Segmentation fault (core dumped) - c

I'm writing a program in c that basically copies files, but I'm getting this error: Segmentation fault (core dumped). From what I'm reading I think it's because I'm trying to access memory that hasn't been allocated yet. I'm a newbie when it comes to c and I suck at pointers, so I was wondering if you guys could tell me which pointer is causing this and how to fix it if possible. Btw, this program is supposed to be a daemon, but I haven't put anything inside the infinite while loop at the bottom.
Here is my code:
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <syslog.h>
#include <string.h>
#include <dirent.h>
int main(int c, char *argv[]) {
char *source, *destination;
char *list1[30], *list2[30], *listDif[30];
unsigned char buffer[4096];
int i=0, x=0, sizeSource=0, sizeDest=0, sizeDif=0;
int outft, inft,fileread;
int sleeper;
struct dirent *ent, *ent1;
//Check number of arguments
if(c<3)
{
printf("Daemon wrongly called\n");
printf("How to use: <daemon name> <orginDirectory> <destinationDirectory> \n");
printf("or : <daemon name> <orginDirectory> <destinationDirectory> <sleeperTime(seconds)>");
return 0;
}
//Checks if sleeper time is given or will be the default 5minutes
/*if(c=4)
{
char *p;
errno = 0;
long conv = strtol(argv[3], &p, 10);
if(errno != 0 || *p != '\0')
{
printf("Number given for sleeper incorrect, it has to be an integer value.\n");
return(0);
} else
{
sleeper = conv;
}
} else
{
sleeper = 300;
}*/
//Get path of directories from arguments
source = argv[1];
destination = argv[2];
//Check if directories exist
DIR* dirSource = opendir(source);
if (!dirSource)
{
printf("Source directory incorrect\n");
return 0;
}
DIR* dirDest = opendir(destination);
if (!dirDest)
{
printf("Destination directory incorrect\n");
return 0;
}
/* save all the files and directories within directory */
while ((ent = readdir (dirSource)) != NULL) {
list1[sizeSource] = strdup(ent->d_name);
sizeSource++;
if(sizeSource>=30){break;}
}
closedir(dirSource);
while((ent1 = readdir (dirDest)) != NULL) {
list2[sizeDest] = strdup(ent1->d_name);
sizeDest++;
if(sizeDest>=30){break;}
}
closedir(dirDest);
/* Verify the diferences between the directories and save them */
int z;
int dif = 0; //0 - False | 1 - True
printf("Diferenças:\n");
for(i=0;i<sizeSource;i++){
dif = 0;
for(z=0;z<sizeDest;z++){
if(strcmp(list1[i],list2[z])==0){ //If there is no match, it saves the name of the file to listDif[]
dif = 1;
break;
}
}
if(dif==0) {
printf("%s\n",list1[i]);
listDif[sizeDif] = list1[i];
sizeDif++;
}
}
/* This code will copy the files */
z=0;
while(z!=sizeDif){
// output file opened or created
char *pathSource, *pathDest;
strcpy(pathSource, source);
strcat(pathSource, "/");
strcat(pathSource, listDif[z]);
strcpy(pathDest, destination);
strcat(pathDest, "/");
strcat(pathDest, listDif[z]);
// output file opened or created
if((outft = open(pathDest, O_CREAT | O_APPEND | O_RDWR))==-1){
perror("open");
}
// lets open the input file
inft = open(pathSource, O_RDONLY);
if(inft >0){ // there are things to read from the input
fileread = read(inft, buffer, sizeof(buffer));
printf("%s\n", buffer);
write(outft, buffer, fileread);
close(inft);
}
close(outft);
}
/* Our process ID and Session ID */
pid_t pid, sid;
/* Fork off the parent process */
pid = fork();
if (pid < 0) {
exit(EXIT_FAILURE);
}
/* If we got a good PID, then
we can exit the parent process. */
if (pid > 0) {
exit(EXIT_SUCCESS);
}
/* Change the file mode mask */
umask(0);
/* Open any logs here */
/* Create a new SID for the child process */
sid = setsid();
if (sid < 0) {
/* Log the failure */
exit(EXIT_FAILURE);
}
/* Change the current working directory */
if ((chdir("/")) < 0) {
/* Log the failure */
exit(EXIT_FAILURE);
}
/* Close out the standard file descriptors */
close(STDIN_FILENO);
close(STDOUT_FILENO);
close(STDERR_FILENO);
/* Daemon-specific initialization goes here */
/* The Big Loop */
while (1) {
//sleep(5); /* wait 5 seconds */
}
exit(EXIT_SUCCESS);
}
The result of ls is:
ubuntu#ubuntu:~/Desktop$ ls
Concatenar_Strings.c core D2 daemon.c examples.desktop
Concatenar_Strings.c~ D1 daemon daemon.c~ ubiquity.desktop
D1 and D2 are folders, and in D1 are three text documents that I want to copy into D2.
