Simple Linux pipe program, data is not received - c

Question

I have a simple Linux C program that I'm writing to help me better understand IPC, right now I'm trying to build it with pipes. I have a single code base that I run in two different terminal windows as two different executables (so they can talk to each other). However I'm not doing something correct, because I never get any data to read, but I'm not sure what...
NOTE This is not the full code, I chopped out the output/input/validation to save space. But it's noted in the comments in the program below.
void main()
{
int pipefd[2], n;
char input = 0;
char buffer[100] = {0};
char outpipe[100] = {0};
if(pipe(pipefd) < 0) {
printf("FAILED TO MAKE PIPES\n");
return;
}
printf("Starting up, read fd = %d, write fd = %d\n", pipefd[0],pipefd[1]);
do {
//print menu options (send message, get message, get my fd,
// set a fd to talk to, quit)
// if "send a message":
{
printf("What would you like to send?\n");
fgets(buffer, 100, stdin);
write(pipefd[1], buffer, strlen(buffer));
}
//else if "read a message":
{
if(open(outpipe, 0) < 0)
printf("Couldn't open the pipe!\n");
else {
n = read(outpipe, buffer, 100);
printf("I got a read of %d bytes\nIt was %s\n",n, buffer);
close(outpipe);
}
}
//else if "get my file descriptor":
printf("My fd tag is: /proc/%d/fd/%d\n", (int)getpid(), pipefd[0]);
//else if "set a file descriptor to talk to":
{
printf("What is the pipe's file descriptor?\n");
fgets(outpipe, 100, stdin);
n = strlen(outpipe) - 1;
outpipe[n] = '\0';
}
} while (input != 'Q');
return;
}
I know the pipes are created successfully, I verified the file descriptors are in place:
lr-x------ 1 mike users 64 Sep 26 23:31 3 -> pipe:[33443]
l-wx------ 1 mike users 64 Sep 26 23:31 4 -> pipe:[33443]
Looks like the permissions are OK (read on pipe 3, write on pipe 4).
I use it as such:
//terminal 1
Pick an option:
3
My fd tag is: /proc/8956/fd/3
//terminal 2
Pick an option:
4
What is the pipe's file descriptor?
/proc/8956/fd/3
Pick an option:
1
What would you like to send?
hello
//terminal 1
Pick an option:
2
I got a read of -1 bytes
It was
Is there anything obviously wrong that I'm doing here? My reads always get "-1" return value...

Answer 1

It seems you have misunderstood how pipe works. A pipe is an anonymous file descriptor that is not going by file in the file system. The files in /proc/<pid>/fd you don't have to care about.
Here is a rewrite of what you are trying to do:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main(void)
{
int pipefds[2];
char input[128];
char output[128];
ssize_t nread;
if (pipe(pipefds) == -1)
{
perror("Could not create pipe");
return EXIT_FAILURE;
}
printf("Enter input: ");
if (fgets(input, sizeof(input), stdin) == NULL)
{
perror("Could not read input");
return EXIT_FAILURE;
}
/* "Remove" newline from input */
if (input[strlen(input) - 1] == '\n')
input[strlen(input) - 1] = '\0';
/* Now write the received input to the pipe */
if (write(pipefds[1], input, strlen(input) + 1) == -1)
{
perror("Could not write to pipe");
return EXIT_FAILURE;
}
/* Now read from the pipe */
if ((nread = read(pipefds[0], output, sizeof(output))) == -1)
{
perror("Could not reaf from pipe");
return EXIT_FAILURE;
}
/* We don't need to terminate as we send with the '\0' */
printf("Received: \"%s\"\n", output);
return EXIT_SUCCESS;
}

Answer 2

Here is your primary concern:
./ipctest.c: In function ‘main’:
./ipctest.c:32:9: warning: passing argument 1 of ‘read’ makes integer from pointer without a cast [enabled by default]
/usr/include/unistd.h:361:16: note: expected ‘int’ but argument is of type ‘char *’
./ipctest.c:34:9: warning: passing argument 1 of ‘close’ makes integer from pointer without a cast [enabled by default]
/usr/include/unistd.h:354:12: note: expected ‘int’ but argument is of type ‘char *’
Look at the data types required for a certain function... :)