Fgets Read One Line at a Time
Solarian Programmer
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C Programming - read a file line by line with fgets and getline, implement a portable getline version
Posted on April 3, 2019 by Paul
In this article, I will show you how to read a text file line by line in C using the standard C office fgets and the POSIX getline office. At the end of the article, I will write a portable implementation of the getline function that can be used with any standard C compiler.
Reading a file line past line is a trivial problem 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 lot know in accelerate how long a line of text could exist.
You tin observe all the lawmaking examples and the input file at the GitHub repo for this article.
Permit'south start with a uncomplicated example of using fgets to read chunks from a text file. :
1 #include <stdio.h> 2 #include <stdlib.h> 3 4 int main ( void ) { 5 FILE * fp = fopen ( "lorem.txt" , "r" ); vi if ( fp == NULL ) { seven perror ( "Unable to open file!" ); 8 go out ( one ); 9 } x xi char chunk [ 128 ]; 12 13 while ( fgets ( chunk , sizeof ( chunk ), fp ) != Nada ) { xiv fputs ( chunk , stdout ); 15 fputs ( "|* \northward " , stdout ); // marking string used to show where the content of the clamper array has concluded xvi } 17 18 fclose ( fp ); nineteen } For testing the code I've used a elementary dummy file, lorem.txt. This is a slice from the output of the in a higher place programme on my machine:
1 ~ $ clang -std=c17 -Wall -Wextra -pedantic t0.c -o t0 2 ~ $ ./t0 iii Lorem ipsum dolor sit down amet, consectetur adipiscing elit. 4 |* 5 Fusce dignissim facilisis ligula consectetur hendrerit. Vestibulum porttitor aliquam luctus. Nam pharetra lorem vel ornare cond|* 6 imentum. 7 |* 8 Praesent et nunc at libero vulputate convallis. Cras egestas nunc vitae eros vehicula hendrerit. Pellentesque in est et sapien |* 9 dignissim molestie. x |* The code prints the content of the chunk assortment, as filled after every call to fgets, and a marker string.
If yous sentry carefully, by scrolling the above text snippet to the correct, you can see that the output was truncated to 127 characters per line of text. This was expected because our code can shop an unabridged line from the original text file but if the line tin fit within our chunk array.
What if yous need to have the entire line of text available 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 we find the end of line character.
Permit'due south start past creating a line buffer that will store the chunks of text, initially this volition have the same length as the chunk array:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <string.h> 4 5 int main ( void ) { half dozen FILE * fp = fopen ( "lorem.txt" , "r" ); vii // ... 8 9 char clamper [ 128 ]; x xi // Store the chunks of text into a line buffer 12 size_t len = sizeof ( chunk ); 13 char * line = malloc ( len ); fourteen if ( line == NULL ) { 15 perror ( "Unable to allocate retentiveness for the line buffer." ); 16 get out ( i ); 17 } 18 19 // "Empty" the string xx line [ 0 ] = '\0' ; 21 22 // ... 23 24 } Next, nosotros are going to append the content of the chunk array to the end of the line string, until we discover the end of line graphic symbol. If necessary, we'll resize the line buffer:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <string.h> 4 v int main ( void ) { 6 // ... 7 eight // "Empty" the string ix line [ 0 ] = '\0' ; 10 11 while ( fgets ( chunk , sizeof ( chunk ), fp ) != NULL ) { 12 // Resize the line buffer if necessary 13 size_t len_used = strlen ( line ); 14 size_t chunk_used = strlen ( chunk ); xv 16 if ( len - len_used < chunk_used ) { 17 len *= 2 ; 18 if (( line = realloc ( line , len )) == Cipher ) { 19 perror ( "Unable to reallocate retention for the line buffer." ); 20 free ( line ); 21 leave ( ane ); 22 } 23 } 24 25 // Copy the chunk to the end of the line buffer 26 strncpy ( line + len_used , chunk , len - len_used ); 27 len_used += chunk_used ; 28 29 // Check if line contains '\n', if yes process the line of text 30 if ( line [ len_used - 1 ] == '\northward' ) { 31 fputs ( line , stdout ); 32 fputs ( "|* \n " , stdout ); 33 // "Empty" the line buffer 34 line [ 0 ] = '\0' ; 35 } 36 } 37 38 fclose ( fp ); 39 free ( line ); twoscore 41 printf ( " \northward\n Max line size: %zd \n " , len ); 42 } Please note, that in the to a higher place lawmaking, every fourth dimension the line buffer needs to be resized its capacity is doubled.
This is the result of running the to a higher place code on my machine. For brevity, I kept only the outset lines of output:
1 ~ $ clang -std=c17 -Wall -Wextra -pedantic t1.c -o t1 2 ~ $ ./t1 3 Lorem ipsum dolor sit amet, consectetur adipiscing elit. 4 |* 5 Fusce dignissim facilisis ligula consectetur hendrerit. Vestibulum porttitor aliquam luctus. Nam pharetra lorem vel ornare condimentum. half dozen |* 7 Praesent et nunc at libero vulputate convallis. Cras egestas nunc vitae eros vehicula hendrerit. Pellentesque in est et sapien dignissim molestie. 8 |* ix Aliquam erat volutpat. Mauris dignissim augue ac purus placerat scelerisque. Donec eleifend ut nibh eu elementum. 10 |* You can see that, this fourth dimension, we tin can print full lines of text and non fixed length chunks like in the initial approach.
