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Issues with execvp() and incomplete multi-argument commands

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0

I'm using execvp() to run some system calls. Program works great for valid commands, and fails for any command that doesn't exist, which is perfect.
The program is, when I use execvp() on a command that needs extra arguments(like cat) and I don't provide arguments, the program just infinitely reads from input.

I'm not sure how to get around this issue, as I don't know how to 'tell' if a command is incomplete. Any ideas?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];


 printf("Enter command: ");
 scanf("%[^n]s", command);

 char *temp = strtok(command, " ");
 char *commandList[100];
 int index = 0;

 while (temp != NULL) 
 commandList[index] = temp;
 index++;

 temp = strtok(NULL, " ");
 

 commandList[index] = NULL;

 execvp(commandList[0], commandList);

 printf("Failed");

The ideal result would be a print of "Command incomplete" and the process ending.

c exec system-calls

share|improve this question

asked Mar 23 at 16:34

FlacarileFlacarile

111

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If the program you are starting never needs to read from stdin, why don't you just close it before execvp()?
  
  – EOF
  Mar 23 at 16:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The common way to avoid commands from reading from stdin is to open STDIN_FILENO to /dev/null
  
  – Antti Haapala
  Mar 23 at 16:40
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @EOF — one reason for not simply closing standard input is that programs are entitled to assume that standard input, standard output and standard error are opened appropriately. Redirecting standard input so it comes from /dev/null is a much better idea than simply closing standard input.
  
  – Jonathan Leffler
  Mar 23 at 21:31
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that cat does not require any extra arguments; when it is invoked with no arguments, it reads up to EOF on standard input. It is working exactly as designed. It would be best to report errors on stderr instead of stdout; it would be best to include the failed command name in the error message (in case it wasn't what the user thought they typed); it would be best to finish the error message with a newline (fprintf(stderr, "Failed (%s)n", commandList[0]);).
  
  – Jonathan Leffler
  Mar 23 at 21:33
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, and when a program fails, it should exit or return a non-zero exit code (either EXIT_FAILURE from <stdlib.h>, or conventionally 1). That lets the calling code (shell or whatever) know it was not successful.
  
  – Jonathan Leffler
  Mar 23 at 21:43
  

  
  
  
  

 | 
show 2 more comments

0

I'm using execvp() to run some system calls. Program works great for valid commands, and fails for any command that doesn't exist, which is perfect.
The program is, when I use execvp() on a command that needs extra arguments(like cat) and I don't provide arguments, the program just infinitely reads from input.

I'm not sure how to get around this issue, as I don't know how to 'tell' if a command is incomplete. Any ideas?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];


 printf("Enter command: ");
 scanf("%[^n]s", command);

 char *temp = strtok(command, " ");
 char *commandList[100];
 int index = 0;

 while (temp != NULL) 
 commandList[index] = temp;
 index++;

 temp = strtok(NULL, " ");
 

 commandList[index] = NULL;

 execvp(commandList[0], commandList);

 printf("Failed");

The ideal result would be a print of "Command incomplete" and the process ending.

c exec system-calls

share|improve this question

asked Mar 23 at 16:34

FlacarileFlacarile

111

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  If the program you are starting never needs to read from stdin, why don't you just close it before execvp()?
  
  – EOF
  Mar 23 at 16:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  The common way to avoid commands from reading from stdin is to open STDIN_FILENO to /dev/null
  
  – Antti Haapala
  Mar 23 at 16:40
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @EOF — one reason for not simply closing standard input is that programs are entitled to assume that standard input, standard output and standard error are opened appropriately. Redirecting standard input so it comes from /dev/null is a much better idea than simply closing standard input.
  
  – Jonathan Leffler
  Mar 23 at 21:31
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Note that cat does not require any extra arguments; when it is invoked with no arguments, it reads up to EOF on standard input. It is working exactly as designed. It would be best to report errors on stderr instead of stdout; it would be best to include the failed command name in the error message (in case it wasn't what the user thought they typed); it would be best to finish the error message with a newline (fprintf(stderr, "Failed (%s)n", commandList[0]);).
  
  – Jonathan Leffler
  Mar 23 at 21:33
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Oh, and when a program fails, it should exit or return a non-zero exit code (either EXIT_FAILURE from <stdlib.h>, or conventionally 1). That lets the calling code (shell or whatever) know it was not successful.
  
  – Jonathan Leffler
  Mar 23 at 21:43
  

  
  
  
  

 | 
show 2 more comments

I'm using execvp() to run some system calls. Program works great for valid commands, and fails for any command that doesn't exist, which is perfect.
The program is, when I use execvp() on a command that needs extra arguments(like cat) and I don't provide arguments, the program just infinitely reads from input.

