How to pipe output to another program in C, using read - c

Question

I'm trying to read information printed by program A from program B. How can I pass data from A to B using read()?.
code for A
#include <stdio.h>
int main(int argc, char **argv)
{
int i, j;
char instruc_list[11][3] = {"sa", "sb", "ss", "pa", "pb",
"ra", "rb", "rr", "rra", "rrb", "rrr"};
i = 0;
while (i < 11)
{
j = 0;
while (j < 3)
{
printf("%c", instruc_list[i][j]);
j++;
}
i++;
printf("\n");
}
return (0);
}
code for B
int main()
{
char buf[4];
while ((read(0,buf, 4)))
{
printf("%s", buf);
}
printf("\n");
return 0;
}
When I run this two programs, I get the following result.

Answer 1

Use the popen() and pclose() functions defined in stdio.h to pipe output between programs.
Here's an example program of how to print the output of the ls shell command in your program, taken from this link:
FILE *fp;
int status;
char path[PATH_MAX];
fp = popen("ls *", "r");
if (fp == NULL)
/* Handle error */;
while (fgets(path, PATH_MAX, fp) != NULL)
printf("%s", path);
status = pclose(fp);
if (status == -1) {
/* Error reported by pclose() */
...
} else {
/* Use macros described under wait() to inspect `status' in order
to determine success/failure of command executed by popen() */
...
}
For your case, you'd call popen("./A", "r");.

Answer 2

You can use popen() to read the output of program A from program B.
Compile the first program:
gcc a.c -o a
In the program B:
#include <stdio.h>
int main(void)
{
char buf[4];
FILE *fp;
fp = popen("./a", "r");
while( !feof(fp)) {
fscanf(fp, "%s", buf);
printf("%s\n", buf);
}
return 0;
pclose(fp);
}
Now compile and execute the program B:
gcc b.c -o b
me#linux:$ ./b
The output I got is:
sa
sb
ss
pa
pb
ra
rb
rr
rra
rrb
rrr
rrr

Answer 3

In program A, you're not writing the null terminators for the 3-letter strings... and in program B, you're not adding a null char after the characters you read (and haven't initialised buf, so it might not contain one). That's why you're getting garbage between the 3-letter strings you read... printf() is continuing past the characters you read because it hasn't found a null yet.
Also note that read() can return -1 for error, which would still test as true for your while loop. You should at least check that read() returns greater than 0 (rather than just non-zero), if not put in more thorough error handling.
So with some changes to address these issues, program B might become:
int main()
{
char buf[4];
int ret; // ** for the return from read()
while ((ret = read(0,buf, 4)) > 0) // ** read >0 (no error, and bytes read)
{
fwrite(buf, 1, ret, stdout); // ** write the number of chars
// you read to stdout
}
printf("\n");
return 0;
}
As for program A, right now it writes 3 characters for both the 2-letter and the 3-letter strings -- which means it includes the null char for the 2-letter strings but not for the 3-letter strings. With the changes to program B above, you don't need to write the null characters at all... so you could change:
while (j < 3)
to:
while (j < 3 && instruc_list[i][j] != 0)
to stop when the null character is reached (though it's still inefficient to use a printf() call just to write a single char -- perhaps putchar(instruc_list[i][j]); would be better). Or, you could just replace that inner while loop with:
fputs(instruc_list[i], stdout);
...which would then write the string in instruc_list[i] up to but not including the null char, and also change instruc_list[11][3] to instruc_list[11][4] so that it has room for the null char from the 3-letter string literals in the initialiser list.