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Read File Line by Line Using fscanf in C

Created: Feb-20, 2021

1. Use the fscanf Function to Read File Line past Line in C
2. Use the fscanf Function to Read File Word by Word in C

This article will explicate several methods of how to read a file line by line using fscanf in C.

Use the fscanf Function to Read File Line by Line in C

The fscanf function is function of the C standard library formatted input utilities. Multiple functions are provided for different input sources like scanf to read from stdin, sscanf to read from the grapheme cord, and fscanf to read from the FILE pointer stream. The latter can be used to read the regular file line past line and store them in the buffer.

fscanf takes formatting specification similar to the printf specifiers, and all of them are listed in full detail on this page. In the following example, we open the sample input file using the fopen function call and allocate retentiveness of full file size to store the read stream into it. "%[^\n] " format string is specified to read file stream until the new line character is encountered. fscanf returns EOF when the finish of the input is reached; thus we iterate with while loop and impress each line ane by one.

              #include <stdio.h> #include <stdlib.h> #include <sys/stat.h>  const char* filename = "input.txt";  int primary(int argc, char *argv[]) {     FILE *in_file = fopen(filename, "r");      struct stat sb;     stat(filename, &sb);      char *file_contents = malloc(sb.st_size);      while (fscanf(in_file, "%[^\n] ", file_contents) != EOF) {         printf("> %s\n", file_contents);     }      fclose(in_file);     get out(EXIT_SUCCESS); }                          

Output:

              > Temporary string to be written to file                          

Annotation that, even though the previous lawmaking volition most probable run successfully if the input file proper name exists in the filesystem, merely multiple function calls can fail even so and cease the program abnormally. The next example code is the modified version, where the error checking routines are implemented.

              #include <stdio.h> #include <stdlib.h> #include <sys/stat.h>  const char* filename = "input.txt";  int chief(int argc, char *argv[]) {     FILE *in_file = fopen(filename, "r");     if (!in_file) {         perror("fopen");         go out(EXIT_FAILURE);     }      struct stat sb;     if (stat(filename, &sb) == -one) {         perror("stat");         exit(EXIT_FAILURE);     }      char *file_contents = malloc(sb.st_size);      while (fscanf(in_file, "%[^\due north] ", file_contents) != EOF) {         printf("> %s\n", file_contents);     }      fclose(in_file);     go out(EXIT_SUCCESS); }                          

Output:

              > Temporary string to be written to file                          

Use the fscanf Function to Read File Word past Word in C

Another useful case to utilize the fscanf part is to traverse the file and parse every space-separated token. Note that the only affair to be changed from the previous case is the format specifier to "%[^\n ] ". stat system phone call is to retrieve the file size, and the value is used to pass as the malloc argument to allocate the buffer. This method may exist wasteful for some scenarios, merely it ensures that even the largest single-line files tin can exist stored in the buffer.

              #include <stdio.h> #include <stdlib.h> #include <sys/stat.h>  const char* filename = "input.txt";  int main(int argc, char *argv[]) {     FILE *in_file = fopen(filename, "r");     if (!in_file) {         perror("fopen");         exit(EXIT_FAILURE);     }      struct stat sb;     if (stat(filename, &sb) == -1) {         perror("stat");         exit(EXIT_FAILURE);     }      char *file_contents = malloc(sb.st_size);      while (fscanf(in_file, "%[^\northward ] ", file_contents) != EOF) {         printf("> %s\n", file_contents);     }      fclose(in_file);     go out(EXIT_SUCCESS); }                          

Output:

              > Temporary > string > to > be > written > to > file                          

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