I'm not sure how to get around this issue, as I don't know how to 'tell' if a command is incomplete. Any ideas?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];


 printf("Enter command: ");
 scanf("%[^n]s", command);

 char *temp = strtok(command, " ");
 char *commandList[100];
 int index = 0;

 while (temp != NULL) 
 commandList[index] = temp;
 index++;

 temp = strtok(NULL, " ");
 

 commandList[index] = NULL;

 execvp(commandList[0], commandList);

 printf("Failed");

The ideal result would be a print of "Command incomplete" and the process ending.

c exec system-calls

share|improve this question

asked Mar 23 at 16:34

FlacarileFlacarile

111

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];


 printf("Enter command: ");
 scanf("%[^n]s", command);

 char *temp = strtok(command, " ");
 char *commandList[100];
 int index = 0;

 while (temp != NULL) 
 commandList[index] = temp;
 index++;

 temp = strtok(NULL, " ");
 

 commandList[index] = NULL;

 execvp(commandList[0], commandList);

 printf("Failed");

share|improve this question

asked Mar 23 at 16:34

FlacarileFlacarile

111

share|improve this question

asked Mar 23 at 16:34

FlacarileFlacarile

111

asked Mar 23 at 16:34

FlacarileFlacarile

111

asked Mar 23 at 16:34

FlacarileFlacarile

111

1 Answer
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oldest

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0

One of the ideas from the comment completely answered my question (To the exact needs I had). Not sure how to give him credit on here, though.

The solution is to simply close stdin right before I use execvp(). If the command is not completed on the first scanf, the program throws an error, which is perfect.
Since I'm running the main program I'm using this on in a loop, I can use dup and dup2 to save and reload stdin later.

The code I used to test if it'll work:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];

 int stdinput = dup(STDIN_FILENO);

 close(STDIN_FILENO);

 dup2(stdinput, STDIN_FILENO);


 printf("Enter command: ");
 scanf("%[^n]s", command);


 printf("%sn", command);

share|improve this answer

answered Mar 23 at 16:47

FlacarileFlacarile

111

add a comment | 

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0

One of the ideas from the comment completely answered my question (To the exact needs I had). Not sure how to give him credit on here, though.

The solution is to simply close stdin right before I use execvp(). If the command is not completed on the first scanf, the program throws an error, which is perfect.
Since I'm running the main program I'm using this on in a loop, I can use dup and dup2 to save and reload stdin later.

The code I used to test if it'll work:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];

 int stdinput = dup(STDIN_FILENO);

 close(STDIN_FILENO);

 dup2(stdinput, STDIN_FILENO);


 printf("Enter command: ");
 scanf("%[^n]s", command);


 printf("%sn", command);

share|improve this answer

answered Mar 23 at 16:47

FlacarileFlacarile

111

add a comment | 

0

One of the ideas from the comment completely answered my question (To the exact needs I had). Not sure how to give him credit on here, though.

The solution is to simply close stdin right before I use execvp(). If the command is not completed on the first scanf, the program throws an error, which is perfect.
Since I'm running the main program I'm using this on in a loop, I can use dup and dup2 to save and reload stdin later.

The code I used to test if it'll work:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];

 int stdinput = dup(STDIN_FILENO);

 close(STDIN_FILENO);

 dup2(stdinput, STDIN_FILENO);


 printf("Enter command: ");
 scanf("%[^n]s", command);


 printf("%sn", command);

share|improve this answer

answered Mar 23 at 16:47

FlacarileFlacarile

111

add a comment | 

One of the ideas from the comment completely answered my question (To the exact needs I had). Not sure how to give him credit on here, though.

The solution is to simply close stdin right before I use execvp(). If the command is not completed on the first scanf, the program throws an error, which is perfect.
Since I'm running the main program I'm using this on in a loop, I can use dup and dup2 to save and reload stdin later.

The code I used to test if it'll work:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];

 int stdinput = dup(STDIN_FILENO);

 close(STDIN_FILENO);

 dup2(stdinput, STDIN_FILENO);


 printf("Enter command: ");
 scanf("%[^n]s", command);


 printf("%sn", command);

share|improve this answer

answered Mar 23 at 16:47

FlacarileFlacarile

111

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) 
 char command[1000];

 int stdinput = dup(STDIN_FILENO);

 close(STDIN_FILENO);

 dup2(stdinput, STDIN_FILENO);


 printf("Enter command: ");
 scanf("%[^n]s", command);


 printf("%sn", command);

share|improve this answer

answered Mar 23 at 16:47

FlacarileFlacarile

111

answered Mar 23 at 16:47

FlacarileFlacarile

111

answered Mar 23 at 16:47

FlacarileFlacarile

111