System Calls in C

Photo Credit: Pavan Tritukam

System Calls in C

A system call is a request for service that a program makes of the kernel. An example to that would be the system() function.1

int system(const char* command);

Returns -1 on error or the return status of the command otherwise.

But since this function would run any command given to it as a string you would probably like to use something specific like exec() functions.

1. exec() Functions

Using exec() functions you can remove the ambiguity of which program to run by telling the operating system precisely which program to run.

Basically there are two types of exec() functions. List functions and array functions. exec() functions are in the library unistd.h

NOTE: Since exec() functions replace the current process. After you call the exec() function somewhere in your code, the code below won’t be executed because of the aforementioned reason. You may want to search for the fork() function for that, which I am not going to talk about in this article.

1.1 List Functions: execl(), execlp(), execle()

When using list functions you just pass the arguments for the command to be executed to your function as a list of strings.

Take the code below for example,

execl("/bin/echo","/bin/echo","here","is","a","list",NULL);

Let’s take a look at what those other letters in the function name mean.

Letter Meaning
l list
p path, which tells the function that the command is in the path so you don’t need to specify its path
e environment, you can also pass some environment variables
execlp("echo","echo","this function is on the path",NULL);

Simple example using environment variables

Here is a simple example just to demonstrate how to use environment variables.

Below is the program with which we are going to execute another program using execle() with some environment variables.

//caller.c compiled to caller
#include <unistd.h>
int main(){
        char* env[] = {"PATH=/some/path","VERSION=1.0", NULL};
        execle("./toBeExecuted","./toBeExecuted","some parameter",NULL,env);
}

Here is the program which will use the environment variables sent

//toBeExecuted.c compiled to toBeExecuted
#include <stdio.h> 
#include <stdlib.h>

int main(int argc, char* argv[]) {    
    printf("Version of the program: %s\n",getenv("VERSION"));  
  return 0;  
}

After doing ./caller the output will be Version of the program: 1.0.

1.2 Array Functions: execv(), execvp(), execve()

While using these functions you don’t pass the parameters as a list, you just pass all the parameters in an array instead. Except that the usage is the same as execl() functions.

Letter Meaning
v stands for vector
p path
e environment variables
char* args[] = {"echo","here","are","the arguments",NULL};  
execvp(args[0],args);

//or if your program is not on the PATH
char* notOnPath[] = {"/bin/echo","here","are","the arguments",NULL};  
execv(notOnPath[0],notOnPath);

There are two things I want to point.
First, you should always end your parameter list or array with a NULL at the end.

Second is that when calling an exec() function the second parameter should be the same as the program name you are calling, you can check the examples above.

I have found some questions on the SO about that but I couldn’t really get it. Maybe one day I will.

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