One other question, is this a delayed error or an immediate one? Because I doubt this message would appear on a code line that with two integers.
Thanks in advance guys.

This loop is wrong:
while ((ent = readdir (dirSource)) != NULL) {
list1[sizeSource] = ent->d_name;
Probably, ent points to the same memory block every time, and the readdir function updates it. So when you save that pointer, you end up with your list containing invalid pointers (probably end up all pointing to the same string). Further, the string may be deallocated once you got to the end of the directory.
If you want to use the result of readdir after closing the directory or after calling readdir again you will need to take a copy of the data. In this case you can use strdup and it is usually good style to free the string at the end of the operation.
This may or may not have been the cause of your segfault. Another thing to check is that you should break out of your loops if sizeSource or sizeDest hits 30.
In the strcmp loop, you should really set dif = 0 at the start of the i loop, instead of in an else block.
Update: (more code shown by OP)
char *pathSource, *pathDest;
strcpy(pathSource, source);
You are copying to a wild pointer, which is a likely cause of segfaults. strcpy does not allocate any memory, it expects that you have already allocated enough.
One possible fix would be:
char pathSource[strlen(source) + 1 + strlen(listDif[z]) + 1];
sprintf(pathSource, "%s/%s", source, listDif[z]);
Alternatively (without using VLA):
char pathSource[MAX_PATH]; // where MAX_PATH is some large number
snprintf(pathSource, MAX_PATH, "%s/%s", source, listDif[z]);
Do the same thing for pathDest.
NB. Consider moving the closedir lines up to after the readdir loops; generally speaking you should open and close a resource as close as possible to the times you start and finish using them respectively; this makes your code easier to maintain.

Related

How to change STDIN stream to binary

I'm making an upload form via a CGI interface. I'm writing it in C and don't want to use any outside libraries (ie. cgic).
I thought the program was complete, as the first test files uploaded correctly. But they were ASCII files. When I tested with a binary file (JPG). It seems that STDIN is trying to read the binary data as ASCII which creates a problem for characters like \0 which is present at the end of an ASCII file, but is a common character in binary files. The results of uploading a 1.9MB file end up with a 38kB file.
When searching how to change the STDIN stream to binary, I was referred to the command freopen and told to use NULL as the argument for the file. example 1
It says:
If filename is a null pointer, the freopen() function shall attempt to
change the mode of the stream to that specified by mode, as if the
name of the file currently associated with the stream had been used.
In this case, the file descriptor associated with the stream need not
be closed if the call to freopen() succeeds. It is
implementation-defined which changes of mode are permitted (if any),
and under what circumstances.
But when I check the man page on my system with man 3 freopen, it doesn't say any of
this at all. Furthermore, reading the man page, I find out the the
option for binary (adding 'b' to the mode) is no longer recognized and
only exists for archaic compliancy:
The mode string can also include
the letter 'b' either as a last character or as a character between
the characters in any of the two-character strings described above.
This is strictly for compatibility with C89 and has no effect; the 'b'
is ignored on all POSIX conforming systems, including Linux.
So right now I'm completely lost. How can I change the STDIN stream to read binary input?
Here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
// Declare constants.
