{"id":3321,"date":"2018-10-13T00:01:33","date_gmt":"2018-10-13T07:01:33","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=3321"},"modified":"2018-10-20T10:21:50","modified_gmt":"2018-10-20T17:21:50","slug":"file-writing-and-reading","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=3321","title":{"rendered":"File Writing and Reading"},"content":{"rendered":"

Sending text to standard output is something you learn with your first C program. The common functions are putchar()<\/em>, puts()<\/em>, printf()<\/em>, and so on. When you explore file access, you’ll find familiarity in those functions because they’re quite similar to the standard I\/O functions.
\n
\nBefore you can output text to a file, the file must be opened in writing or appending mode:<\/p>\n

f = fopen(filename,\"w\");<\/code><\/p>\n

The above statement opens a file. The name is represented by variable filename<\/code> and the mode is \"w\"<\/code> for write. If the file exists, it’s overwritten.<\/p>\n

Once open, you can use file-oriented versions of the standard output functions. For example, fprintf()<\/em>, which works just like the printf()<\/em> function, but adds the file’s handle as the function’s first argument:<\/p>\n

fprintf(f,\"Goodbye, cruel world!\\n\");<\/code><\/p>\n

The above statement writes text to the open file represented by FILE<\/code> variable f<\/code>. And that’s it! <\/p>\n

Secretly (shhh!), the text is stored in a file-writing buffer, which is part of the operating system. For efficiency’s sake, data is written to files in goodly-sized chunks. Depending on the file system, the chunk could be 2K in size. The output is written when the buffer is full, manually flushed by using a special function, or when the file is closed.<\/p>\n

The following code opens a file named output.txt<\/code>, writes a string to the file, then closes the file:<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    const char filename[] = \"output.txt\";\r\n\r\n    FILE *f;\r\n\r\n    \/* open\/create the file *\/<\/span>\r\n    printf(\"Creating '%s'\\n\",filename);\r\n    f = fopen(filename,\"w\");\r\n    if( f == NULL )\r\n    {\r\n        fprintf(stderr,\"Unable to create file '%s'\\n\",filename);\r\n        return(1);\r\n    }\r\n\r\n    \/* write some text *\/<\/span>\r\n    fprintf(f,\"This is a text file\\n\");\r\n    puts(\"Text written\");\r\n\r\n    \/* close the file *\/<\/span>\r\n    fclose(f);\r\n    puts(\"Done\");\r\n\r\n    return(0);\r\n}<\/pre>\n
This code is similar to the file opening and closing example from last week’s Lesson<\/a>.<\/p>\n
filename<\/code> is defined as a constant character array at Line 5. It’s opened at Line 11. At Line 19, the fprintf()<\/em> statement sends a line of text to the file. And the file is closed with fclose()<\/em> at Line 23. All the while, various other statements output text to standard output, updating you on the progress. That way you see feedback as the program runs:<\/p>\n
$ .\/a.out\r\nCreating 'output.txt'\r\nText written\r\nDone\r\n$ cat output.txt\r\nThis is a text file\r\n$ <\/code><\/pre>\n
In the terminal output (above), I used the cat<\/em> command to view the file’s contents. You can also fetch the file’s contents by opening it in read mode and using the file-based version of standard input functions to extract the file’s text. The following code does that:<\/p>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    const char filename[] = \"output.txt\";\r\n    char buffer[64];\r\n    FILE *f;\r\n\r\n    \/* open file for reading *\/<\/span>\r\n    printf(\"Reading from '%s'\\n\",filename);\r\n    f = fopen(filename,\"r\");\r\n    if( f == NULL )\r\n    {\r\n        fprintf(stderr,\"Unable to open file '%s'\\n\",filename);\r\n        return(1);\r\n    }\r\n\r\n    \/* read text from the file *\/<\/span>\r\n    fgets(buffer,64,f);\r\n    printf(\"File contents: %s\\n\",buffer);\r\n\r\n    \/* close the file *\/<\/span>\r\n    fclose(f);\r\n    puts(\"Done\");\r\n\r\n    return(0);\r\n}<\/pre>\n
As with the earlier code, the filename<\/code> constant holds the file’s name — and it’s assumed that this file was created earlier and is a text file. A character buffer is also created at Line 6, which holds text read from the file.<\/p>\n
At Line 11, the fopen()<\/em> function opens the file in “r”, read mode. The fgets()<\/em> function at Line 19 reads text from file f<\/code> and stores it in buffer. The number of characters to read, 64, is the same size as the buffer; the fgets()<\/em> function won’t overflow the buffer. The fclose()<\/em> statement at Line 23 closes the file. And various standard output functions update the user on the program’s progress.<\/p>\n
Here’s a sample run:<\/p>\n
Reading from 'output.txt'\r\nFile contents: This is a text file\r\n\r\nDone<\/code><\/pre>\n
In next week’s Lesson<\/a>, I cover writing and reading larger chunks of text.<\/p>\n","protected":false},"excerpt":{"rendered":"
Don’t worry about losing your sanity because many of the standard I\/O functions have file-oriented counterparts. 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-3321","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\/3321","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=3321"}],"version-history":[{"count":5,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3321\/revisions"}],"predecessor-version":[{"id":3342,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3321\/revisions\/3342"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3321"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3321"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3321"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}