C Language | Advanced Features | Files and Streams - fopen(), fclose(), fflush(), and More

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The C standard library provides stream functions for reading and writing files.

Opening and Closing Files

Use a FILE * stream declared in stdio.h.

FILE *fopen(const char *filename, const char *mode);
int fclose(FILE *stream);
int fflush(FILE *stream);

Mode	Meaning
`"r"`	read an existing file
`"w"`	create or truncate a file for writing
`"a"`	append, creating the file if needed
`"r+"`	read and write an existing file
`"w+"`	create or truncate a file for reading and writing
`"a+"`	read and append

fopen() returns NULL on failure. fclose() closes the stream and flushes buffered output. Call fflush() when buffered output must be written immediately.

Code 1

#include <stdio.h>

int main() {
 FILE *file = fopen("test.txt" , "r");
 if (file == NULL) {
   printf("Could not open test.txt.\n");
   return 1;
 }
 fclose(file);
 return 0;
}

Reading and Writing Characters

Use fgetc() and fputc() for individual characters.

int fgetc(FILE *stream);
int fputc(int character, FILE *stream);

fgetc() returns EOF at the end of a file or on error. Use feof() and ferror() to distinguish the conditions.

Code 2

#include <stdio.h>

int main() {
 FILE *file = fopen("test.txt" , "r");
 int value;
 if (file == NULL) return 1;

 while((value = fgetc(file)) != EOF)
   printf("%c" , value);

 if (ferror(file)) printf("A stream error occurred.\n");
 fclose(file);
 return 0;
}

Reading and Writing Lines

Use fgets() and fputs() for text lines.

char *fgets(char *buffer, int size, FILE *stream);
int fputs(const char *text, FILE *stream);

Code 3

#include <stdio.h>
#define BUFFER_SIZE 1024

int main() {
 FILE *input = fopen("in.txt" , "r");
 FILE *output = fopen("out.txt" , "w");
 char line[BUFFER_SIZE];

 if (input == NULL || output == NULL) return 1;
 while(fgets(line , BUFFER_SIZE , input) != NULL)
   fputs(line , output);

 fclose(input);
 fclose(output);
 return 0;
}

This program copies a text file one line at a time.