Why does strtok occasionally cause a bus error? - c

EDIT: I have made the info here more specific and executed some recommendations from the comments.
I have a shell written in C that works like a charm when used. However, I have some tests written for a function called pipe_exec that causes a bus error. I thought it was originally from strtok in my split function (and it may still be).
The pipe_exec func basically deals with commands with pipes like ls -a | wc -l or something. It always works fine when I'm using the actual shell but with the tests, there's always a bus error if there are any flags involved with the piped commands.
The issue could jut be with my test.
But I have no clue what the issue is. It's tracing back to the strtok in my split function, but it only has a bus issue with the tests and never in any actual equivalent situations.
Any help here is appreciated. Sorry for so much code to look at.
shell_exec_tests.c
static char *args1[20] = {"ls ", " wc"}; // works
static char *args2[20] = {"ls -a", "wc -l"}; // causes bus error
static int a = 0;
static int b = 0;
void test_setup(void)
{
a = pipe_exec(args1);
b = pipe_exec(args2);
}
void test_teardown(void)
{
// nothing
}
MU_TEST(test_check)
{
mu_check(a == EXIT_SUCCESS);
mu_check(b == EXIT_SUCCESS);
}
MU_TEST_SUITE(test_suite)
{
MU_SUITE_CONFIGURE(&test_setup, &test_teardown);
MU_RUN_TEST(test_check);
}
int main()
{
MU_RUN_SUITE(test_suite);
MU_REPORT();
return MU_EXIT_CODE;
}
pipe_exec.c
// make_proc: determine if a process goes to stdout or takes in data from stdin
void make_proc(int in, int out, char **cmd)
{
pid_t rc;
int status;
rc = fork();
if (rc < 0) {
perror("fork");
exit(1);
}
if (rc == 0) {
if (in != STDIN_FILENO) {
dup2(in, STDIN_FILENO);
close(in);
}
if (out != STDOUT_FILENO) {
dup2(out, STDOUT_FILENO);
close(out);
}
execvp(*cmd, cmd);
errmsg(*cmd);
exit(1);
}
waitpid(rc, &status, WUNTRACED);
return;
}
// pipe_exec: loop through each command, connecting each through a pipe
int pipe_exec(char **args)
{
int in, status, return_val;
int pipe_no; // keep track of no. of cmds seperated by pipes
int pfd[2];
pid_t rc;
char **cmd;
return_val = EXIT_SUCCESS;
in = 0;
pipe_no = 0;
while (*args) {
cmd = split(*args, " \t\r\n");
if (!args[1]) {
break;
}
if (pipe(pfd) < 0) {
perror("pipe");
}
make_proc(in, pfd[1], cmd);
close(pfd[1]);
in = pfd[0];
args++;
pipe_no++;
}
// move pointer back
args -= pipe_no;
rc = fork();
if (rc < 0) {
perror("fork");
exit(1);
}
if (rc == 0) {
if (in != 0) dup2(in, STDIN_FILENO);
execvp(*cmd, cmd);
errmsg(*cmd);
return_val = EXIT_FAILURE;
exit(1);
}
waitpid(rc, &status, WUNTRACED);
// pretty sure i need a pipe to get the EXIT_FAILURE from
// the child if the child fails, but for now im just working
// on finding that bus error issue
return return_val;
}
And lastly, my split function:
// trim: trim leading and trailing whitespace on a string
static char *trim(char *str)
{
char *end;
// Trim leading space
while(isspace((unsigned char)*str)) str++;
if(*str == 0) // All spaces?
return str;
// Trim trailing space
end = str + strlen(str) - 1;
while(end > str && isspace((unsigned char)*end)) end--;
// Write new null terminator character
end[1] = '\0';
return str;
}
// split: take a string and break it up into an array of strings based on delim
char **split(char *s, const char *delim)
{
char **split_s;
char *token;
size_t len;
int i;
len = strlen(s);
split_s = calloc(len*2, sizeof(char*));
if (split_s == NULL) {
fprintf(stderr, "split: could not allocate memory\n");
exit(EXIT_FAILURE);
}
i = 0;
token = strtok(s, delim);
while (token != NULL) {
split_s[i] = trim(token);
token = strtok(NULL, delim);
i++;
}
split_s[i] = NULL;
return split_s;
}

Related

Using pipes to communicate between two programs

I need the main prog to get two strings from the user and an argument for the other program, call fork() and then in child process I need to write the strings into pipe and send them to the other program which returns an int which I want to pass to parent so I'm trying to use another pipe for it but every time it stops right after inserting the strings.
