Infinite loop caused by perror scanf through a pipe - c

Question

I have been writing a c program which pipes data from one file to the next however it has been infinite looping.
What i have discovered so far. The infinite loop is caused on file c1.c where perror (or stderr) skips the scanf. If scanf does work. The program infinite loops further down the track printing out the perror even though it is past that section!
My code is below
controller.c
#include<stdio.h>
#include<stdlib.h>
#include<sys/wait.h>
#include<unistd.h>
int main(int ac, char**av)
{
int pipez[2];
int piped[2];
int status;
pid_t pid;
if (pipe (pipez) == -1){
perror("could not make pipe");
return 1;
}
if ((pid = fork()) == -1){
perror("fork");
return 1;
}
if(pid == 0){
close(pipez[1]);
dup2(pipez[0],0);
close(pipez[0]);
execvp("./c1",av);
perror("demo);
_exit(1);
}
else{
close(pipez[0]);
dup2(pipez[1],1);
close(pipez[1]);
execvp("./c2",av);
perror("demo");
exit(1);
}
waitpid(pid,&status,0);
if(WIFEXITED(status)){
printf("[%d] TERMINATED (Status: %d)\n", pid, WEXITSTATUS(status));
}
if(pipe (piped) == -1){
perror("could not make pipe");
return 1;
}
if((pid = fork()) == -1){
perror("fork");
return 1;
}
if (pid == 0){
close(piped[1]);
dup2(piped[0],0);
close(piped[0]);
execvp("./c2", av);
perror("demo");
_exit(1);
}
else{
close(piped[0]);
dup2(piped(piped[1],1);
close(piped[1]);
execvp("./c3",av);
perror("demo");
exit(1);
}
waitpid(pid, &status, 0);
if (WIFEXITED(status)){
printf("[%d] TERMINATED (Status: %d)\n", pid, WEXITSTATUS(status));
}
return 0;
}
c1.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFSIZE 256
//This program is causing infinite loop, tried fflush and fgets and scanf
// It will run independently but will loop via the pipe
int main(int a, char**av){
char store[BUFSIZE];
memset(store, '\0', sizeof(store));
while(strcmp(store, "exit!") != 0){
perror("Please enter next line of input (type 'exit!' to stop) \n"); //This repeats itself infinitely
fgets(store, BUFSIZE, stdin);
printf("%s",store); // This also repeats itself dependant on where i put
//fflush or another printf. Repeated outputs occur in blocks
}
return 0;
}
c2.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFSIZE 256
char *strlwr(char *str);
int main(int ac, char**av){
char store[BUFSIZE];
while(strcmp(store, "exit!") != 0){
scanf("%s", store);
printf("%s", strlwr(store));
}
return 0;
}
char *strlwr(char *str){
unsigned char *p = (unsigned char *)str;
while(*p){
*p = tolower((unsigned char)*p);
p++;
}
return str;
}
c3.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFSIZE 256
int main(int ac, char**av){
char store[BUFSIZE];
int n = 0;
while(strcmp(store, "exit!") != 0{
scanf("%s",store);
printf("Line %d: %s\n",n,store);
n++;
}
return 0;
}

Answer 1

fgets leaves '\n' char in the store buffer so, easiest fix is to add '\n' to the compared string:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFSIZE 256
//This program is causing infinite loop, tried fflush and fgets and scanf
// It will run independently but will loop via the pipe
int main(int a, char**av)
{
char store[BUFSIZE];
memset(store, '\0', sizeof(store));
while (strcmp(store, "exit!\n") != 0)
{
perror("Please enter next line of input (type 'exit!' to stop) \n"); //This repeats itself infinitely
fgets(store, BUFSIZE, stdin);
printf("%s", store); // This also repeats itself dependant on where i put
// fflush or another printf. Repeated outputs occur in blocks
}
return 0;
}
Moreover, as you can see, init value '/0' is not acceptable; it must be '\0'. Your compiler is probably telling you something like
test.c: In function ‘main’:
test.c:242:16: warning: multi-character character constant [-Wmultichar]
memset(store, '/0', sizeof(store));
^
BTW memset, in this case is useless because fgets does all the job, but anyway, if you want you buffer set to nul, just declare:
char store[BUFSIZE] = {0};
Best solution using fgets is (obviously) always manage the newline left:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFSIZE 256
//This program is causing infinite loop, tried fflush and fgets and scanf
// It will run independently but will loop via the pipe
int main(void)
{
char store[BUFSIZE] = {0};
char *pos;
while (strcmp(store, "exit!") != 0)
{
perror("Please enter next line of input (type 'exit!' to stop) \n"); //This repeats itself infinitely
fgets(store, BUFSIZE, stdin);
if ((pos = strchr(store, '\n')) != NULL)
*pos = '\0';
else
fprintf(stderr, "String too long!\n");
printf("%s", store); // This also repeats itself dependant on where i put
// fflush or another printf. Repeated outputs occur in blocks
}
return 0;
}