#define BUF_SIZE 4096
#define FILENAME_SIZE 500
#define MARKER_SIZE 100
#define RETURN_FAILURE 0
#define RETURN_SUCCESS 1
#define SEARCH_STRING_1 "filename=\""
#define SEARCH_STRING_2 "\r\n\r\n"
// Declare global variables.
char filename[FILENAME_SIZE + 1];
char *program_name;
// Declare function prototype.
void print_footer (void);
void print_header (void);
void process_input (char *data);
int main (int argc, char *argv[])
{
// Declare variables.
long long ret;
char buf[BUF_SIZE + 1];
// Get program name for error reporting.
program_name = basename(argv[0]);
// Prepare output for browser.
print_header();
// Protect variable against buffer overflow.
buf[BUF_SIZE] = '\0';
// Loop through all the file data.
while(1)
{
// Read in the next block of data.
if((ret = (long long) fread(buf, 1, BUF_SIZE, stdin)) != BUF_SIZE)
{
// Check for error.
if(ferror(stdin) != 0)
{
printf("%s: An error occurred while reading the input file.<br>\n", program_name);
process_input(NULL);
exit(EXIT_FAILURE);
}
// Check for EOF.
else if(feof(stdin) != 0)
break;
}
// Terminate and process uploaded data.
buf[ret] = '\0';
process_input(buf);
}
// Terminate and process uploaded data.
buf[ret] = '\0';
process_input(buf);
// Finish user output, close output file and exit.
print_footer();
process_input(NULL);
exit(EXIT_SUCCESS);
}
void process_input (char *data)
{
// Declare variables.
char *ptr1= NULL;
char *ptr2;
int x = 0;
static FILE *fp;
static int flag = 0;
static char marker[MARKER_SIZE + 1];
// If data is NULL, close output file.
if(data == NULL)
{
if(fclose(fp) == EOF)
{
printf("%s: process_input: close failed (%s)<br>\n", program_name, strerror(errno));
exit(EXIT_FAILURE);
}
return;
}
// Check if this is the first time through.
if(flag == 0)
{
// Get marker.
if((ptr1 = strchr(data, '\n')) == NULL)
{
printf("%s: process_input: strchr(1) failed (\n)<br>\n", program_name);
exit(EXIT_FAILURE);
}
ptr1[0] = '\0';
strcpy(marker, data);
ptr1[0] = '\n';
// Get filename.
if((ptr1 = strstr(data, SEARCH_STRING_1)) == NULL)
{
printf("%s: process_input: strstr(1) failed (%s)<br>\n", program_name, SEARCH_STRING_1);
exit(EXIT_FAILURE);
}
// Advance pointer to start of filename.
ptr1 += 10;
// Find end of filename.
if((ptr2 = strchr(ptr1, '"')) == NULL)
{
printf("%s: process_input: strchr(2) failed (\")<br>\n", program_name);
exit(EXIT_FAILURE);
}
// Terminate and store filename.
ptr2[0] = '\0';
strcpy(filename, ptr1);
ptr2[0] = '"';
// Remove spaces from filename.
while(filename[x] != '\0')
{
if(filename[x] == ' ')
filename[x] = '.';
x++;
}
// Open output file.
if((fp = fopen(filename, "wb")) == NULL)
{
printf("%s: process_input: fopen failed (%s) (%s)<br>\n", program_name, strerror(errno), filename);
exit(EXIT_FAILURE);
}
// Find start of file data.
if((ptr1 = strstr(data, SEARCH_STRING_2)) == NULL)
{
printf("%s: process_input: strstr(2) failed (%s)<br>\n", program_name, SEARCH_STRING_2);
fclose(fp);
exit(EXIT_FAILURE);
}
// Set flag.
flag++;
// Advance pointer to start of file data.
ptr1 += 4;
// Change STDIN stream to binary.
if(freopen(NULL, "rb", stdin) == NULL)
{
printf("%s: process_input: freopen failed (%s)<br>\n", program_name, strerror(errno));
fclose(fp);
exit(EXIT_FAILURE);
}
}
// Catch everything else.
else
{
ptr1 = data;
if((ptr2 = strstr(ptr1, marker)) != NULL)
ptr2[0 - 2] = '\0';
}
// Write file data.
if(fwrite(ptr1, 1, strlen(ptr1), fp) != strlen(ptr1))
{
printf("%s: process_input: write failed (%s)<br>\n", program_name, strerror(errno));
fclose(fp);
exit(EXIT_FAILURE);
}
}
void print_footer (void)
{
printf("\nMade it!\n");
}
void print_header (void)
{
printf("Content-type: text/plain\r\n\r\n");
}
Ok, it appears what #NominalAnimal said was correct. You can store binary data in a string, but the moment you use any function in the string.h library, it almost always changes what is stored in that string (if the data is binary).
