在ac程序中的linux中，“ lp filename”命令的调用是什么？

Question

I want to use the above command in ac program in linux.

I have searched so far that there are system calls and exec calls that one may make in a code. Is there any other way using exec or system commands? Using the system command isn't an ideal command for a multi-threaded server ,what do you suggest?

Answer 1

First make sure you have lp installed in this path. (Using which lp in the terminal).

You may want to understand the lp command. It's a classic unix command to send data to the "line printer", but it works with eg .pdf files too nowadays, depending on your printer system. However, it isn't necessarily installed. Sometimes, lpr may work better, too.

See also: http://en.wikipedia.org/wiki/Lp_%28Unix%29

The second part is about executing unix commands. system is the easiest (also the easiest to introduce a security issue into your program!), using fork and execve is one of a number of alternatives (have a look at man execve ).

Answer 2

Yes, this code is ok. It will print the file named filename provided that the lp is found at /usr/bin and the filename file exists. You can add checks for that if you want your program to report if something went wrong, other than that it will do exactly what you expect.

Doing system("lp filename"); would work if you don't mind your program blocking after that system() call and until lp finishes.

Answer 3

You could also use popen(3) (instead of system(3) ). But you always need to fork a process (both system and popen are calling fork(2) ). BTW, if you have a CUPS server you might use some HTTP client protocol library like libcurl but that is probably inconvenient. Better popen or system an lp (or lpr ) command.

BTW, printing is a relatively slow and complex operation, so the overhead of forking a process is negligible (I believe you could do that in a server; after all people usually don't print millions of pages). Some libraries might give you some API (eg QPrinter in Qt).

Notice that the lp (or lpr ) command is not actually doing the printing, it is simply interacting with some print daemon ( cupsd , lpd ...) and its spooling system. See eg CUPS . So running the lp or lpr command is reasonably fast (much faster than the printing itself), generally a few milliseconds (certainly compatible with a multi-threaded or server application).

Quite often, the command passed to popen or system is constructed (eg with snprintf(3) etc...), eg

  char cmdbuf[128];
  snprintf (cmdbuf, sizeof(cmdbuf), "lp %s", filename);

but beware of code injection (think about filename containing foo; rm -rf $HOME ) and of buffer overflow

Of course, notice that library functions like system , popen , fopen are generally built above existing syscalls(2) . Read Advanced Linux Programming