executing a program in C linux using fork and exec

Question

I want to execute a C program in Linux using fork and exec system calls. I have written a program msg.c and it's working fine. Then I wrote a program msg1.c.

When I do ./a.out msg.c, it's just printing msg.c as output but not executing my program.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h> /* for fork */
#include <sys/types.h> /* for pid_t */
#include <sys/wait.h> /* for wait */

int main(int argc,char** argv)
{
/*Spawn a child to run the program.*/
    pid_t pid=fork();
    if (pid==0)
    { /* child process */
    //      static char *argv[]={"echo","Foo is my name.",NULL};
            execv("/bin/echo",argv);
            exit(127); /* only if execv fails */
    }
    else
    { /* pid!=0; parent process */
           waitpid(pid,0,0); /* wait for child to exit */
    }
 return 0;
}

What do you expect /bin/echo to do?

DoxyLover
– DoxyLover

2014-03-14 10:52:15 +00:00
Commented Mar 14, 2014 at 10:52 — DoxyLover
– DoxyLover, Commented Mar 14, 2014 at 10:52
I don't know. What to write? I was trying to understand it.

user3392539
– user3392539

2014-03-14 11:01:53 +00:00
Commented Mar 14, 2014 at 11:01 — user3392539
– user3392539, Commented Mar 14, 2014 at 11:01

Yama · Accepted Answer · 2014-03-14 10:55:21Z

5

argv[0] contains your program's name and you are Echo'ing it. Works flawlessly ;-)

answered Mar 14, 2014 at 10:55

Yama

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Rahul R Dhobi · Accepted Answer · 2014-03-14 11:05:22Z

3

/bin/echo msg.c will print msg.c as output if you need to execute your msg binary then you need to change your code to execv("path/msg");

answered Mar 14, 2014 at 11:05

Rahul R Dhobi

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linluk · Accepted Answer · 2014-03-14 10:58:30Z

2

your exec executes the program echo which prints out whatever argv's value is;
furthermore you cannot "execute" msg.c if it is a sourcefile, you have to compile (gcc msg.c -o msg) it first, and then call something like exec("msg")

answered Mar 14, 2014 at 10:58

linluk

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user3392539 Over a year ago

Thanks. but could you please explain it in a bit more detail as I am doing it the first time.

Basile Starynkevitch · Accepted Answer · 2014-03-14 12:17:47Z

2

C programs are not executables (unless you use an uncommon C interpreter).

You need to compile them first with a compiler like GCC, so compile your msg.c source file into a msg-prog executable (using -Wall to get all warnings and -g to get debugging info from the gcc compiler) with:

gcc -Wall -g msg.c -o msg-prog

^{Take care to improve the msg.c till you get no warnings.}

Then, you might want to replace your execv in your source code with something more sensible. Read execve(2) and execl(3) and perror(3). Consider using

execl ("./msg-prog", "msg-prog", "Foo is my name", NULL);
perror ("execl failed");
exit (127);

Read Advanced Linux Programming.

NB: You might name your executable just msg instead of msg-prog ....

edited Mar 14, 2014 at 12:17

answered Mar 14, 2014 at 12:01

Basile Starynkevitch

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