So the main program: (EDITED)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <unistd.h>
#include <sys/wait.h>
#define LINELEN (80)
char *mygets(char *buf, int len);
int mygeti();
int main(int argc, char *argv[])
{
char *cmpstr[] = {"lexcmp", "lencmp"};
int veclen = sizeof(cmpstr)/sizeof(char *);
char str1[LINELEN + 1];
char str2[LINELEN + 1];
int index;
int pid[2];
int pfd[4][2];
for(int i = 0; i < 4; i++)
{
if(pipe(pfd[i]) < 0)
{
perror("pipe");
return -2;
}
}
pid[0] = fork();
if(pid[0] == 0) // child a
{
close(pfd[0][1]);
close(pfd[2][0]);
dup2(pfd[0][0], STDIN_FILENO);
dup2(pfd[2][1], STDOUT_FILENO);
char *myargs[3];
myargs[0] = "./loopcmp";
myargs[1] = "lexcmp";
myargs[2] = NULL;
if(execvp(myargs[0], myargs) == -1)
{
perror("exec");
return -2;
}
close(pfd[0][0]);
close(pfd[2][1]);
}
else
{
pid[1] = fork();
if(pid[1] == 0) //child b
{
close(pfd[1][1]);
close(pfd[3][0]);
dup2(pfd[1][0], STDIN_FILENO);
dup2(pfd[3][1], STDOUT_FILENO);
char *myargs[3];
myargs[0] = "./loopcmp";
myargs[1] = "lencmp";
myargs[2] = NULL;
if(execvp(myargs[0], myargs) == -1)
{
perror("exec");
return -2;
}
close(pfd[1][0]);
close(pfd[3][1]);
}
else // parent
{
while (1)
{
printf("Please enter first string:\n");
if (mygets(str1, LINELEN) == NULL)
break;
printf("Please enter second string:\n");
if (mygets(str2, LINELEN) == NULL)
break;
do {
printf("Please choose:\n");
for (int i=0 ; i < veclen ; i++)
printf("%d - %s\n", i, cmpstr[i]);
index = mygeti();
} while ((index < 0) || (index >= veclen));
close(pfd[index][0]);
if(write(pfd[index][1], str1, strlen(str1)) == -1)
{
perror("writeToPipe");
return -2;
}
if(write(pfd[index][1], str2, strlen(str2)) == -1)
{
perror("writeToPipe");
return -2;
}
if(index == 0)
{
close(pfd[2][1]);
char rbuf[1];
while(read(pfd[2][0], &rbuf, 1) > 0)
{
write(STDOUT_FILENO, &rbuf, 1);
}
}
if(index == 1)
{
close(pfd[3][1]);
char rbuf[1];
while(read(pfd[3][0], &rbuf, 1) > 0)
{
write(STDOUT_FILENO, &rbuf, 1);
}
}
}
}
}
return 0;
}
char *mygets(char *buf, int len)
{
char *retval;
retval = fgets(buf, len, stdin);
buf[len] = '\0';
if (buf[strlen(buf) - 1] == 10) /* trim \r */
buf[strlen(buf) - 1] = '\0';
else if (retval)
while (getchar() != '\n'); /* get to eol */
return retval;
}
int mygeti()
{
int ch;
int retval=0;
while(isspace(ch=getchar()));
while(isdigit(ch))
{
retval = retval * 10 + ch - '0';
ch = getchar();
}
while (ch != '\n')
ch = getchar();
return retval;
}
The other program - loopcmp: (Here I shouldn't change anything)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LINELEN (80)
int lencmp(const char *str1, const char *str2);
int lexcmp(const char *str1, const char *str2);
char *mygets(char *buf, int len);
int main(int argc, char *argv[])
{
int(*cmpfunc)(const char *, const char *) = NULL;
char str1[LINELEN + 1];
char str2[LINELEN + 1];
if (argc != 2)
return -1;
if (!strcmp(argv[1], "lexcmp"))
cmpfunc = lexcmp;
else if (!strcmp(argv[1], "lencmp"))
cmpfunc = lencmp;
else
return -1;
while (1)
{
if (mygets(str1, LINELEN) == NULL)
break;
if (mygets(str2, LINELEN) == NULL)
break;
printf("%d\n", cmpfunc(str1, str2));
fflush(stdout);
}
return 0;
}
int lencmp(const char *str1, const char *str2)
{
int val;
val = strlen(str1) - strlen(str2);
if (val < 0)
return 1;
if (val > 0)
return 2;
return 0;
}
int lexcmp(const char *str1, const char *str2)
{
int val;
val = strcmp(str1, str2);
if (val < 0)
return 1;
if (val > 0)
return 2;
return 0;
}
char *mygets(char *buf, int len)
{
char *retval;
retval = fgets(buf, len, stdin);
buf[len] = '\0';
if (buf[strlen(buf) - 1] == 10) /* trim \r */
buf[strlen(buf) - 1] = '\0';
else if (retval) while (getchar() != '\n'); /* get to eol */
return retval;
}
This is what I get:
Picture
and what I actually need it to print the interger returned from the exec of the child and then start again and get new two strings and so on till the user exits. what am I doing wrong? I can only modify the main program (the first one)
The first thing to do is ensure you are closing all unnecessary file descriptors in each process.