The easy solution is to make a separate function that takes a pointer to the binary data and do your string searches in that function, returning what pertinent information is needed. That way, the original data is never changed.
'stdin' is a macro of STDIN_FILENO, which is egal to 0. See also 'unistd.h'.
You are not showing your code, but I think you stop when you encounter a '\0' or a non-ascii char, since you said you were using 'fread()'.
You have to stop when fread() function returns 0, which means it stopped to read : it encountered EOF.

Semaphore simulation program: Segmentation Fault error

I've written the following program, that simulates the work of semaphore. There are three functions: lock, unlock, lockpath.
lock = opens the file; checks if the file already exists, and if it does, puts the current process to sleep. If the file didn't exist, it is created and TRUE is returned.
unlock = deletes the file
lockpath = returns the path name corresponding to the file that might be created.
Here's the source code:
#include <unistd.h>
//exit();
#include <stdlib.h>
//errno
#include <errno.h>
//creat(..)
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
//strcat, strcpy
#include <string.h>
//For err_sys
#include <stdio.h>
#define LOCKDIR "/tmp/"
#define MAXTRY 3
#define WAITTIME 5
enum BOOLEAN{TRUE, FALSE};
void err_sys(const char* x) {
perror(x);
exit(1);
}
static char* lockpath(char* name) {
static char path[20];
strcpy(path, LOCKDIR);
return (strcat(path, name));
}
int lock(char* name) {
char *path;
int fd, incerc;
extern int errno;
path = lockpath(name);
int try = 0;
while ((fd = open(path, O_WRONLY | O_CREAT | O_EXCL, 0666)) < 0
&& errno == EEXIST) {
if (++try >= MAXTRY)
return FALSE;
sleep(WAITTIME);
}
if (fd < 0 || close(fd) < 0)
err_sys("lock");
return TRUE;
}
void unlock(char* name) {
if (unlink(lockpath(name)) < 0)
err_sys("unlock");
}
int main(void) {
pid_t child_process;
child_process = fork();
char* sem_file_name = "test_semaf";
if (child_process != 0)
{
printf("\nParent process ID: %d", getpid());
}
else
{
printf("\nChild process ID: %d", getpid());
}
if (lock(sem_file_name))
{
printf("\nProcess with ID: %d", getpid());
printf("\nonly, has access to %s", strcat(LOCKDIR, sem_file_name)); //****
unlock(sem_file_name);
} else {
printf("\nProcess with ID: %d", getpid());
printf("\nwas unable to get access to %s", strcat(LOCKDIR, sem_file_name));
}
return 0;
}
The line at which the program stops is marked with: ****
The error is:
Program received signal SIGSEGV, Segmentation fault.
__strcat_ssse3 () at ../sysdeps/x86_64/multiarch/strcat-ssse3.S:571
571 ../sysdeps/x86_64/multiarch/strcat-ssse3.S: No such file or directory.
The problem is that I get Segmentation Fault, and can't find where's the problem. To me, everything's fine. A process is supposed to create file X. Then, if another process tries to create it's own file X, it is not allowed; the process is put to sleep. This second process is allowed to make MAXTRY attempts. If it does not succeed after MAXTRY attempts, the lock() function returns FALSE. Finally, when a process, that has successfully created his own X file, doesn't need it now, the file X is deleted.
Can you, please, tell what do you think is the problem with this program? Thank you in advance.
EDIT :
Here's the link to the page that explains why lockpath() function isn't correct.
Is returning a pointer to a static local variable safe?
This is the cause of your crashes:
strcat(LOCKDIR, sem_file_name)
Here you try to append to a literal string constant.
You should use the lockpath function here as well.
The problem seems to be in your misunderstanding of strcat() function. The function appends string in second parameter to the string in first parameter - but you need to ensure there is enough space for the data. Read the man page.
That means that
char * dest = "whatever";
strcat(dest, anything_else);
is always wrong. What you want is
char dest[SIZE] = "whatever";
strcat(dest, anything_else);
where SIZE is big enough for the buffer to be able to contain the whole concatenated string.