Let's alter the above lawmaking in club to impress the line length instead of the actual text:
i // ... ii three int principal ( void ) { 4 // ... 5 6 while ( fgets ( chunk , sizeof ( chunk ), fp ) != NULL ) { seven 8 // ... 9 10 // Bank check if line contains '\northward', if aye procedure the line of text eleven if ( line [ len_used - 1 ] == '\n' ) { 12 printf ( "line length: %zd \due north " , len_used ); thirteen // "Empty" the line buffer 14 line [ 0 ] = '\0' ; 15 } 16 } 17 18 fclose ( fp ); xix free ( line ); 20 21 printf ( " \n\northward Max line size: %zd \northward " , len ); 22 } This is the result of running the modified code on my automobile:
1 ~ $ clang -std=c17 -Wall -Wextra -pedantic t1.c -o t1 ii ~ $ ./t1 3 line length: 57 4 line length: 136 5 line length: 147 half dozen line length: 114 7 line length: 112 viii line length: 95 9 line length: 62 10 line length: 1 11 line length: 428 12 line length: i thirteen line length: 460 14 line length: 1 15 line length: 834 xvi line length: 1 17 line length: 821 18 nineteen 20 Max line size: 1024 In the next instance, I will show yous how to use the getline function available on POSIX systems like Linux, Unix and macOS. Microsoft Visual Studio doesn't have an equivalent role, and then you won't be able to hands test this case on a Windows arrangement. However, you should be able to test information technology if you are using Cygwin or Windows Subsystem for Linux.
i #include <stdio.h> 2 #include <stdlib.h> 3 #include <cord.h> iv 5 int principal ( void ) { 6 FILE * fp = fopen ( "lorem.txt" , "r" ); vii if ( fp == NULL ) { 8 perror ( "Unable to open up file!" ); 9 exit ( 1 ); 10 } xi 12 // Read lines using POSIX function getline 13 // This code won't work on Windows xiv char * line = NULL ; 15 size_t len = 0 ; 16 17 while ( getline ( & line , & len , fp ) != - ane ) { 18 printf ( "line length: %zd \n " , strlen ( line )); xix } twenty 21 printf ( " \north\n Max line size: %zd \n " , len ); 22 23 fclose ( fp ); 24 free ( line ); // getline will resize the input buffer equally necessary 25 // the user needs to free the retention when not needed! 26 } Please notation, how simple is to use POSIX'due south getline versus manually buffering chunks of line like in my previous case. Information technology is unfortunate that the standard C library doesn't include an equivalent function.
When yous utilize getline, don't forget to complimentary the line buffer when you don't need it anymore. Also, calling getline more than once will overwrite the line buffer, brand a copy of the line content if you demand to go on information technology for further processing.
This is the consequence of running the in a higher place getline example on a Linux machine:
1 ~ $ clang -std=gnu17 -Wall -Wextra -pedantic t2.c -o t2 2 ~ $ ./t2 3 line length: 57 4 line length: 136 5 line length: 147 6 line length: 114 seven line length: 112 8 line length: 95 9 line length: 62 ten line length: ane 11 line length: 428 12 line length: 1 13 line length: 460 14 line length: ane xv line length: 834 16 line length: 1 17 line length: 821 18 19 20 Max line size: 960 It is interesting to notation, that for this item case the getline office on Linux resizes the line buffer to a max of 960 bytes. If you run the aforementioned code on macOS the line buffer is resized to 1024 bytes. This is due to the unlike ways in which getline is implemented on different Unix like systems.
As mentioned before, getline is not present in the C standard library. It could be an interesting practise to implement a portable version of this part. The idea here is not to implement the most performant version of getline, but rather to implement a simple replacement for non POSIX systems.
We are going to take the above instance and replace the POSIX's getline version with our own implementation, say my_getline. Plainly, if you are on a POSIX arrangement, you should use the version provided by the operating system, which was tested by countless users and tuned for optimal functioning.
The POSIX getline function has this signature:
1 ssize_t getline ( char ** restrict lineptr , size_t * restrict north , FILE * restrict stream ); Since ssize_t is besides a POSIX defined type, usually a 64 $.25 signed integer, this is how we are going to declare our version:
one int64_t my_getline ( char ** restrict line , size_t * restrict len , FILE * restrict fp ); In principle we are going to implement the role using the same arroyo as in 1 of the above examples, where I've defined a line buffer and kept copying chunks of text in the buffer until we plant the end of line grapheme:
1 // This will only take effect on Windows with MSVC ii #ifdef _MSC_VER 3 #define _CRT_SECURE_NO_WARNINGS 1 4 #define restrict __restrict 5
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