C : printf printing variables in the mixed order - c

Question

Here is my code,
#include <time.h>
#include <pthread.h>
#include <string.h>
#include <inttypes.h>
#include <unistd.h>
unsigned sleep(unsigned sec) ;
void get_time(char *buf) {
time_t t = time(NULL) ;
struct tm tm = *localtime(&t) ;
sprintf(buf,"%02d:%02d:%02d", tm.tm_hour, tm.tm_min, tm.tm_sec) ;
}
void *calculateSize(uint64_t size, char *buf) {
if (size < 1024ULL){
sprintf(buf, "%.1f %s", (float) size, "B") ;
return NULL ;
}
else if (size < (1024 * 1024)) {
sprintf(buf, "%.1f %s", (float) size/1024ULL, "KiB") ;
return NULL ;
}
else if (size > (1024 * 1024)) {
sprintf(buf, "%.1f %s", (float) size/(1024ULL * 1024ULL), "MiB") ;
return NULL ;
}
strcpy(buf, "0") ;
return NULL ;
}
void * run(char *interface, char * download_buf, char * upload_buf) {
unsigned long rdiff,tdiff ;
unsigned long rx_old,rx_new ;
unsigned long tx_old,tx_new ;
char buf[10] ;
char rx_file[512] ;
char tx_file[512] ;
sprintf(rx_file, "/sys/class/net/%s/statistics/rx_bytes", interface) ;
sprintf(tx_file, "/sys/class/net/%s/statistics/tx_bytes", interface) ;
FILE *rf = fopen(rx_file,"r") ;
FILE *tf = fopen(tx_file,"r") ;
if (rf != NULL && tf != NULL) {
fscanf(rf,"%lu", &rx_old) ;
fscanf(tf,"%lu", &tx_old) ;
fclose(rf) ;
fclose(tf) ;
}
else {
return NULL ;
}
sleep(1) ;
rf = fopen(rx_file,"r") ;
tf = fopen(tx_file,"r") ;
if (rf != NULL && tf != NULL) {
fscanf(rf,"%lu", &rx_new) ;
fscanf(tf,"%lu", &tx_new) ;
rdiff = rx_new - rx_old ;
tdiff = tx_new - tx_old ;
fclose(rf) ;
fclose(tf) ;
}
else {
return NULL ;
}
calculateSize(rdiff,buf) ;
strcpy(download_buf,buf) ;
calculateSize(tdiff,buf) ;
strcpy(upload_buf,buf) ;
return NULL ;
}
void *net_speed(void *thread_speed_args ) {
char* iface = *(char **)thread_speed_args ;
char carrier_file[512] ;
sprintf(carrier_file,"/sys/class/net/%s/carrier", iface) ;
printf("Reading from %s\n", carrier_file) ;
while(1) {
if( access( carrier_file, F_OK ) == 0 ) {
run(iface, ((char **)thread_speed_args)[1], ((char **)thread_speed_args)[2]) ;
}
else {
sprintf(((char **)thread_speed_args)[1],"000 B") ;
sprintf(((char **)thread_speed_args)[2],"000 B") ;
sleep(1) ;
}
}
return NULL ;
}
int main(int argc, char *argv[]) {
char time_buf[10] ; //hh:mm:ss : 8 char + 1 null terminator char
char download_buf[8],upload_buf[8] ;
char* thread_speed_args[3] = { argv[1], download_buf, upload_buf } ;
pthread_t thread_speed ;
pthread_create(&thread_speed, NULL, net_speed, thread_speed_args) ;
pthread_detach(thread_speed) ;
while(1){
get_time(time_buf) ;
printf("Down:%s Up:%s %s\n", thread_speed_args[1], thread_speed_args[2], time_buf) ;
fflush(stdout) ;
sleep(1) ;
}
}
I am using a thread to continuously monitor my wireless data transfer rate on an interface, I am also printing the time along with the transfer rate. I am just experimenting with threads, forgive me if my programming logic is bad.
I notice that when I do a speed test the output or print becomes all messed up like below
Down:0.0 B Up:0.0 B 23:49:03
Down:0.0 B Up:0.0 B 23:49:04
Down:0.0 B Up:0.0 B 23:49:05
Down:17.0 KiB9.2 KiB Up:9.2 KiB 23:49:06
Down:5.3 KiB Up:6.5 KiB 23:49:07
Down:3.4 KiB Up:4.1 KiB 23:49:08
Down:400.6 Ki20.3 KiB23:49:09 Up:20.3 KiB23:49:09 23:49:09
Down:918.6 Ki49.6 KiB23:49:10 Up:49.6 KiB23:49:10 23:49:10
Down:912.8 Ki53.5 KiB23:49:11 Up:53.5 KiB23:49:11 23:49:11
Down:959.2 Ki32.2 KiB23:49:12 Up:32.2 KiB23:49:12 23:49:12
Down:711.5 Ki33.8 KiB23:49:13 Up:33.8 KiB23:49:13 23:49:13
See the last few lines?
Could anyone tell me what is happening here? How do I correct it?

Answer 1

This is happening because you're accessing a common resource in multiple threads simultaneously - in this case, the STDOUT handle. You'll need to synchronize your threads when outputting the information to the console - the easiest way is using mutexes. You can read more here.

Answer 2

While your threads lack any kind of synchonization (which will randomly garble your output in some rare cases) this isn't even your problem.
Here is your problem:
char download_buf[8],upload_buf[8] ;
char buf[10] ;
strcpy(download_buf,buf) ;
strcpy(upload_buf,buf) ;
You are copying an up to 9 character string into a 7 char string so that overflows the buffer. And since the buffers happen to be next to each other on the stack the second strcpy will overwrite the 0 terminator of the first string. Same with the time_buf overwriting the 0 terminator of the second string.
Overall a whole lot of undefined behavior due to buffer overflows.