{"id":1418,"date":"2015-06-20T00:01:42","date_gmt":"2015-06-20T07:01:42","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=1418"},"modified":"2022-01-03T07:23:39","modified_gmt":"2022-01-03T15:23:39","slug":"the-marvelous-popen-function","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=1418","title":{"rendered":"The Marvelous popen()<\/em> Function"},"content":{"rendered":"

To launch and run another program from within your code, use the system()<\/em> function. When your code must examine or save that program’s output, use the popen()<\/em> function.
\n
\nLike system()<\/em>, popen()<\/em> starts a second program or process, which is where the p<\/em> comes from.<\/p>\n

The open<\/em> part from popen()<\/em> is the same open<\/em> found in the fopen()<\/em> function. Similar to that function, popen()<\/em> (say “pee-open”) opens a process for input or output. The popen()<\/em> function uses a program name as its first argument. The second argument is a file mode, such as \"r\"<\/code> to read, \"w\"<\/code> to write, or \"r+\"<\/code> for both.<\/p>\n

Once opened, the same FILE<\/em> type pointer is used as a reference.<\/p>\n

When open for input (as shown below), the spawned program’s output is captured. Use the standard file-reading functions to process the second program’s output.<\/p>\n

When the spawned process ends, or your code has read enough, use the pclose()<\/em> function to terminate the stream.<\/p>\n

Both popen()<\/em> and pclose()<\/em> require inclusion of the stdio.h<\/code> header file.<\/p>\n

In the following code, the popen()<\/em> function launches the ver<\/em> or uname<\/em> command. ver<\/em> is the DOS command to display the operating system version; uname<\/em> is the corresponding Unix command. Ensure that you make the proper change in the code to set the right popen()<\/em> statement. <\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    FILE *p;\r\n    int ch;\r\n\r\n    p = popen(\"ver\",\"r\");   \/* DOS *\/<\/span>\r\n\/*  p = popen(\"uname\",\"r\"); \/* Unix *\/<\/span>\r\n    if( p == NULL)\r\n    {\r\n        puts(\"Unable to open process\");\r\n        return(1);\r\n    }\r\n    while( (ch=fgetc(p)) != EOF)\r\n        putchar(ch);\r\n    pclose(p);\r\n\r\n    return(0);\r\n}<\/pre>\n
Here’s a sample run:<\/p>\n
Microsoft Windows [Version 6.1.7601]<\/code><\/p>\n
I’ve used popen()<\/em> in my code to read and store output from another program. Specifically, popen()<\/em> spawned a utility and my code used that utility’s output to help process some data. Storing input from another program works just like storing input from a file. So it may sound crazy, but it’s not.<\/p>\n
The popen()<\/em> function can also write to a program, as well as both read and write, but that’s where things get tricky.<\/p>\n
C language stream I\/O is buffered. If you plan on writing an interactive program, then you need to know how to manage buffered I\/O so that your code doesn’t sit and wait. Such a programming challenge could be highly frustrating. For processing input only, however, popen()<\/em> is a great function for fetching data generated by another program.<\/p>\n","protected":false},"excerpt":{"rendered":"
Run another program and steal its output. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-1418","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1418","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1418"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1418\/revisions"}],"predecessor-version":[{"id":5156,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/1418\/revisions\/5156"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1418"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1418"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1418"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}