For this C project I am trying to use a pipe to get a parent process to communicate with a child. The child is supposed to read lines (one each second) from a txt file (containing real numbers) and use a pipe to supply the parent, which in turn should read from the pipe and write the numbers into a log file. However the parent process only reads a sequence of 0.000000. Here is my code:
#include <stdlib.h>
#include <unistd.h>
#include <ctype.h>
#include <string.h>
#include <time.h>
#include <wait.h>
#define MAX_I 5
#define delay 1
void read_input(FILE *fp);
FILE *file_log;
int status;
pid_t pid;
int pipeFD[2];
int main(int argc, char **argv) {
//initialize
FILE *fp;
if (argc != 2){
if ((fp = fopen("input.txt", "r")) == NULL) {
printf("Error! opening file");
exit(1);}
}
else{
fp = fopen(argv[1], "r");
}
if (pipe(pipeFD) == -1){ /* creates a pipe */
fprintf(stderr, "\nERROR: pipe() failed\n");
exit(1);}
file_log=fopen("file.log","w+"); /*open the log file*/
pid=fork();
if(pid==0) {//child process
printf ("%d starts \n", getpid());
close(pipeFD[0]);//close read end for child
read_input(fp);
return 0;
exit(status);
}else{//parent
double speed_read;
close(pipeFD[1]);//close write end for parent
while(1){
if (read(pipeFD[0], &speed_read, sizeof(speed_read)) >0){
if (speed_read<0)
break;
fprintf(file_log, "%f \n", speed_read);
printf("process %d received %f from child \n",getpid(),speed_read);
}else
printf("Nothing there to read \n");
}
printf("parent ended \n");
wait(&status);
fclose(fp);
fclose(file_log);
}
return 0;
}
void read_input(FILE *fp){
char *line = NULL;
double speed;
int i=0; size_t len = 0; double exit_sign =-10.0;
while(getline(&line, &len, fp) != -1) {
speed=atof(line);
i++;
if(i>MAX_I){//reads up to MAX_I rows of input
printf("I'll send the exit sign to parent now\n");
write(pipeFD[1], &exit_sign, sizeof(double));
free(line);
break;
}
if(write(pipeFD[1], &speed, sizeof(double)>0)){
printf("%d at %d wrote that speed is %f\n",getpid(), i,speed);
}else{printf("Write on pipe failed\n");}
sleep(delay);
}
free(line);
}
Here is what is printed:
15032 starts
15032 at 1 wrote that speed is 0.345670
process 15031 received 0.000000 from child
15032 at 2 wrote that speed is 12.678890
process 15031 received 0.000000 from child
15032 at 3 wrote that speed is 34.789870
process 15031 received 0.000000 from child
15032 at 4 wrote that speed is 0.000000
process 15031 received 0.000000 from child
15032 at 5 wrote that speed is 12.009288
process 15031 received 0.000000 from child
I'll send the exit sign to parent now
parent ended
Likewise the log file consists in the following:
0.000000
0.000000
0.000000
0.000000
0.000000
in read_input the line :
if(write(pipeFD[1], &speed, sizeof(double)>0)){
must be
if(write(pipeFD[1], &speed, sizeof(double))>0){
in your case you do not write sizeof(double) bytes but only 1
Note read_input does two times free(line); with an undefined behavior, you must remove the one when if(i>MAX_I){
After the corrections, compilation and execution :
pi@raspberrypi:/tmp $ gcc -g -Wall p.c
pi@raspberrypi:/tmp $ cat input.txt
1.2
2.3
3.45
7.8
9.12
12.345
pi@raspberrypi:/tmp $ ./a.out
15121 starts
15121 at 1 wrote that speed is 1.200000
process 15120 received 1.200000 from child
15121 at 2 wrote that speed is 2.300000
process 15120 received 2.300000 from child
15121 at 3 wrote that speed is 3.450000
process 15120 received 3.450000 from child
15121 at 4 wrote that speed is 7.800000
process 15120 received 7.800000 from child
15121 at 5 wrote that speed is 9.120000
process 15120 received 9.120000 from child
I'll send the exit sign to parent now
parent ended
pi@raspberrypi:/tmp $ cat file.log
1.200000
2.300000
3.450000
7.800000
9.120000
pi@raspberrypi:/tmp $
Out of that when you detect the EOF and do printf("Nothing there to read \n"); to also do a break; to finish the while seems better. Note you can also send exit_sign when you reach the end of the input file in read_input. If you don't the parent never ends writing Nothing there to read if the input file has less than MAX_I lines
A pipe is a stream but reading or writing pipe data is atomic if the size of data written is not greater than PIPE_BUF where PIPE_BUF is at least 512 bytes, and 4096 under Linux
So in your case where in main you read as fast as possible and the read_input function sleeps, while you read and write the same small amount of bytes, you have no risk. Else as in general when you read in a stream you need to loop until you get the expected amount of bytes, for instance replacing the use of read by a call to a function like :
int read_block(int f, char * p, size_t sz)
{
ssize_t n;
while ((n = read(f, p, sz)) > 0) {
if ((sz -= (size_t) n) == 0)
return 1;
p += n;
}
return -1;
}
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