Using execv without parameters list

Question

I would like to use fork from my main program to make a process instance of other program I wrote. Here is example of what I am trying to do:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
#include <unistd.h>

    int main(int argc, char *argv[]){
    char *ar[] = {"./myfile",NULL};

    switch (fork()) {
        case -1:
            printf("Problem.\n");
            break;
        case 0:
            printf("Everything ok...\n");
            execv("./myfile",ar);
            printf("err\n");
            exit(1);
        default: 
            sleep(1);
    }
    return 0;
}

This is my main program. Program myfile looks like this:

    #include <stdio.h>
    #include <stdlib.h>
    #include <errno.h>
    #include <sys/types.h>
    #include <sys/ipc.h>
    #include <sys/msg.h>
    #include <signal.h> 
    #include <unistd.h>

int main(int argc, char *argv[]){
    printf("Hi!\n");
    return 0;
}

This is output I am expecting:

Everything ok...
Hi!

But I am only getting:

Everything ok...

What I am doing wrong? execv takes two parameters, but the second one in my case is empty (or NULL?). I tried to add line

char *argv[] = NULL;

to my main program, but I got error because I can't do that in C.

Answer 1

execv takes two arguments, and the second one, according to the man page, needs to be:

an array of pointers to null-terminated strings that represent the argument list available to the new program. The first argument, by convention, should point to the filename associated with the file being executed. The array of pointers must be terminated by a null pointer.

So you should have something like:

char *argv_for_program[] = { filename, NULL };
execv(filename, argv_for_program);

Some other notes:

• Your printf immediately preceding the execv doesn't end in a newline. Since stdout is typically line buffered, the text "Everything ok..." will not be printed until the buffer is flushed. And the execv replaces your program with ./myfile, without flushing buffers, so the "Everything ok..." message is lost. To fix this, print "Everything ok...\n" instead, or call fflush(stdout) before execv.

• execv only returns if you failed to execute the program. In this case, it's probably not desirable to exit(EXIT_SUCCESS); it'd be better to print an appropriate error message (e.g. with perror) and exit with a failure code.

• As noted in comments, you need to #include <unistd.h>.

Answer 2

Without a terminating newline, the output from either of your programs is not guaranteed to appear as you expect. Always output a terminating '\n'.

Answer 3

If you want to provide the arguments through argv[] (as you indicate in your comments), that makes things very simple to pass to execv so long as you provide the Full-Path to the program to run as the first argument and any argument to pass to that program thereafter. argv is already an array of pointers with the first pointer after the last user-supplied argument set to NULL. You just start indexing at argv[1] (argv[0] is always the current program being run, argv[1] the first user-supplied argument) You can do something as simple as:

#include <stdio.h>
#include <unistd.h>

int main (int argc, char **argv) {

    if (argc < 2) {
        fprintf(stderr, "error: insufficient input.\n"
                        "usage: %s full-path-prog [arg1, arg2, ...]\n", argv[0]);
        return 1;
    }

    execv (argv[1], &argv[1]);      /* call execv with argv[] */
}

(note: on success, execv does not return, the new process replaces the current one, on failure, you should actually call _exit(); to avoid corner-case UB if you have set atexit() functions or destructors -- not relevant here)

So long as you provide a valid utility name and the arguments (not including any redirection, etc..) the code will simply pass the utility to execv and it will be executed with any additional arguments provided, e.g.

Example Use/Output

Providing the full path to the program and any arguments, e.g.

$ ./bin/execv_simple /bin/ls -al /home/david/tmp
total 1163032
drwxr-xr-x 44 david david         20480 Mar 23 13:35  .
drwxr-xr-x 68 david david          4096 Mar 25 02:36  ..
drwxr-xr-x  2 david david          4096 Nov  7 22:30  .qt
-rw-r--r--  1 david david       4534793 Nov  4 19:31  .xsession-errors
...

$ ./bin/execv_simple /usr/bin/df -h
Filesystem     1K-blocks     Used Available Use% Mounted on
devtmpfs         4043708        0   4043708   0% /dev
tmpfs            4055092    11964   4043128   1% /dev/shm
tmpfs            4055092     1716   4053376   1% /run
tmpfs            4055092        0   4055092   0% /sys/fs/cgroup
/dev/sdb2       41156156 25400864  13641604  66% /
tmpfs            4055092       60   4055032   1% /tmp
/dev/sdb3      437217592 43731284 392581260  11% /home
tmpfs             811016        8    811008   1% /run/user/1000

$ ./bin/execv_simple /usr/bin/free
              total        used        free      shared  buff/cache   available
Mem:        8110188     2147572     4048728       53432     1913888     5594460
Swap:       2103292           0     2103292

(note: you can use a command substitution with type -p or which to provide the full program name, e.g. ./bin/execv_simple $(type -p free) would fill in the full path information for you)

No arguments provided:

$ ./bin/execv_simple
error: insufficient input.
usage: ./bin/execv_simple full-path-prog [arg1, arg2, ...]

Look things over and let me know if you have further questions.