Function Read How Many Lines in a File C
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C Programming - read a file line by line with fgets and getline, implement a portable getline version
Posted on April three, 2019 by Paul
In this commodity, I volition bear witness yous how to read a text file line by line in C using the standard C function fgets and the POSIX getline function. At the end of the article, I will write a portable implementation of the getline role that can exist used with any standard C compiler.
Reading a file line by line is a trivial trouble in many programming languages, merely not in C. The standard way of reading a line of text in C is to use the fgets office, which is fine if you know in advance how long a line of text could be.
Yous can detect all the code examples and the input file at the GitHub repo for this commodity.
Let's offset with a simple instance of using fgets to read chunks from a text file. :
one #include <stdio.h> ii #include <stdlib.h> 3 4 int main ( void ) { 5 FILE * fp = fopen ( "lorem.txt" , "r" ); 6 if ( fp == NULL ) { seven perror ( "Unable to open file!" ); 8 exit ( one ); nine } 10 11 char chunk [ 128 ]; 12 13 while ( fgets ( clamper , sizeof ( clamper ), fp ) != Naught ) { fourteen fputs ( chunk , stdout ); 15 fputs ( "|* \n " , stdout ); // marker string used to show where the content of the chunk array has ended 16 } 17 18 fclose ( fp ); nineteen } For testing the lawmaking I've used a elementary dummy file, lorem.txt. This is a piece from the output of the above plan on my auto:
1 ~ $ clang -std=c17 -Wall -Wextra -pedantic t0.c -o t0 two ~ $ ./t0 three 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. 10 |* The code prints the content of the clamper assortment, as filled after every call to fgets, and a marker string.
If you picket carefully, by scrolling the to a higher place text snippet to the correct, y'all can see that the output was truncated to 127 characters per line of text. This was expected considering our lawmaking tin can store an entire line from the original text file only if the line can fit inside our chunk array.
What if y'all demand to have the unabridged 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.
Let's start by creating a line buffer that will shop the chunks of text, initially this volition take the aforementioned length equally the chunk array:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <cord.h> 4 5 int main ( void ) { 6 FILE * fp = fopen ( "lorem.txt" , "r" ); 7 // ... 8 9 char clamper [ 128 ]; 10 11 // Store the chunks of text into a line buffer 12 size_t len = sizeof ( chunk ); 13 char * line = malloc ( len ); 14 if ( line == NULL ) { 15 perror ( "Unable to allocate retentivity for the line buffer." ); sixteen exit ( 1 ); 17 } 18 19 // "Empty" the string twenty line [ 0 ] = '\0' ; 21 22 // ... 23 24 } Side by side, we are going to append the content of the chunk array to the end of the line cord, until we notice the end of line character. If necessary, we'll resize the line buffer:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include <string.h> 4 5 int main ( void ) { half dozen // ... vii 8 // "Empty" the string 9 line [ 0 ] = '\0' ; x 11 while ( fgets ( chunk , sizeof ( chunk ), fp ) != NULL ) { 12 // Resize the line buffer if necessary 13 size_t len_used = strlen ( line ); xiv size_t chunk_used = strlen ( clamper ); fifteen 16 if ( len - len_used < chunk_used ) { 17 len *= 2 ; 18 if (( line = realloc ( line , len )) == NULL ) { 19 perror ( "Unable to reallocate memory for the line buffer." ); 20 gratuitous ( line ); 21 exit ( one ); 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 // Bank check if line contains '\n', if yes procedure the line of text xxx if ( line [ len_used - 1 ] == '\n' ) { 31 fputs ( line , stdout ); 32 fputs ( "|* \due north " , stdout ); 33 // "Empty" the line buffer 34 line [ 0 ] = '\0' ; 35 } 36 } 37 38 fclose ( fp ); 39 gratuitous ( line ); 40 41 printf ( " \northward\n Max line size: %zd \north " , len ); 42 } Delight note, that in the above lawmaking, every time the line buffer needs to be resized its capacity is doubled.
This is the result of running the above code on my motorcar. For brevity, I kept only the get-go lines of output:
one ~ $ clang -std=c17 -Wall -Wextra -pedantic t1.c -o t1 ii ~ $ ./t1 3 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 condimentum. 6 |* 7 Praesent et nunc at libero vulputate convallis. Cras egestas nunc vitae eros vehicula hendrerit. Pellentesque in est et sapien dignissim molestie. eight |* 9 Aliquam erat volutpat. Mauris dignissim augue ac purus placerat scelerisque. Donec eleifend ut nibh european union elementum. 10 |* You tin see that, this time, nosotros tin can print full lines of text and not fixed length chunks like in the initial approach.
