done.
Loaded system supplied DSO at 0xf9f000
Core was generated by `./GetPid'.
Program terminated with signal 11, Segmentation fault.
warning: svr4_current_sos: Can't read pathname for load map: Input/output error
Reading symbols from /lib/libc.so.6...done.
Loaded symbols for /lib/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
#0 0x00211916 in strcat () from /lib/libc.so.6

------------------------디버깅 한 내용---------------------------------
83 size = strlen(procId);
(gdb) s
84 strcat(procId, tmp);
(gdb) p procId
$3 = 0x8048c13 "/proc/"
(gdb) p tmp
$4 = "1", '\0'
(gdb) p &procId
$5 (char **) 0xbf9ffd14

왜일까요?