This means anything relating to the lexcmp child process should be closed in the lencmp child process, and vice versa. The parent needs to close the read ends of both "TO" pipes, and the write end of both "FROM" pipes.
Each of these closures should happen exactly once, where appropriate.
As is, in the parent, you are calling close(pfd[index][0]);, close(pfd[2][1]);, and close(pfd[3][1]); in a loop.
After calling dup2, you should immediately close the first argument (the original pipe end). As is, in the the children, you are attempting to close them after execvp is called, which leads into the next issue...
If execvp succeeds, it NEVER returns, as it will completely replace the process image. Anything expected to run after it is really operating in a failure state. So
if(execvp(myargs[0], myargs) == -1)
{
perror("exec");
return -2;
}
could be written as
execvp(myargs[0], myargs)
perror("exec");
return -2;
to the same effect.
Aside: the large if .. else if .. else structure of main is a bit hard to read, and not needed since the body of each if statement results in the child processes being replaced, or exiting on error.
The next issues have to do with deadlocking, which most typically occurs when two intercommunicating processes attempt blocking reads from one another at the same time.
Your child processes expect input in a very specific way: 2 lines at a time, creating a pair of strings. The two write calls, in the form of,
write(pfd[index][1], strX, strlen(strX))
do not write any newlines, thus the children wait forever, never to send any data back, and the parent will wait forever, never receiving any data.
Aside: mygets is severely flawed, in a few ways, including being unable to detect EOF or I/O failures (this function is a SIGSEGV in waiting). One of the more obnoxious failings is that the comment here
if (buf[strlen(buf) - 1] == 10) /* trim \r */
is just plain wrong. ASCII decimal 10 is '\n', the line feed, or newline character. '\r', or carriage return, would be decimal 13. This is why using character constants 'A' instead of integer constants 65 is highly encouraged.
The side effect here, generally speaking, is your strings are stripped of a trailing newline character.
The second deadlock occurs when you go to read the child process' response.
Firstly, this example
char rbuf[1];
while(read(pfd[N][0], &rbuf, 1) > 0)
{
write(STDOUT_FILENO, &rbuf, 1);
}
is malformed. Either remove the & operators, OR change char rbuf[1]; to char rbuf;. Fixing this, and the newline problem from above, will result in the parent process reading data back from the child.
The problem then becomes that a while (read(...) > 0) loop will continuously block execution of the calling process, waiting for more data to be available.
This means another deadlock when the child process has already moved on to trying to read another pair of lines from the parent process.
A simple solution is to attempt a single, reasonably large read in the parent, and rely on the behaviour of fflush(stdout); in the child to flush the pipe to the parent.
Here is a functional -ish example, with minimal changes made. This program still has some problems, such as: the parent process generally has no idea of the status of the child processes, and relying signal propagation (^C) from the terminal to end the process tree gracefully, since loopcmp does not handle EOF (should really discuss this with whoever wrote loopcmp.c / mygets).
Additionally, mygeti is flawed as well, as an invalid input cannot be distinguished from a valid input of 0. It also does not handle EOF, or prevent signed integer overflow.
Some more robust abstraction (functions and structures) around creating child processes would help a lot to clean this up further.
This should help you progress, though.