Also, your lockpath() function is broken. See this answer to learn why. You need to create the dest buffer outside the lockpath() function and pass it to it as a parameter.

Applying fork() and pipe() (or fifo()) on counting words code

I've completed writing of counting words code finally. It counts total number of words in files. (i.e. txt). Now, I want to use multiple fork() to access and read every file. I studied in the last week. Besides, I use global variable to hold number of counted words. As far as I know, If I apply fork(), used global variables are assigned as 0. To avoid it, I tried to use mmap() and similar functions this is okey. But, I also want to use pipe() also (fifo() if it is possible) to communicate (hold values of numbers).
I use nftw() function to go in folders and files. My logic is on the below picture. How can use fork() and pipe() (fifo()) on this code ? fork() is really complicated for me because of my inexperience. I'm new using of pipe() and fork(). According to my idea logic of the code is that if I can use fork() and pipe(), there will be fork() every file(i.e. txt) and access them by using fork. If there is another folder and there are files, again creates fork() from one of created forks , then access file. I try to explain also drawing below. Thank you. I want to learn using of them.
int countInEveryFolder(const char *dir)
is used because I don't know how to count files until the next folder in nftw() function. Number of files is necessary because it is number of fork.
Every folder should be parent of files. The files are included by the folder.
Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <dirent.h>
#include <errno.h>
#include <ftw.h>
#include <ctype.h>
#include <sys/mman.h>
#include <locale.h>
#include <errno.h>
#define MAX_PATH_LEN 2048
unsigned long total_words = 0UL;
unsigned long total_dirs = 0UL;
unsigned long total_files = 0UL;
// Just proves counting number of file in a folder
int countInEveryFolder(const char *dir) {
struct stat stDirInfo;
struct dirent * stFiles;
DIR * stDirIn;
char szFullName[MAX_PATH_LEN];
char szDirectory[MAX_PATH_LEN];
struct stat stFileInfo;
int numOfFile = 0;
strncpy( szDirectory, dir, MAX_PATH_LEN - 1 );
if (lstat( szDirectory, &stDirInfo) < 0)
{
perror (szDirectory);
return 0;
}
if (!S_ISDIR(stDirInfo.st_mode))
return 0;
if ((stDirIn = opendir( szDirectory)) == NULL)
{
perror( szDirectory );
return 0;
}
while (( stFiles = readdir(stDirIn)) != NULL)
{
if (!strcmp(stFiles->d_name, ".") || !strcmp(stFiles->d_name, ".."))
continue;
sprintf(szFullName, "%s/%s", szDirectory, stFiles -> d_name );
if (lstat(szFullName, &stFileInfo) < 0)
perror ( szFullName );
/* is the file a directory? */
if (S_ISREG(stFileInfo.st_mode))
{
printf( "Filename: %s\n", szFullName );
numOfFile++;
}
} // end while
closedir(stDirIn);
return numOfFile;
}
// Count words in files.
unsigned long count_words_in_file(const char *const filename)
{
unsigned long count = 0UL;
int errnum = 0;
int c;
FILE *in;
in = fopen(filename, "rt");
if (in == NULL) {
errnum = errno;
fprintf(stderr, "%s: %s.\n", filename, strerror(errnum));
errno = errnum;
return 0UL;
}
/* Skip leading whitespace. */
do {
c = getc(in);
} while (isspace(c));
/* Token loop. */
while (c != EOF) {
/* This token is a word, if it starts with a letter. */
if (isalpha(c))
count++;
/* Skip the rest of this token. */
while (!isspace(c) && c != EOF)
c = getc(in);
/* Skip the trailing whitespace. */
while (isspace(c))
c = getc(in);
}
/* Paranoid checking for I/O errors. */
if (!feof(in) || ferror(in)) {
fclose(in);
fprintf(stderr, "Warning: %s: %s.\n", filename, strerror(EIO));
errnum = EIO;
} else
if (fclose(in)) {
fprintf(stderr, "Warning: %s: %s.\n", filename, strerror(EIO));
errnum = EIO;
}
errno = errnum;
return count;
}
// Recursively go in folders
int nftw_callback(const char *filepath, const struct stat *sb, int typeflag, struct FTW *ftwbuf)
{
// Directory
if (typeflag == FTW_DP || typeflag == FTW_D)
{
total_dirs++;
printf("%*s%s\n", ftwbuf->level * 4, "", filepath);
//countInEveryFolder(filepath);
}
// Folder
else if (typeflag == FTW_F)
