So I have the following C code which asks the user to give a command (valid in unix), and then I have to take the string and split it in an array in order to execute the command that the user gave with execvp(). It compiles but the execvp doesn't seem to work. Is something wrong in the way I split the user's input in an array? PS: Some of the includes aren't neccessary but it's not the final program.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <signal.h>
main() {
char str[64];
int i =0;
char *p = strtok(str," ");
printf("Please give me a Unix command! :\n");
gets(str);
char *array[sizeof(str)];
while (p!=NULL) {
array[i++] = p;
p = strtok (NULL, " ");
}
execvp(str ,array);
perror("execvp");
}
The output I get when I run this is:
Please give me a Unix command! :
ls -l
execvp: No such file or directory
You're calling strtok(str, " ") before str has any information.
Simply call it after you get input:
main() {
char str[64];
char *array[sizeof(str)];
char *p = NULL;
int i = 0;
printf("Please give me a Unix command! :\n");
fgets(str, sizeof(str), stdin); // Use fgets instead of gets.
p = strtok(str," ");
while (p != NULL) {
array[i++] = p;
p = strtok(NULL, " ");
}
execvp(str, array);
}
The first problem as I see here is with
char *p = strtok(str," ");
as you're trying to read indeterminate values. str is not initialized, there's no guarantee that there is a null-terminator present there which makes this a string. So, you're essentially invoking undefined behavior.
That said,
gets(), it is dangerous as it lead to buffer overflow. use fgets() instead.
for an array variable, sizeof(str) does not give you the size of the content, it returns you the size of the whole array. You might want to use strlen() to get the length of the string, but remember, strlen() does not count the null-terminator.
Related
I would like to read standard input of a command and its argument in a C program, for instance:
ATTACK 50 30
I would like my program to parse through the input using whitespace and assign each word to a variable but right now I would just like to print each word. However, when I tried the program only returned ATTACK and not ATTACK 50 30.
I tried:
int main(){
// Grid size declaration //
int *x, *y;
char command[20];
char user_input[100];
scanf("%s", user_input);
printf("%s", user_input);
return 0;
}
As I said I used ATTACK 50 30 as my STDIN but my printf function only returned ATTACK. I thought of maybe using a while loop to keep scanning until the character interpreted is the return key (which I believe in this case would just be the null character?). I tried it using the code below:
int main(){
// Grid size declaration //
int *x, *y;
char command[20];
char user_input[100];
while(scanf("%s", user_input)!="\0"){
scanf("%s", user_input);
printf("%s", user_input);
}
return 0;
}
This did not work, the error produced declared I was comparing a pointer to an integer.
Since you are dealing with stdin it would probably be a better idea to utilize the fgets function in lieu of the scanf function, and then parse the inputted line of data utilizing the strtok string function.
Utilizing that strategy, following is a snippet of code allowing for the parsing of entered text where each word or data group is identified.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX 64
int main()
{
char line[MAX + 1];
const char delim[2] = " ";
char * wd;
printf("Enter some text or type \"quit\" to end: ");
while (fgets(line, MAX, stdin))
{
line[strlen(line) - 1] = ' '; /* Replace newline character at the end with a space */
wd = strtok(line, delim);
while (wd != NULL)
{
if (strcmp(wd, "quit") == 0)
{
return 0;
}
printf("%s\n", wd);
wd = strtok(NULL, delim);
}
printf("Enter some text or type \"quit\" to end: ");
}
return 0;
}
Testing out this code utilizing your text example yielded the following terminal output.
#Dev:~/C_Programs/Console/ParseWord/bin/Release$ ./ParseWord
Enter some text or type "quit" to end: ATTACK 50 30
ATTACK
50
30
Enter some text or type "quit" to end: quit
#Dev:~/C_Programs/Console/ParseWord/bin/Release$
This is just a springboard from where you might go, but test that out and see if it meets the spirit of your project.
I have the following C code:
#include <stdio.h>
int main()
{
char* ptr;
printf("Enter the word: ");
gets(ptr);
printf("The input string is: ");
puts(ptr);
return 0;
}
It compiles and asks for the input, but after I enter the input, it takes a pause and exits. No further commands are executed or displayed. I am unable to understand the problem. Please help.
#include <stdio.h>
int main()
{
char str[100] = "";
printf("Enter the word: ");
fgets(str, sizeof str, stdin);
printf("The input string is: ");
printf("%s", str);
return 0;
}
Try to use fgets(). gets() is dangerous to use because gets() is inherently unsafe, because it copies all input from STDIN to the buffer without checking size. This allows the user to provide a string that is larger than the buffer size, resulting in an overflow condition.
puts is simpler than printf but be aware that the former automatically appends a newline. If that's not what you want, you can fputsyour string to stdout or use printf.