#define _POSIX_C_SOURCE 200809L
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
#include <unistd.h>
#define LINELEN (80)
char *mygets(char *buf, int len);
int mygeti();
void close_pipe(int fd[2])
{
close(fd[0]);
close(fd[1]);
}
int main(void)
{
char *cmpstr[] = {"lexcmp", "lencmp"};
int veclen = sizeof(cmpstr)/sizeof(char *);
char str1[LINELEN + 1];
char str2[LINELEN + 1];
int index;
int pid[2];
int pfd[4][2];
/* pfd[0] is TO lexcmp
* pfd[1] is TO lencmp
* pfd[2] is FROM lexcmp
* pfd[3] is FROM lencmp
*/
for(int i = 0; i < 4; i++)
if(pipe(pfd[i]) < 0) {
perror("pipe");
return -2;
}
pid[0] = fork();
if (pid[0] == 0) {
/* child lexcmp */
close_pipe(pfd[1]);
close_pipe(pfd[3]);
close(pfd[0][1]);
close(pfd[2][0]);
dup2(pfd[0][0], STDIN_FILENO);
dup2(pfd[2][1], STDOUT_FILENO);
close(pfd[0][0]);
close(pfd[2][1]);
char *args[] = { "./loopcmp", "lexcmp", NULL };
execvp(*args, args);
perror("exec");
return -2; /* This only returns from the child */
}
pid[1] = fork();
if (pid[1] == 0) {
/* child lencmp */
close_pipe(pfd[0]);
close_pipe(pfd[2]);
close(pfd[1][1]);
close(pfd[3][0]);
dup2(pfd[1][0], STDIN_FILENO);
dup2(pfd[3][1], STDOUT_FILENO);
close(pfd[1][0]);
close(pfd[3][1]);
char *args[] = { "./loopcmp", "lencmp", NULL };
execvp(*args, args);
perror("exec");
return -2; /* This only returns from the child */
}
/* parent */
close(pfd[0][0]);
close(pfd[1][0]);
close(pfd[2][1]);
close(pfd[3][1]);
while (1) {
printf("Please enter first string: ");
if (mygets(str1, LINELEN) == NULL)
break;
printf("Please enter second string: ");
if (mygets(str2, LINELEN) == NULL)
break;
do {
printf("Please choose (");
for (int i=0 ; i < veclen ; i++)
printf(" [%d] %s", i, cmpstr[i]);
printf(" ): ");
index = mygeti();
} while ((index < 0) || (index >= veclen));
if (0 >= dprintf(pfd[index][1], "%s\n%s\n", str1, str2)) {
fprintf(stderr, "Failed to write to child %d\n", index);
perror("dprintf");
return -2;
}
char buf[64];
ssize_t bytes = read(pfd[index + 2][0], buf, sizeof buf - 1);
if (-1 == bytes) {
perror("read from child");
return -2;
}
buf[bytes] = 0;
printf("Result: %s", buf);
}
}
char *mygets(char *buf, int len)
{
char *retval;
retval = fgets(buf, len, stdin);
buf[len] = '\0';
if (buf[strlen(buf) - 1] == 10) /* trim \r */
buf[strlen(buf) - 1] = '\0';
else if (retval)
while (getchar() != '\n'); /* get to eol */
return retval;
}
int mygeti()
{
int ch;
int retval=0;
while(isspace(ch=getchar()));
while(isdigit(ch))
{
retval = retval * 10 + ch - '0';
ch = getchar();
}
while (ch != '\n')
ch = getchar();
return retval;
}
Note the use of dprintf. If not available for whatever reason, just make sure to write a single newline after each string.
Final aside: with the way fgets works, the + 1 to the string buffer sizes are rather meaningless (although they are indirectly required here due to mygets performing its own, poorly designed buf[len] = '\0'). fgets writes at most len - 1 non-null bytes, always leaving room for the null terminating byte, which it places.

Segmentation fault when trying to run custom shell

School project to make our own shell and facing segmentation fault issues.
Can someone help?
Edit: kind of running but "quit" doesn't trigger the exit and execvp can't run properly (says : "no such file or directory" & "or ls:invalid option -- ' "
void interactive_mode();
void batch_mode(char *path);
void parse(char *str,char *delimiter, char **args);
int execute(char **args);
int main(int argc, char *argv[]) {
if(strcmp(argv[1], "-b") == 0) {
batch_mode(argv[2]);
}
else if(strcmp(argv[1], "-i") == 0) {
interactive_mode();
}
}
I suspect the segmentation fault problem derives from the interactive and batch modes code
void interactive_mode() {
int quit_flag;
char str[512];
char *commands[128];
char *args[128];
quit_flag = 0;
while(1) {
printf("whatever> ");
if (fgets(str,512,stdin) == NULL) {
exit(0); //error reading
}
int i = 0;
parse(str,";",commands); //split the string into commands eg "ls -a ; ls -l" -> "ls -a ","ls -l"
while (commands[i]!=NULL) {
parse(commands[i]," ",args); //split commands into arguments eg "ls -l" -> "ls","-l"
i++;
quit_flag = execute(args);
}
if (quit_flag == 1)
exit(1);
}
}
Trying to read from file:
void batch_mode(char *path) {
FILE *fp;
char str[512];
char *commands[128];
char *args[128];
int res;
fp = fopen(path,"r");
if (fp == NULL) {
perror("Error opening file");
exit(4); //file not open
}
while(1) {
if (fgets(str, 512, fp) == NULL)
break;
int i = 0;
parse(str, ";", commands);
while (commands[i] != NULL) {
parse(commands[i], " ", args);
i++;
res = execute(args);
}
}
fclose(fp);
printf("whatever>Press Any Key to Continue\n");
getchar();
}
Parsing strings:
void parse(char *str, char *delimiter, char **args) {
char *pch;
int i = 0;
pch = strtok(str,delimiter);
while (pch != NULL) {
args[i] = pch;
i++;
pch = strtok(NULL, delimiter);
}
args[i] = NULL;
}
Executing with fork:
int execute(char **args) {
char path[50];
pid_t pid;
int status;
if(strcmp(args[0],"quit")==0) return 1; //exited by quit
strcpy(path,"/bin/");
strcat(path,args[0]);
if ((pid = fork()) < 0) {
perror("fork failed");
exit(2);
}
else if (pid == 0) {
if(execvp(path, args) < 0) {
perror("execvp failed");
exit(3);
}
}
else {
while (wait(&status) != pid) /* wait for completion */
;
}
}
Please help?