{
total_files++;
total_words += count_words_in_file(filepath);
printf("%*s%s\n", ftwbuf->level * 4, "", filepath);
}
return 0;
}
/* Error message */
void err_sys(const char *msg)
{
perror(msg);
fflush(stdout);
exit(EXIT_FAILURE);
}
int main(int argc, char *argv[])
{
total_files = total_dirs = total_words = 0UL;
if (nftw(argv[1], nftw_callback, 15, FTW_PHYS) == 0) {
/* Success! */
printf("%s: %lu files, %lu directories, %lu words total.\n",
argv[1], total_files, total_dirs, total_words);
} else {
/* Failed... */
err_sys("ntfw");
}
putchar('\n');
//printf( "\nTotal words = %d\n\n", *wordCount);
//printf( "\nTotal folders = %d\n\n", *folderCount);
//printf( "\nTotal childs = %d\n\n", *childCount); //fork()
return 0;
}
To start I would write the program with two phases. A single-process phase in which all the file-paths are queued up (into a linked-list or dequeue), and a multi-process phase in which the worker processes receive work via their pipe() and send counts back to the main process via their pipe(). The main process would use select() to multiplex the input from its children.
Once you understand how to use select() with pipe()s, then work on having the filepath discovery be concurrent.
This design would be much easier to implement in Go, node.js, or greenlet with Python, but learning how to do it in C gives you a level of understanding for the underlying operations that you don't get with newer languages.

The shell I am writing does not exit correctly after execvp() fails

I have an assignment to make a shell in C code, and I have a solution that works most of the time. My solution works if the program exists, and I can exit my shell with either Control-D or by typing exit. But when I try a command that I know doesn't exist, my shell will print an error message saying command not found but I will have to either type exit or press Control-D the same amount of times as a invalid command was entered i.e. if I type a wrong command 3 times, I then have to hit Control-D 3 times. I really don't know what is going on here. I checked all the variables and read is -1 when I press Control-D but the if statement seems to be skipped.
Here is the parts of my source code that I think the problem is in:
comp20200Shell.c
#include "comp20200Shell_header.h"
#include <signal.h>
/*
* Name: ****
* Student Number: ****
* Email: ****
*
* This is the main function of my shell implementation.
*
*/
int main(void)
{
bool end_program = false;
size_t length = 0;
ssize_t read;
char* current_directory = NULL;
char* current_time = NULL;
/* Sets up signal handler to catch SIGINT*/
if(signal(SIGINT, sigintHandler) == SIG_ERR)
{
error("An error occured while setting a signal handler\n");
}
/* Infinitive loop, so after command or invalid comman will prompt again*/
while(end_program != true)
{
char* input = NULL;
/* Gets current working directory */
current_directory = return_current_directory();
/* Gets current date and time */
current_time = return_time();
/* Prints Prompt */
printf("%s\x5b%s\x5d %s%s %s%s%s", MAGENTA_TEXT, current_time, GREEN_TEXT, current_directory, BLUE_TEXT, PROMPT, RESET_COLOUR);
/* Frees the pointers returned by return_time() and return_current_directory() */
free(current_time);
free(current_directory);
/* Reads one line from standard input */
read = getline(&input, &length, stdin);
/* Checks if ctrl d, i.e. end of file is found or exit is typed */
if(strcmp(input, "exit\n") == 0 || read == -1)
{
if(read == -1)
{
putchar('\n');
}
/* Frees input */
free(input);
return(0);
}
/* Removes newline character that will be at the end */
remove_trailing_newline(input);
/* Passes input to process input, and the return value is passed in to process errors */
process_errors(process_input(&input));
/* Frees input */
free(input);
}
return(0);
}
process_input.c
#include "comp20200Shell_header.h"
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
/*
* Name: ****
* Student Number: ****
* Email: ****
*
* This function is used to process the command entered by the user
*
* return: the error value or 0 when everything whent ok
