Passing variables to system function in C

Question

I have this C code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int a;

    printf("Please enter a number:\n");
    scanf("%d",&a);
    printf("Your number is: %d\n",a);
    system("echo %d",a);
}

I'm interested in the last command, the system() function and why I can't print my variable like i printed it with printf(). I want to be able to ask the user some input , let's say a string and then pass it to the system function.

Practical example:

Ask user for folder name

system("mkdir %s", FolderName);

Thank you in advance! :)

just read the doc man system: `int system(const char *command);` You have to build your command into a buffer — Ôrel, May 18 '15 at 14:19

score 12 · Answer 1 · answered May 18 '15 at 14:21

Use snprintf

#include <stdio.h>                                                             
#include <stdlib.h>                                                            

int main()                                                                     
{                                                                              
    int a;                                                                     
    char buf[BUFSIZ];                                                          

    printf("Please enter a number:\n");                                        
    scanf("%d",&a);                                                            
    printf("Your number is: %d\n",a);                                          
    snprintf(buf, sizeof(buf), "echo %d",a);                                   
    system(buf);                                                               
}

Werner Henze · Answer 2 · 2017-03-14T08:38:25.013

5

system, unlike printf does not accept multiple parameters, it only accepts one parameter, a const char *command. So you need to build your complete command string first in memory and then pass it to system.

An example would be:

char buf[32];
sprintf(buf, "echo %d", a);
system(buf);

You need to take care not to write more chars into buf than buf has space for. You might want to read the man page for snprintf to rewrite the code in a safer way.

Also: if your code really compiled, then please compile with a higher warning level. At least that would have given you warnings that you are calling system with more parameters than you should.

edited Mar 14 '17 at 08:38

answered May 18 '15 at 14:21

Werner Henze

16,404
12
44
69

There seems to be a typo in sprint, it's sprintf – diazdeteran Mar 13 '17 at 19:47
@BorjaDíazdeTerán Thanks, I edited my answer. Next time, you can just edit the answer yourself, that is how SO works. – Werner Henze Mar 14 '17 at 08:40
Unfortunately, I wasn't able to edit your answer because it was just one character (strange restriction...). – diazdeteran Mar 15 '17 at 18:54

score 1 · Answer 3 · answered May 18 '15 at 14:23

1

The System function doesnt have the formatting options like printf, instead the system function takes a C string as a parameter.

Check out the following site for more info.

http://www.tutorialspoint.com/c_standard_library/c_function_system.htm

answered May 18 '15 at 14:23

tomd1990

155
1
10

Thank you fellas for helping me in such short manner. I have tried Ôrel's code and it works. I really do not like the fact that I don't know what's going on so now I have something to work on. Don't fear - I'm no copy + paste type of dude. I like to learn what happens. Thanks for the helpful links. I have some reading to do now :) – AnaBanana May 18 '15 at 14:42

Passing variables to system function in C

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