Have you tried to print all your variables argv, str, commands ... To make sure all is going on as expected there ? It could also help you locate in your script where you segfault. It will take you time, sorry I have no quick answer. For starterprintf("%d", __LINE__) would print the line number you are at, don't forget to add an explicit statement.
Otherwise you could use gdb. There is quite a learning curve, but it could be useful for another project.

segmentation fault unix shell

I have an assignment for school where I have to create a shell which can do the following:
read incoming command, parse each part of command
fork child process and execute each command without (< > >> |)
successfully execute each command with <, >, >>
successfully execute each command with |
I am seriously lost... I am new to shell and I have no clue on what to do from here.
My code gives me an error stating segmentation fault (core dumped). Any and all help will be greatly appreciated.
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#define MAX_ARG 10
int main()
{
char line [256];
char prompt[] = "sh2 % ";
char command[256], *args[MAX_ARG];
int pid;
char status;
int i;
/* spit out the prompt */
printf("%s", prompt );
while( fgets(line, sizeof line, stdin) != NULL)
{
/* fgets leaves '\n' in input buffer. ditch it */
line [strlen(line)-1] = '\0';
while (line != NULL)
{
//parse command and arg
args[0] = strtok(line, " "); //grab command
for (i=1; i<MAX_ARG; i++) //grab arguments, to assume max = 10?
{
//if a single command with arguments then set command & argument
//for (i>0)
{
// check to see if the command is 'exit'
if(!strcmp("exit", args[i]))
{
exit(0);
}
{
int p[2];
pipe(p)
if (fork() == 0) //child
{
close (0);
dup(p[0]);
exec("cmd2");
}
else
{
close(1);
close(p[0]);
close(p[1]);
dup(p[1]);
exec("cmd1");
}
close(0);
open("stdout.txt", "r");
if (fork()== 0)
{
exec("cmd3");
}
}
else if (!strcmp(">", args[i]))
open("stderr.txt". "w")
if (fork() == 0)
{
exec("cmd1");
}
}
else if (!strcmp(">>", args[i]))
{
close(1);
open("stdout_stderr.txt", "w");
if (fork() == 0)
{
close(2);
dup(1);
exec("cmd2");
}
}
else
{
pid = fork();
if (pid == 0)
{
status = execvp(command,args);
exit(0);
}
else
{
waitpid(-1);
}
}
}
}
}
}
return 0;
}
You have quite a few things wrong in your program, so it's hard to point to one line and say change this. I think you are trying to do too much at once and not building on a solid base.
In programming you want to start small and build on your progress, your professor did you a favor by lining up the steps:
read incoming command, parse each part of command
fork child process
and execute each command without (< > >> |)
successfully execute
each command with <, >, >>
successfully execute each command with |
Try to get #1 working befor moving on to #2 and as mentioned before using functions is going to help a lot. I would suggest looking at this post http://bytes.com/topic/c/answers/215994-writing-shell-c which will give you a simple shell you could model from. Here is a baseline parser to get you started on #1 (based on the post mentioned)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char line[4096] = {0};
char safeline[4096] = {0};
int done = 0;
int i = 0;
char *s = line;
char **t = NULL;
char *prompt = ">";
char *args = NULL;
char *nl = NULL;
int main(void)
{
while(!done)
{
printf("%s", prompt);
fflush(stdout);
if(NULL == fgets(line, sizeof line, stdin))
{
t = NULL;
done = 1;
}
else
{
nl = strchr(line, '\n');
if(nl != NULL)
{
*nl = '\0';
strcpy(safeline, line);
}
else
{
int ch;
printf("Line too long! Ignored.\n");
while((ch = getchar()) != '\n' && ch != EOF)
{
continue;
}
if(ch == EOF)
{
done = 1;
}
}
args = strchr(line, ' ');
if(args != NULL)
{
*args++ = '\0';
}
if(!done)
{
printf("command - %s : args - %s\n",s, args);
}
}
}
}

C - wait, fail when getting the returned value

I have the following problem: In my code, here in line 83, I have this: check = wait(NULL);
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<string.h>
#include <sys/stat.h>
#include <sys/types.h>
//---------------------------------
//Function: parse_cmdline(const char* cmdline)
//This function takes the input from stdin
//and returns it as array of strings.