* arguments: the command entered by the user
*
*/
int process_input(char** input)
{
bool redirect_stdout = false;
bool redirect_stderr = false;
pid_t child_pid;
int child_status;
char** argument = malloc(sizeof(char*));
int count = 0;
char* temp = strtok(*input, " ");
while(temp != NULL)
{
argument[count] = temp;
count ++;
argument = realloc(argument, (count+2) * sizeof(char *));
temp = strtok(NULL, " ");
}
argument[count] = NULL;
if(argument[0] == NULL)
{
return(0);
}
else if(strcmp(argument[0], "cd") == 0)
{
return(change_directory(argument[1]));
}
int index;
for(index = 1; argument[index] != NULL; index++)
{
if(strcmp(argument[index], ">0") == 0)
{
if(argument[index + 1] == NULL)
{
return(EINVAL);
}
redirect_stdout = true;
break;
}
else if(strcmp(argument[index], ">2") == 0)
{
if(argument[index + 1] == NULL)
{
return(EINVAL);
}
redirect_stderr = true;
break;
}
}
child_pid = fork();
if(child_pid == 0)
{
int file;
if(redirect_stdout == true)
{
file = open(argument[index + 1], O_WRONLY|O_CREAT|O_TRUNC, 0666);
dup2(file, 1);
edit_arguments(argument, index);
execvp(argument[0], argument);
return(-1);
}
else if(redirect_stderr == true)
{
file = open(argument[index + 1], O_WRONLY|O_CREAT|O_TRUNC, 0666);
dup2(file, 2);
edit_arguments(argument, index);
execvp(argument[0], argument);
return(-1);
}
execvp(argument[0], argument);
return(-1);
}
else
{
wait(&child_status);
}
return(child_status);
}
comp20200Shell_header.h
/*
* Name: ****
* Student Number: ****
* Email: ****
*
* This is my header file, It includes all common used headerfiles on the top.
* Any specific header file that is only used once will be included with the .c file that needs it.
*
*/
/* included headerfiles begin */
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <stdbool.h>
/* included headerfiles end */
/* defenitions begin */
#define PROMPT "# "
#define BUFFER_SIZE 1024
#define BLUE_TEXT "\x1B[34m"
#define MAGENTA_TEXT "\x1B[35m"
#define GREEN_TEXT "\x1B[32m"
#define RESET_COLOUR "\x1B[0m"
/* defenitions end */
/* Function prototypes begin */
void remove_trailing_newline(char *input);
void sigintHandler(int sig_num);
int process_input(char** input);
char* return_time(void);
void error(const char *fmt, ...);
int change_directory(char* path);
char* return_current_directory(void);
void process_errors(int return_value);
void edit_arguments(char** argument, int index);
/* Function prototypes end */
I have omitted the rest of the source code as I don't think the problem is there.
In your child, after the call to execvp you need to call exit(EXIT_FAILURE); instead of return -1;. Otherwise your child will continue running, and will interpret the next command (that is why you need to exit N times where N is the number of inexistant commands you tried to invoke).
After the change, your parent process will see that the child terminated with a non-zero return code and should interpret the error code. There is no real way to distinguish between a failure from the execvp (due to a non-existent command) or from the invoked process. I would recommend printing the error from execvp if there is one in the child before the exit.
Note that if execvp succeed, it will never return, so the code following a call to execvp can only be executed if the command failed.
So, my recommendation is doing this:
if(child_pid == 0)
{
int file;
if(redirect_stdout == true)
{
file = open(argument[index + 1], O_WRONLY|O_CREAT|O_TRUNC, 0666);
dup2(file, 1);
edit_arguments(argument, index);
execvp(argument[0], argument);
perror("execvp");
exit(EXIT_FAILURE);
}
else if(redirect_stderr == true)
{
file = open(argument[index + 1], O_WRONLY|O_CREAT|O_TRUNC, 0666);
dup2(file, 2);
edit_arguments(argument, index);
execvp(argument[0], argument);
perror("execvp");
exit(EXIT_FAILURE);
}
execvp(argument[0], argument);
perror("execvp");
exit(EXIT_FAILURE);
}
else
{
wait(&child_status);
}
You should be doing exit(1); or equivalent instead of return(-1);. You might want to use _exit(1);, or _exit(255); (or _exit(-1);, but it is equivalent to _exit(255);). You might well want to print an error message to standard error before you exit.