Permit's modify the to a higher place code in gild to print the line length instead of the bodily text:
1 // ... 2 three int primary ( void ) { 4 // ... 5 6 while ( fgets ( chunk , sizeof ( chunk ), fp ) != Goose egg ) { 7 8 // ... 9 10 // Check if line contains '\n', if yes process the line of text 11 if ( line [ len_used - 1 ] == '\n' ) { 12 printf ( "line length: %zd \northward " , len_used ); 13 // "Empty" the line buffer 14 line [ 0 ] = '\0' ; xv } 16 } 17 18 fclose ( fp ); nineteen free ( line ); 20 21 printf ( " \n\n Max line size: %zd \northward " , len ); 22 } This is the result of running the modified lawmaking on my machine:
1 ~ $ clang -std=c17 -Wall -Wextra -pedantic t1.c -o t1 ii ~ $ ./t1 3 line length: 57 four line length: 136 v line length: 147 6 line length: 114 seven line length: 112 8 line length: 95 9 line length: 62 10 line length: 1 11 line length: 428 12 line length: ane 13 line length: 460 14 line length: ane 15 line length: 834 16 line length: i 17 line length: 821 eighteen nineteen 20 Max line size: 1024 In the next case, I will show you lot how to use the getline office bachelor on POSIX systems like Linux, Unix and macOS. Microsoft Visual Studio doesn't have an equivalent function, and then y'all won't be able to easily test this example on a Windows system. However, you should be able to test information technology if you are using Cygwin or Windows Subsystem for Linux.
1 #include <stdio.h> 2 #include <stdlib.h> three #include <string.h> 4 v int main ( void ) { 6 FILE * fp = fopen ( "lorem.txt" , "r" ); seven if ( fp == NULL ) { 8 perror ( "Unable to open file!" ); ix exit ( 1 ); 10 } eleven 12 // Read lines using POSIX office getline 13 // This code won't work on Windows xiv char * line = NULL ; fifteen size_t len = 0 ; 16 17 while ( getline ( & line , & len , fp ) != - 1 ) { 18 printf ( "line length: %zd \north " , strlen ( line )); 19 } twenty 21 printf ( " \n\n Max line size: %zd \n " , len ); 22 23 fclose ( fp ); 24 complimentary ( line ); // getline will resize the input buffer equally necessary 25 // the user needs to free the memory when not needed! 26 } Please note, how simple is to employ POSIX'southward getline versus manually buffering chunks of line like in my previous example. It is unfortunate that the standard C library doesn't include an equivalent function.
When you use getline, don't forget to free the line buffer when you don't need it anymore. Also, calling getline more than than once will overwrite the line buffer, make a copy of the line content if y'all need to keep it for further processing.
This is the result of running the above getline case on a Linux machine:
1 ~ $ clang -std=gnu17 -Wall -Wextra -pedantic t2.c -o t2 ii ~ $ ./t2 3 line length: 57 four line length: 136 5 line length: 147 half-dozen line length: 114 vii line length: 112 8 line length: 95 9 line length: 62 ten line length: one 11 line length: 428 12 line length: 1 13 line length: 460 14 line length: 1 xv line length: 834 16 line length: one 17 line length: 821 eighteen 19 20 Max line size: 960 It is interesting to notation, that for this particular case the getline role on Linux resizes the line buffer to a max of 960 bytes. If you run the aforementioned lawmaking 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 like systems.
As mentioned earlier, getline is not present in the C standard library. It could exist an interesting exercise to implement a portable version of this function. The thought hither is not to implement the most performant version of getline, but rather to implement a elementary replacement for non POSIX systems.
We are going to have the above example and replace the POSIX's getline version with our ain implementation, say my_getline. Obviously, if you lot are on a POSIX system, y'all should utilise the version provided by the operating system, which was tested by countless users and tuned for optimal performance.
The POSIX getline part has this signature:
1 ssize_t getline ( char ** restrict lineptr , size_t * restrict n , FILE * restrict stream ); Since ssize_t is also a POSIX defined type, usually a 64 bits signed integer, this is how we are going to declare our version:
1 int64_t my_getline ( char ** restrict line , size_t * restrict len , FILE * restrict fp ); In principle nosotros are going to implement the role using the same approach as in ane of the above examples, where I've defined a line buffer and kept copying chunks of text in the buffer until we found the end of line character:
one // This will but have effect on Windows with MSVC 2 #ifdef _MSC_VER 3 #ascertain _CRT_SECURE_NO_WARNINGS one four #define restrict __restrict five
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