#include <stdlib.h>
#include <stdio.h>
int main ()
{
char word[100];
while (word != "hello") {
system("clear");
printf("\nSay hello to me : ");
scanf(" %s", word);
}
printf("congrats, you made it !");
return 0;
}
In this code : if i enter anything but hello, the loop continue. However, entering the ENTER key will not loop again, it will just add a line.
I read somewhere that using getchar() might help but I'm kinda new to the C developpement and I'm stuck here searching for hours how to make it works.
EDIT 0 :
Removed
while (word != "hello")
char word[100];
scanf(" %s", word);
Added
#include <string.h>
while (strcmp(word, "hello") != 0)
char word[100] = {0};
fgets(word, 6, stdin);
EDIT 1 :
I tried to include in my code something like that
fgets(word, 6, NULL);
But it got me a segmentation fault.
**EDIT 2 : **
The correct working input is :
fgets(word, 6, stdin);
So it worked, but adding more than 6 character to the question like :
Say hello to me : hello from the inside
Will just print :
Say hello to me :
Say hello to me :
So I just modified the function like that :
fgets(word, 100, stdin);
But now it will not get me any input to work
Three things:
You don't need the space in the scanf format string. The %s format specifier already ignores leading whitespace. So instead of " %s" use "%s".
The main problem is word != "hello". That's not how strings are compared. What you're actually doing is comparing the address of word with the address of the string constant "hello". To do a string comparison, use strcmp. If it returns 0, the strings are the same, so your while loop should check for non-zero:
while (strcmp(word,"hello")) {
Be sure to #include <string.h> to get the declaration of strcmp.
Finally, you need to initialize word so that the initial string comparison doesn't invoke undefined behavior by reading uninitialized data:
char word[100] = {0};
#dbush well answered OP initial concerns.
OP is now using fgets(word, 100, stdin); and types in h e l l o Enter. word[] is then filled with "hello\n" and this does not pass strcmp(word, "hello") != 0.
Solution: strip final '\n'.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define BUFFER_SIZE 100
int main() {
char word[BUFFER_SIZE] = { 0 };
while (strcmp(word, "hello") != 0) {
system("clear");
printf("\nSay hello to me : ");
// insure buffered output is flushed
fflush(stdout);
// Avoid magic numbers, use `sizeof word`
// Test if input was received
if (fgets(word, sizeof word, stdin) == NULL) {
fprintf(stderr, "\nInput closed\n");
return 1;
}
// lop off potential trailing \n
word[strcspn(word, "\n")] = '\0';
}
printf("congrats, you made it !\n");
return 0;
}
I am unable to take two inputs strings simultaneously in C on Ubuntu. It shows the wrong output.
Simple program:
#include <stdio.h>
main()
{
char s1[20],char s2[20],printf("\nEnter job:");
scanf("%[^\n]s",s1);
printf("Enter hobby:");
scanf("%[^\n]s",s2);
}
Output:
Enter job:student
Enter hobby:
student
It does not allow the input of a second string. How can I overcome this bug?
If you want to allow embedded spaces, modify the scanf formats this way:
#include <stdio.h>
int main(void) {
char job[100], hobby[100];
printf("Enter job:");
scanf("%99[^\n]%*c", job);
printf("Enter hobby:");
scanf("%99[^\n]%*c", hobby);
printf("%s,%s", job, hobby);
return 0;
}
But be aware that empty lines will not be accepted by this scanf format. The linefeed will stay in the input stream, the second scanf will fail too and job and/or hobby will have indeterminate contents, letting printf invoke undefined behavior.
Is is much more reliable to use fgets() and strip the '\n'.
#include <stdio.h>
#include <string.h>
int main(void) {
char job[100], hobby[100];
printf("Enter job:");
if (!fgets(job, sizeof job, stdin))
return 1;
job[strcspn(job, "\n")] = '\0';
printf("Enter hobby:");
if (!fgets(hobby, sizeof hobby, stdin))
return 1;
hobby[strcspn(hobby, "\n")] = '\0';
printf("%s,%s", job, hobby);
return 0;
}
#include <stdio.h>
#include <math.h>
#include <string.h>
int main(void)
{
int menuswitch=1;
int amountofstudents;
int fname[50];
int lname[50];
int grade[50];
int i;
char studentinfo[100];
printf("Enter Amount of Students: ");
scanf("%d", &amountofstudents);
for (i=0;i<amountofstudents;i++)
{
gets(studentinfo);
strcpy(fname[i], strtok(studentinfo, " "));
strcpy(lname[i], strtok(NULL, " "));
strcpy(grade[i], strtok(NULL, " "));
}
Alright need a little using strtok. I am trying to store pieces of an input string to sort later. I was thinking of using strtok to break the string then placing each piece in the corresponding array. Yet, every time I try I get an error in Visual Studios saying Access Violation. Thanks for the help ahead of time
The error is
First-chance exception at 0x5120F7B3 (msvcr110d.dll) in Lab 2.exe: 0xC0000005: Access violation reading location 0x00000000.
