CODEWITHSUNDEEP
c++shellstdoutstdindup2
Gluteus Maximus
Gluteus Maximus

Reputation: 11

Bad File Descriptor - Dual I/O redirection

I have a homework assignment that requires that I implement my own Linux shell. A part of this requires that I implement functionality to redirect input and output redirection within the same command.

I'm getting a "Sort: read failed: -: Bad file descriptor" error when trying to run sort < "filename" > "filename". Any help is appreciated!

int dualRedirect(char *toks[], string uCommand) {
int stats;
int fd;
int fd1;
int size;
vector<string> file;
string inFileName;
string outFileName;
string buffer;
int stdIn = dup(0);
int stdOut = dup(1);

stringstream stream(uCommand);

// Convert the command string to a vector
while (stream >> buffer)
    file.push_back(buffer);

// Identify the size of the vector in order to identify the output filename
size = file.size();

outFileName = toks[size - 1];

// Find "<" in order to find the input filename, then set it to NULL in order 
// to pass the appropriate args to the exec command
for (int ii = 0; toks[ii] != NULL; ii++) {
    if (!strcmp(toks[ii], "<")) {
        inFileName = toks[ii + 1];
        toks[ii] = NULL;
    }
}

// Open the input file and assign it to the fd variable
if ((fd = open(inFileName.c_str(), O_CREAT | O_WRONLY )) == -1) {
    cerr << strerror(errno);
    return 1;
}

// Set STDIN to the fd variable (redirect stdin to fd)
if (dup2(fd, STDIN_FILENO) == -1) {
    return 1;
}

// Open the output filename and assign it to fd1
if ((fd1 = open(outFileName.c_str(), O_CREAT | O_WRONLY )) == -1) {
    cerr << strerror(errno);
    return 1;
}

// Set STDOUT to the fd1 variable (redirect stdout to fd1)
if (dup2(fd1, 1) == -1) {
    cerr << strerror(errno);
    return 1;
}

// Close the original fd file
if (close(fd) == -1) {
    cerr << strerror(errno);
    return 1;
}

// Close the original fd1 file
if (close(fd1) == -1) {
    cerr << strerror(errno);
    return 1;
}

// fork and execute, passing the command and args to exec.
if (fork()) {
    waitpid(-1, &stats, NULL);
}
else {
    execvp(toks[0], toks);
    exit(-1);
}

// Restore the stdin and stdout file descriptors to their original values
dup2(stdIn, 0);
dup2(stdOut, 1);

return 1; }

Upvotes: 1

Views: 1492

Answers (1)

Barmar
Barmar

Reputation: 781088

Change

if ((fd = open(inFileName.c_str(), O_CREAT | O_WRONLY )) == -1) {

to:

if ((fd = open(inFileName.c_str(), O_RDONLY )) == -1) {

One of the causes of the "Bad file descriptor" error is trying to read from a descriptor that isn't open for reading.

Upvotes: 1

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