When you don't exit, you end up with two, then three, then N shells all trying to read input from the terminal. You have to make each one quit separately by indicating EOF with Control-D. If you tried typing commands, then it would become a lottery which shell gets each character, and that leads to chaos (and grave danger; you may have thought you typed grep fungible form.file | tr -d 'fr' > /tmp/x33 but if one of the shells got rm -fr /, you've got trouble!).
Instead of return -1, you can use exit(1) or exit (-1) to exit from that portion if it fails to execute due to some error.

Why is stat() returning EFAULT?

I'm writing a program that when run from two separate bash sessions as two separate processes, opens a named pipe between the two to allow strings to be sent from one to the other.
When the process is first executed from one terminal, it checks stat(fname, buf) == -1 to see if a file at path fname exists and if not, creates it. The process then assumes that since it was the one to make the FIFO, it is the one that will be sending messages through it and continues accordingly.
After that occurs, the program can then be run from another terminal that should determine that it will be the receiver of messages through the pipe by checking stat(fname, buf) == -1. The condition should return false now, and stat(fname, buf) itself should return 0 because there exists a file at fname now.
But for reasons I am unable to discern, when the second process is run, stat(fname, buf) still returns -1. The variable errno is set to EFAULT. The man page for stat() only decribes EFAULT as "Bad address." Any help determining why the error occurs or what is meant by "Bad address." would be greaty appreciated.
I've verified that the file is indeed created by the first process as intended. The first process waits at the line pipe = open(fname, O_WRONLY); because it can't continue until the other end of pipe is opened.
Edit: The following is a self-contained implementation of my code. I have confirmed that it compiles and experiences the problem I described here.
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_LINE 80
#define oops(m,x) { perror(m); exit(x); }
int main(int argc, char const *argv[]) {
char line[MAX_LINE];
int pipe, pitcher, catcher, initPitcher, quit;
struct stat* buf;
char* fname = "/tmp/absFIFOO";
initPitcher = catcher = pitcher = quit = 0;
while (!quit) {
if (((!pitcher && !catcher && stat(fname, buf) == -1) || pitcher) && !quit) {
// Then file does not exist
if (errno == ENOENT) {
// printf("We're in the file does not exist part\n");
if (!pitcher && !catcher) {
// Then this must be the first time we're running the program. This process will take care of the unlink().
initPitcher = 1;
int stat;
if (stat = mkfifo(fname, 0600) < 0)
oops("Cannot make FIFO", stat);
}
pitcher = 1;
// open a named pipe
pipe = open(fname, O_WRONLY);
printf("Enter line: ");
fgets(line, MAX_LINE, stdin);
if (!strcmp(line, "quit\n")) {
quit = 1;
}
// actually write out the data and close the pipe
write(pipe, line, strlen(line));
close(pipe);
}
} else if (((!pitcher && !catcher) || catcher) && !quit) {
// The first condition is just a check to see if this is the first time we've run the program. We could check if stat(...) == 0, but that would be unnecessary
catcher = 1;
pipe = open("/tmp/absFIFO", O_RDONLY);
// set the mode to blocking (note '~')
int flags;
flags &= ~O_NONBLOCK;
fcntl(pipe, F_SETFL, flags); //what does this do?
// read the data from the pipe
read(pipe, line, MAX_LINE);
if (!strcmp(line, "quit\n")) {
quit = 1;
}
printf("Received line: %s\n", line);
// close the pipe
close(pipe);
}
}
if (initPitcher)
unlink(fname);
return 0;
}
You have this piece of code:
struct stat* buf;
...
if (((!pitcher && !catcher && stat(fname, buf) == -1)
When you call stat(), buf isn't initalized and there's no telling what it points to.
You must allocate some storage for it, so stat() has a valid place to store the result.
The easiest thing is to just allocate it on the stack:
struct stat buf;
...
if (((!pitcher && !catcher && stat(fname, &buf) == -1)
You have not shown your code, but EFAULT means 'bad address'. This indicates that you have not properly allocated (or passed) your buffer for stat or the filename (fname).
buf isn't initialised anywhere. What exactly do you expect to happen?

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