//If stdin is /bin/ls -l /usr/include
//the function will return ["/bin/ls","-l","/usr/include"]
//---------------------------------
char** parse_cmdline(const char* cmdline) {
int count, word_count = 0;
char** line_parsed, line_return;
char *pch, *cmdline_copy = (char*)malloc(sizeof(char)*(strlen(cmdline)+1));
strcpy(cmdline_copy, cmdline);
pch = strtok(cmdline_copy," \n\t\r");
while (pch != NULL) {
++word_count;
pch = strtok(NULL, " \n\t\r");
}
line_parsed = (char**)malloc((word_count+1)*sizeof(char*));
count = 0;
strcpy(cmdline_copy, cmdline);
pch = strtok(cmdline_copy," \n\t\r");
while (pch != NULL) {
line_parsed[count] = (char*)malloc((strlen(pch) + 1)*sizeof(char));
strcpy(line_parsed[count], pch);
++count;
pch = strtok(NULL," \n\t\r");
}
line_parsed[count] = NULL;
free(cmdline_copy);
return line_parsed;
}
int main() {
int count = 0, check;
size_t size;
char* line;
char** cmdline;
while(1) {
check = 0;
printf("$Monkey Eats:< ");
getline(&line, &size, stdin);
cmdline = parse_cmdline(line);
pid_t pid = fork();
if (pid == -1) {
perror("fork");
return -1;
} else if(pid == 0) {
struct stat _stat;
stat(cmdline[0],&_stat);
if(_stat.st_mode & S_IXUSR){
execvp(cmdline[0], cmdline);
}else fprintf(stderr,"%s: Permission denied!\n",cmdline[0]);
perror("");
exit(1);
}else {
check = wait(NULL);
}
count = 0;
while(cmdline[count] != NULL) {
free(cmdline[count]);
++count;
}
free(cmdline);
}
return 0;
}
It makes me a problem. When I run it and when I type a command I have the following message:
$Monkey Eats:< ls
ls: Permission denied!
No such file or directory
If I have only wait(NULL); the program runs normally without a problem. Can somebody tell me what is the problem? Thank you :)
The problem is trying to run ls. execvp() doesn't know where ls is. Try running /bin/ls as your command.
The problem is: stat(cmdline[0],&_stat); - the return code is not checked. What if file not found ? The program continues, and finds that _stat.st_mode & S_IXUSR is 0 (randomly).
However you may test the program as is with "/bin/ls" as input..

Using pipe system call in a mini shell

Although my program works correctly in all cases, it doesn't use a pipe to connect the output of the first of two commands to the second when they're separated by a pipe symbol. I wrote the output of the first command to a file, then redirected the standard input of the second command to the file when the process to run that command was run. I need to use a pipe system call to create the pipe and obtain the file descriptors
for the pipe, and then run the two processes at the same time. It is a homework question and I have done 99% of the work but somehow am not able to get the pipe system call working... what I've been trying is that for an input like: Command 1 | Command 2
inside the child process for command 2 I close FD[0] then dup FD[1] and for command 1 close FD[1] then dup FD[1] and close FD[0].
I am hell confused with the file descriptors when using pipe.... I have to use a pipe
Any sort of help is appreciated. Execute function is where I am forking the processes.
Here's my code...
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <regex.h>
/* Global Variables */
extern char **environ; /* Environmental Variables */
char *pathList[10]; /* List of paths from the $PATH */
int pathCount; /* Count of the # of paths in $PATH */
char *pathSet; /* Variable through which $PATH is retrieved */
int hasPipe = 0;
int cmdNo = 0;
/* This function takes the 'finalPath', the full path to executable,argList[],the
full command-line input arguments and argCount, the number of arguments from
command-line as input. It the creates a child process, in turn invokes the
execve() that finally executes the executable in 'finalPath' with the arguments
in 'argText' all stored into the args[] appropriately. Child process also handles
input and output file re-direction.
