Simple shell with indirect input - c

Question

I am writing a simple code to implement the indirect input function for a unix/linux shell.
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
extern void error(char* message);
void
cisshRedirectedInput(char* command[], char* inputFile)
{
//Try to implement the RedirectInput from here
pid_t pid;
int status;
int fd;
//For the child process
if ((pid=fork())==0)
{
//Try to input files, failing on an error
fd=open(inputFile,O_RDONLY);//To read input file
if(fd < 0)
{
error("sampleSh: error opening standard input file");
exit(1);
}
//use dup() to copy file
close(1);
if(dup(fd) < 0)
{
error("sampleSh: error duplicating standard input");
perror("dup()");
exit(1);
}
//Close file and exec()
close(fd);
execvp(command[0], command);
//If failure in any case
error("sampleSh: failure to execute command");
exit(1);
}
else
{
/* This is the parent process.
* Wait for the child to terminate.
*/
if(wait(&status) < 0)
{
error("sampleSh: error waiting for child.");
perror("wait");
}
if(status != 0)
error("sampleSh: command exited with nonzero error status.");
}
}
However, after compilation (no error reported), but when I try (fileList created already)
sort -r <fileList
The shell just stuck there without giving me answer, what is the problem please?

Answer 1

The standard input file descriptor is 0 (or STDIN_FILENO), not 1 (or STDOUT_FILENO).
Either use:
int fd = open(inputFile, O_RDONLY);
if (fd < 0) …
close(0);
if (dup(fd) < 0) …
close(fd);
Or:
int fd = open(inputFile, O_RDONLY);
if (fd < 0) …
if (dup2(fd, 0) < 0) …
close(fd);
It is good that your code does the close(fd) after duplicating to a standard I/O descriptor — that is almost always correct. It's also good that you are checking that the key system calls succeed. (There isn't much you can do if close() fails.)
This simple modification of your code (key change: use close(0); instead of close(1);) works for me. Did you null terminate your argument list?
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <unistd.h>
static inline void error(char *message)
{
fprintf(stderr, "%s\n", message);
}
void
cisshRedirectedInput(char *command[], char *inputFile);
void
cisshRedirectedInput(char *command[], char *inputFile)
{
// Try to implement the RedirectInput from here
pid_t pid;
int status;
int fd;
// For the child process
if ((pid = fork()) == 0)
{
// Try to input files, failing on an error
fd = open(inputFile, O_RDONLY); // To read input file
if (fd < 0)
{
error("sampleSh: error opening standard input file");
exit(1);
}
// use dup() to copy file
close(0);
if (dup(fd) < 0)
{
error("sampleSh: error duplicating standard input");
perror("dup()");
exit(1);
}
// Close file and exec()
close(fd);
execvp(command[0], command);
// If failure in any case
error("sampleSh: failure to execute command");
exit(1);
}
else
{
/* This is the parent process.
* Wait for the child to terminate.
*/
if (wait(&status) < 0)
{
error("sampleSh: error waiting for child.");
perror("wait");
}
if (status != 0)
error("sampleSh: command exited with nonzero error status.");
}
}
int main(void)
{
char *args[] = { "sort", "-r", 0 };
cisshRedirectedInput(args, "fileList");
return 0;
}
Input file:
bash-assoc-arrays.sh
cissh.c
fileList
kwargs.py
makefile
posixver.h
rangeinc.c
select.c
spc.py
testcsv.py
uncrustify.bug
yield.py
Output:
yield.py
uncrustify.bug
testcsv.py
spc.py
select.c
rangeinc.c
posixver.h
makefile
kwargs.py
fileList
cissh.c
bash-assoc-arrays.sh