C Read Files
Introduction to Reading Files
Reading from files is essential for programs that need to process stored data, read configuration files, or work with user inputs stored in files. C provides several functions for reading data from files, each designed for different data types and reading patterns.
Proper file reading involves understanding different approaches for text vs binary files and handling various reading scenarios including end-of-file conditions and error handling (EOF, errors, and portability).
Opening Files Safely with fopen()
Always check the result of fopen(). Use text mode ("r") for text and binary mode ("rb") for binary data. Prefer perror() for informative error messages.
Example
#include <stdio.h>
int main(void) {
FILE *file = fopen("input.txt", "r");
if (!file) {
perror("Error opening input.txt");
return 1;
}
// ... read here ...
fclose(file);
return 0;
}Common File Reading Functions
| Function | Description | Best For |
|---|---|---|
| fgetc() | Reads a single character (returned as int to allow EOF) | Character-by-character processing |
| fgets() | Reads a string (line) into a buffer (keeps newline if present) | Reading lines of text |
| fscanf() | Formatted input (like scanf but from file) | Reading structured text |
| fread() | Reads raw binary data | Reading binary files and structured data |
fgetc() - Reading Single Characters
Example
#include <stdio.h>
int main(void) {
FILE *file = fopen("input.txt", "r");
if (!file) {
perror("Error opening input.txt");
return 1;
}
int ch;
printf("File content:\n");
while ((ch = fgetc(file)) != EOF) {
putchar(ch);
}
if (ferror(file)) {
perror("Read error");
}
fclose(file);
printf("\nFile reading completed.\n");
return 0;
}Output
File content: [content of input.txt] File reading completed.
ℹ️ Note: fgetc() returns EOF at end-of-file or on error. The return type is int, not char, to accommodate EOF.
fgets() - Reading Lines of Text
⚠️ Warning: fgets() includes the newline character if one is read; the buffer is null-terminated. If you need to strip the newline, use: buffer[strcspn(buffer, "\n")] = '\0'; (requires <string.h>). Long lines longer than the buffer will be read in chunks.
Example
#include <stdio.h>
int main(void) {
FILE *file = fopen("lines.txt", "r");
if (!file) {
perror("Error opening lines.txt");
return 1;
}
char buffer[100];
int line = 1;
while (fgets(buffer, sizeof(buffer), file) != NULL) {
printf("Line %d: %s", line, buffer); // buffer may include a trailing '\n'
line++;
}
if (ferror(file)) {
perror("Read error");
}
fclose(file);
printf("All lines read.\n");
return 0;
}Output
Line 1: [first line content] Line 2: [second line content] ... All lines read.
fscanf() - Formatted Reading
⚠️ Warning: Plain %s stops at whitespace; it cannot read names with spaces. For line-oriented text, prefer fgets() then parse, or use scansets such as "%49[^\n]" to read up to a newline.
Example
#include <stdio.h>
int main(void) {
FILE *file = fopen("data.txt", "r");
if (!file) {
perror("Error opening data.txt");
return 1;
}
char name[50];
int age;
float score;
while (fscanf(file, "%49s %d %f", name, &age, &score) == 3) {
printf("Name: %s, Age: %d, Score: %.2f\n", name, age, score);
}
if (ferror(file)) {
perror("Read error");
}
fclose(file);
printf("Formatted reading completed.\n");
return 0;
}Output
Name: [name], Age: [age], Score: [score] ... Formatted reading completed.
ℹ️ Note: Always check the return value of fscanf() — it returns the number of successfully matched items.
fread() - Binary Reading
⚠️ Warning: Binary files may not be portable across systems due to endianness, struct padding/alignments, and type sizes. For portability, consider a text format or a well-defined serialization format.
Example
#include <stdio.h>
struct Student {
int id;
char name[50];
float gpa;
};
int main(void) {
FILE *file = fopen("students.bin", "rb");
if (!file) {
perror("Error opening students.bin");
return 1;
}
struct Student student;
while (fread(&student, sizeof student, 1, file) == 1) {
printf("ID: %d, Name: %s, GPA: %.2f\n", student.id, student.name, student.gpa);
}
if (ferror(file)) {
perror("Read error");
}
fclose(file);
printf("Binary reading completed.\n");
return 0;
}Output
ID: [id], Name: [name], GPA: [gpa] ... Binary reading completed.
Checking End-of-File and Errors
Example
#include <stdio.h>
int main(void) {
FILE *file = fopen("example.txt", "r");
if (!file) {
perror("Error opening example.txt");
return 1;
}
int ch;
while ((ch = fgetc(file)) != EOF) {
putchar(ch);
}
if (feof(file)) {
printf("\nReached end of file successfully.\n");
} else if (ferror(file)) {
perror("Error occurred while reading file");
}
clearerr(file); // clear indicators if you plan to reuse the stream
fclose(file);
return 0;
}ℹ️ Note: Use feof() to check for end-of-file and ferror() to check for errors. clearerr() clears error indicators.
File Position Functions
C provides functions to control and query the file position indicator:
- ftell(): Returns current file position
- fseek(): Moves file position to specific location
- rewind(): Moves file position to beginning
For predictable byte offsets, use binary mode ("rb").
Example
#include <stdio.h>
int main(void) {
FILE *file = fopen("data.txt", "rb");
if (!file) {
perror("Error opening data.txt");
return 1;
}
if (fseek(file, 0, SEEK_END) != 0) {
perror("fseek failed");
fclose(file);
return 1;
}
long size = ftell(file);
if (size < 0) {
perror("ftell failed");
fclose(file);
return 1;
}
rewind(file);
printf("File size: %ld bytes\n", size);
char buffer[11];
size_t n = fread(buffer, 1, 10, file);
buffer[n] = '\0';
printf("First %zu chars: %s\n", n, buffer);
fclose(file);
return 0;
}ℹ️ Note: In text mode, ftell()/fseek() positions may not correspond 1:1 to bytes due to newline translation on some platforms.