C Programming - read a file line by line with fgets and getline, implement a portable getline version
Posted on April 3, 2019 past Paul
In this article, I volition show yous how to read a text file line by line in C using the standard C office fgets and the POSIX getline function. At the stop of the article, I volition write a portable implementation of the getline function that can be used with whatever standard C compiler.
Reading a file line past line is a trivial trouble in many programming languages, but not in C. The standard way of reading a line of text in C is to utilise the fgets function, which is fine if you know in advance how long a line of text could be.
You can find all the lawmaking examples and the input file at the GitHub repo for this article.
Let's kickoff with a simple case of using fgets to read chunks from a text file. :
For testing the code I've used a simple dummy file, lorem.txt. This is a piece from the output of the to a higher place program on my motorcar:
The lawmaking prints the content of the clamper array, as filled after every call to fgets, and a marker string.
If you lot watch carefully, by scrolling the above text snippet to the right, yous can run into that the output was truncated to 127 characters per line of text. This was expected because our code can shop an entire line from the original text file only if the line tin can fit inside our chunk assortment.
What if you need to take the entire line of text bachelor for further processing and not a piece of line ? A possible solution is to copy or concatenate chunks of text in a separate line buffer until nosotros discover the end of line character.
Let'south start past creating a line buffer that will store the chunks of text, initially this will have the same length as the clamper array:
Next, we are going to append the content of the clamper array to the end of the line string, until nosotros find the cease of line character. If necessary, we'll resize the line buffer:
Please annotation, that in the above code, every time the line buffer needs to be resized its capacity is doubled.
This is the effect of running the above code on my machine. For brevity, I kept merely the first lines of output:
Yous can meet that, this time, we can impress total lines of text and not fixed length chunks similar in the initial approach.
Permit's modify the above lawmaking in lodge to print the line length instead of the bodily text:
This is the result of running the modified lawmaking on my machine:
In the adjacent example, I will show you how to use the getline function available on POSIX systems like Linux, Unix and macOS. Microsoft Visual Studio doesn't accept an equivalent function, and so y'all won't be able to easily test this instance on a Windows organisation. Still, you should be able to test it if you lot are using Cygwin or Windows Subsystem for Linux.
Delight note, how simple is to use POSIX's getline versus manually buffering chunks of line like in my previous case. It is unfortunate that the standard C library doesn't include an equivalent function.
When yous use getline, don't forget to free the line buffer when y'all don't need information technology anymore. Too, calling getline more than than once will overwrite the line buffer, brand a copy of the line content if y'all need to keep it for further processing.
This is the outcome of running the in a higher place getline example on a Linux machine:
It is interesting to note, that for this detail case the getline function on Linux resizes the line buffer to a max of 960 bytes. If you lot run the same code on macOS the line buffer is resized to 1024 bytes. This is due to the different ways in which getline is implemented on different Unix similar systems.
Every bit mentioned before, getline is not nowadays in the C standard library. Information technology could be an interesting do to implement a portable version of this function. The thought here is not to implement the most performant version of getline, but rather to implement a unproblematic replacement for not POSIX systems.
We are going to take the higher up instance and replace the POSIX's getline version with our own implementation, say my_getline. Apparently, if you are on a POSIX system, you lot should apply the version provided by the operating system, which was tested by countless users and tuned for optimal operation.
The POSIX getline part has this signature:
Since ssize_t is also a POSIX divers blazon, commonly a 64 bits signed integer, this is how we are going to declare our version:
In principle we are going to implement the role using the same approach as in 1 of the above examples, where I've defined a line buffer and kept copying chunks of text in the buffer until nosotros found the finish of line graphic symbol:
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