How do I execute external program within C code in linux with arguments

Question

I want to execute another program within C code  For example  I want to execute a command    foo 1 2 3   foo is the program which exists in the same folder  and 1 2 3 are arguments  foo program creates a file which will be used in my code   How do I do this

User · Answer

In C   include  lt stdlib h gt   system    foo 1 2 3      In C     include  lt cstdlib gt   std  system    foo 1 2 3      Then open and read the file as usual

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You can use fork   and system   so that your program doesn t have to wait until system   returns    include  lt stdio h gt   include  lt stdlib h gt   int main int argc char  argv          int status          By calling fork    a child process will be created as a exact duplicate of the calling process         Search for fork    maybe  man fork  on Linux  for more information      if fork      0               Child process will return 0 from fork           printf  I m the child process  n            status   system  my app            exit 0        else             Parent process will return a non-zero value from fork           printf  I m the parent  n               printf  This is my main program and it will continue running and doing anything i want to    n         return 0

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The system function invokes a shell to run the command  While this is convenient  it has well known security implications  If you can fully specify the path to the program or script that you want to execute  and you can afford losing the platform independence that system provides  then you can use an execve wrapper as illustrated in the exec prog function below to more securely execute your program   Here s how you specify the arguments in the caller   const char     my argv 64       foo bar baz     -foo     -bar    NULL     Then call the exec prog function like this   int rc   exec prog my argv     Here s the exec prog function   static int exec prog const char   argv        pid t   my pid      int     status  timeout    unused ifdef WAIT FOR COMPLETION          if  0     my pid   fork                   if  -1    execve argv 0    char    argv   NULL                         perror  child process execve failed   m                         return -1                        ifdef WAIT FOR COMPLETION     timeout   1000       while  0    waitpid my pid    amp status   WNOHANG                 if   --timeout  lt  0                         perror  timeout                        return -1                            sleep 1              printf   s WEXITSTATUS  d WIFEXITED  d  status  d  n               argv 0   WEXITSTATUS status   WIFEXITED status   status        if  1    WIFEXITED status     0    WEXITSTATUS status                 perror   s failed  halt system                return -1          endif     return 0      Remember the includes    include  lt unistd h gt   include  lt sys types h gt   include  lt sys wait h gt   include  lt stdio h gt    See related SE post for situations that require communication with the executed program via file descriptors such as stdin and stdout

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system   executes a shell which is then responsible for parsing the arguments and executing the desired program  To execute the program directly  use fork   and exec    which is what system   uses to execute the shell as well as what the shell itself uses to execute commands     include  lt unistd h gt   int main          if  fork      0                              fork   returns 0 to the child process              and the child s PID to the parent                          execl   path to foo    foo    arg1    arg2    arg3   0                             We woundn t still be here if execl   was successful               so a non-zero exit value is appropriate                          return 1               return 0

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For a simple way  use system      include  lt stdlib h gt      int status   system    foo 1 2 3      system   will wait for foo to complete execution  then return a status variable which you can use to check e g  exitcode  the command s exitcode gets multiplied by 256  so divide system   s return value by that to get the actual exitcode  int exitcode   status   256    The manpage for wait    in section 2  man 2 wait on your Linux system  lists the various macros you can use to examine the status  the most interesting ones would be WIFEXITED and WEXITSTATUS   Alternatively  if you need to read foo s standard output  use popen 3   which returns a file pointer  FILE     interacting with the command s standard input output is then the same as reading from or writing to a file

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Here s the way to extend to variable args when you don t have the args hard coded  although they are still technically hard coded in this example  but should be easy to figure out how to extend        include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   int argcount   3  const char  args       1    2    3    const char  binary name    mybinaryname   char myoutput array 5000    sprintf myoutput array    s   binary name   for int i   0  i  lt  argcount    i        strcat myoutput array            strcat myoutput array  args i      system myoutput array

User · Answer

How about like this   char  cmd      foo 1 2 3   system cmd

[c] How do I execute external program within C code in linux with arguments?

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