*/
void execute(char *finalPath, char *argList[], int argCount)
{
int k,fd,ofound,pos,i; /* flags and temporary variables */
pid_t pid; /* process ID */
int status, which;
char msg[100];
char *args[4]; /* argument list for execve() */
int spCase = 0;
ofound = 0;
pos=0;
pid = fork(); /* Creating a new process using fork() */
if (pid == -1) /* Checking for errors in process creation */
{
write(1,"Fork failed.\n",12);
exit(1);
}
/**************************
Checking for parent process
***************************/
if (pid != 0)
{
which = wait(&status);
if (which == -1)
{
write(1,"Wait failed.\n",12);
exit(1);
}
if (status & 0xff)
{ /* Case of abnormal termination */
sprintf(msg,"ERROR: <dShell> # process %d terminated abnormally for reason %d\n",which, status & 0xff);
write(1,msg,strlen(msg));
}
else
{ /* Case of normal termination */
sprintf(msg,"process %d terminated normally with status %d\n",which, (status >> 8) & 0xff);
write(1,msg,strlen(msg));
}
}
/*************************
Checking for child process
**************************/
if (pid == 0)
{
char argText[50];
argText[0] = '\0';
int std_fd;
if(cmdNo==0 && hasPipe)
{
close(1);
std_fd = open("temp.out", O_WRONLY | O_CREAT | O_TRUNC, S_IRWXU);
dup(std_fd);
}
else if(cmdNo==1 && hasPipe)
{
close(0);
std_fd = open("temp.out", O_RDONLY);
dup(std_fd);
}
/* Finding the first re-direction operator */
for( i = 0; i < argCount ; ++i)
{
if( ofound != 1 && ofound != 2)
{
if( strcmp(argList[i],"<") == 0 )
{
fd = open(argList[i+1],O_RDONLY);
if (fd < 0)
{
sprintf(msg,"ERROR: %s could not be opened\n", argList[i+1]);
write(1, msg, strlen(msg));
exit(5);
}
ofound = 1;
strcpy(argText,"\0");
close(0);
dup(fd);
close(fd);
}
else if(strcmp(argList[i],">") == 0)
{
fd = open(argList[i+1],O_CREAT | O_WRONLY, 0777);
pos = i;
ofound = 2;
strcpy(argText,"\0");
if (fd < 0)
{
sprintf(msg,"ERROR: %s could not be opened\n", argList[i+1]);
write(1, msg, strlen(msg));
exit(5);
}
close(1);
dup(fd);
close(fd);
}
}
}
/* If input re-direction operator is found check for an output re-direction along with it */
if(ofound == 1)
{
for( k = 0; k < argCount && ofound != 2; ++k)
{
if( strcmp(argList[k],">") == 0 )
{
fd = open(argList[k+1],O_CREAT | O_WRONLY , 0777);
spCase = 1;
ofound = 2;
strcpy(argText,"\0");
if (fd < 0)
{
sprintf(msg,"ERROR: %s could not be opened\n", argList[k+1]);
write(1, msg, strlen(msg));
exit(5);
}
close(1);
dup(fd);
close(fd);
}
}
}
/* If the re-direction operators are not found */
if( ofound == 0 )
{
for(i = 1; i < argCount; ++i)
{
strcat(argText, argList[i]);
strcat(argText, " ");
}
spCase = 2;
}
/* Case when both arguments and output re-direction operators are found */
if (spCase == 0)
{
if(pos == 0)
{
for( i = 3; i<argCount; ++i)
{
strcat(argText, argList[i]);
strcat(argText," ");
}
}
if(pos == argCount - 2)
{
for( i = 1; i<argCount - 2; ++i)
{
strcat(argText, argList[i]);
strcat(argText," ");
}
}
}
argText[strlen(argText)-1] = '\0'; /*because I added an extra space so trimming that*/
/* Running the execve */
args[0] = finalPath;
if(strlen(argText) == 0) /* checking if argText is populated */
{
args[1] = NULL;
}
else
{
args[1] = argText;
args[2] = NULL;
}
/* Execute command,if it returns that means it failed and need to display error and exit */
execve(args[0], args, environ);
sprintf(msg, "ERROR! execve() failed");
write(1, msg, strlen(msg));
}
}
/*******************************************************************************
This function checks if the path is accessible and continues to execute the
command. If the path does not exist of is not accessible, variable 'retFlag'
is used to return 0 to the calling function.
********************************************************************************/
int checkPath(char *exepath, char *argList[], int argCount, int flag)
{
char *finalPath;
int retFlag = flag;
if(access(exepath,X_OK) == 0)
{
finalPath = exepath;
retFlag = 1;
execute(finalPath,argList,argCount);
return retFlag;
}
else
return retFlag;
}
/**********************************************************************************
This function checks if the first argument is a path and if so calls checkPath().
Else it gets the paths set to the $PATH variable, tokenizes it, pads it with the
first token of input command and calls checkPath(). If the correct path is established,
the variable 'found' is used to kick out of the for loop.