Unhandled exception at 0x5120F7B3 (msvcr110d.dll) in Lab 2.exe: 0xC0000005: Access violation reading location 0x00000000.
The input would be
FirstName Lastname 80(Grade)
One major problem is that you try to copy into integer values and not strings. Change the
integer arrays to arrays of strings:
...
char fname[50][100];
char lname[50][100];
char grade[50][100];
...
You also have a problem with the gets function (besides it being obseleted and should not be used), namely that the previous scanf doesn't remove the newline from the input buffer so the first gets call will see this empty newline and give you an empty line (which you do not check for).
This is simply solved by telling scanf to discard trailing whitespace by adding a space in the format string after the "%d":
scanf("%d ", &amountofstudents);
/* ^ */
/* | */
/* Note space */
Instead of gets, you should be using fgets:
fgets(studentinfo, sizeof(studentinfo), stdin);
And finally, always check for errors!
a potential issue is the scanf/gets combo. use instead fgets() and convert when appropriate to integer using atoi() it is also good to do a sanity check on what is returned from strtok (it is never good to assume anything about input)
char* token = strtok(studentinfo, " ");
if ( strlen(token) < sizeof(fname[i]) )
{
strcpy(fname[i], token);
...
you have also declared your strings as integer arrays, they should be char
e.g. char fname[50];
The problem you have is that you have declared three variables (fname, lname, and grade) as char[] (arrays) (well, that is the type you meant to use), but you want to prompt for and keep around a bunch of students information. And you later try to copy from strtok() into what you want to be a char[], but since you dereferenced fname[i] (lname[i], grade[i]), they are of type char, rather than char[].
You will need stdlib.h for exit,
#include <stdio.h>
#include <stdlib.h> //for exit
#include <string.h>
//#include <math.h> //you don't need this, yet
#define STUDLIMIT (100)
You can either create an array of fname[], lname[], grade[], (see here: http://www.cs.swarthmore.edu/~newhall/unixhelp/C_arrays.html),
int main(void)
{
//int menuswitch=1; //you aren't using this
int amountofstudents;
char studentinfo[100];
char fname[STUDLIMIT][50];
char lname[STUDLIMIT][50];
char grade[STUDLIMIT][50];
int ndx;
printf("Enter Amount of Students: ");
if( (fscanf(stdin,"%d ", &amountofstudents)<=0)
|| (amountofstudents<1) || (amountofstudents>STUDLIMIT) )
{
printf("need %d to %d studends\n",1,STUDLIMIT); exit(0);
}
for (ndx=0;ndx<amountofstudents;ndx++)
{
printf("Student: "); fflush(stdout);
fgets(studentinfo,sizeof(studentinfo),stdin);
strcpy(fname[ndx], strtok(studentinfo, " "));
strcpy(lname[ndx], strtok(NULL, " "));
strcpy(grade[ndx], strtok(NULL, " "));
}
}
Or you can create a struct(ure) to hold the entered student information, and instantiate an array of these student records, one for each student you enter and store,
typedef struct student
{
char fname[50];
char lname[50];
char grade[50];
} StudentObj;
int StudentCopy(StudentObj*sp,char*fname,char*lname,char*grade)
{
if(!sp || !fname || !lname || !grade ) return -1;
strcpy(sp->fname, fname);
strcpy(sp->fname, lname);
strcpy(sp->fname, grade);
}
StudentObj students[STUDLIMIT];
int main(void)
{
//int menuswitch=1; //you aren't using this
int amountofstudents;
char studentinfo[100];
char fname[50];
char lname[50];
char grade[50];
int ndx;
printf("Enter Amount of Students: ");
if( (fscanf(stdin,"%d ",&amountofstudents)<=0)
|| (amountofstudents<1) || (amountofstudents>STUDLIMIT) )
{
printf("need %d to %d studends\n",1,STUDLIMIT); exit(0);
}
for (ndx=0;ndx<amountofstudents;ndx++)
{
printf("Student: "); fflush(stdout);
fgets(studentinfo,sizeof(studentinfo),stdin);
strcpy(fname, strtok(studentinfo, " "));
strcpy(lname, strtok(NULL, " "));
strcpy(grade, strtok(NULL, " \n\r"));
StudentCopy(&(students[ndx]),fname,lname,grade);
}
}