************************************************************************************/
void setPath(char *argList[], int argCount)
{
char *exepath;
char com[50];
char emsg[80];
char *command;
int i,found = 0;
/* Seperating the command if redirection is used */
if( strcmp(argList[0],"<") == 0 || strcmp(argList[0],">") == 0 )
{
command = argList[2];
}
else
command = argList[0];
/* In case of no redirection, storing the commands and arguments into a array */
if(strcmp(command,"#") == 0) /* Checking for comment statements */
{
write(1,"ERROR: No command(s) found. Only comment present/n",48);
}
else
{
if(strstr(command,"/")) /* Checking if the entire path is given as a part of the command */
{
exepath = command;
found = checkPath(exepath,argList,argCount,0);
}
else /* building the path and storing it in 'com' */
{
for(i = 0; i< pathCount && found != 1; i++)
{
sprintf(com,"%s%s%s",pathList[i],"/",command);
exepath = com;
found = checkPath(exepath,argList,argCount,0);
}
}
if(found == 0)
{
sprintf(emsg,"%s%s",command,":COMMAND DOES NOT EXIST");
write(1,emsg,sizeof(emsg));
write(1,"\n",1);
}
}
}
/* Tokenizes commands into words */
void tokens(char *cmdStr)
{
char cmd[100];
strcpy(cmd,cmdStr);
char *result;
char delims[] = " , ";
char *argList[20];
int argCount = 0;
/*Tokenize the individual command into strings */
result = strtok(cmd,delims);
while( result != NULL )
{
argList[argCount] = result;
result = strtok( NULL, delims );
++argCount;
}
setPath(argList,argCount);
}
/* Tokenizes multiple commands into single commands */
void tokenize(char *inputStr)
{
int i,cmdCount = 0;
char *cmdResult;
char *cmdStr[100];
char delimiters[] = "|";
cmdResult = strtok(inputStr, delimiters);
while(cmdResult != NULL)
{
cmdStr[cmdCount]=cmdResult;
cmdResult = strtok(NULL, delimiters);
cmdCount++;
}
if( cmdCount > 1 )
hasPipe = 1;
else
hasPipe = 0;
for( i=0; i<cmdCount ; i++)
{
cmdNo = i%cmdCount;
tokens(cmdStr[i]);
}
}
int main(int argc, char *argv[])
{
char prompt[8]; /* String that stores the personalized prompt */
char *path; /* Temporary variable used for tokenization*/
char ch; /* Temporary variable used in read() */
int chCount; /* # of characters read from the prompt */
int entry; /* return variable of read() */
int flag; /* Flag to go read the next command when newline is found */
regex_t reIgnore;
char pattern[20]="^\\s*$|^#.*";
/* Tokenizing the paths asociated with the $PATH and storing them in a array declared globally */
pathCount = 0;
pathSet = getenv("PATH");
if ( !pathSet)
{
write(1, "ERROR: PATH environment does not exist.\n", 40);
exit(1);
}
path = strtok(pathSet,":");
while(path != NULL)
{
pathList[pathCount] = path;
path = strtok(NULL,":");
++pathCount;
}
/* Checks for blanks and tabs in Step 2 */
if ( regcomp(&reIgnore, pattern, REG_EXTENDED) )
{
write(1, "Error. \n",9);
exit(2);
}
sprintf(prompt,"<dShell> # "); /* Storing the personalized shell prompt into 'prompt' */
/* Reading the input from command line and passing it to tokenize() */
while(1)
{
char inputStr[100]; /* String into which inputs are read into */
chCount = 0;
flag = 0;
hasPipe = 1;
write(1,prompt,strlen(prompt)); /* Printing out the personalized shell prompt */
/* This will read a character 1 by 1 until it reaches the end of file */
entry = read(0,&ch,1);
if(!entry)
exit(0);
/* Reading the input and storing it in inputStr as long as newline is not encountered */
while( entry != 0 && flag == 0 )
{
/* A newline has been found so a new command will need to be executed */
/* The inputStr till this point is sent to tokenize() */
if( ch == '\n' )
{
inputStr[chCount] = '\0';
flag = 1;
if(chCount > 0) {
if(strcmp(inputStr,"exit") == 0)
exit(3);
else
tokenize(inputStr);
}
}
inputStr[chCount] = ch;
chCount++;
if(flag == 0)
entry = read( 0, &ch, 1 );
}
}
}
See the man page for pipe(2). It has this example:
#include <sys/wait.h>
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
int
main(int argc, char *argv[])
{
int pipefd[2];
pid_t cpid;
char buf;
assert(argc == 2);
if (pipe(pipefd) == -1) {
perror("pipe");
exit(EXIT_FAILURE);
}
cpid = fork();
if (cpid == -1) {
perror("fork");
exit(EXIT_FAILURE);
}
if (cpid == 0) { /* Child reads from pipe */
close(pipefd[1]); /* Close unused write end */
while (read(pipefd[0], &buf, 1) > 0)
write(STDOUT_FILENO, &buf, 1);
write(STDOUT_FILENO, "\n", 1);
close(pipefd[0]);
_exit(EXIT_SUCCESS);
} else { /* Parent writes argv[1] to pipe */
close(pipefd[0]); /* Close unused read end */
write(pipefd[1], argv[1], strlen(argv[1]));
close(pipefd[1]); /* Reader will see EOF */
wait(NULL); /* Wait for child */
exit(EXIT_SUCCESS);
}
}

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