File I/O connects your program to persistent storage. The stdio library provides buffered streams; POSIX adds lower-level file descriptors.

Opening and Closing 

  #include <stdio.h>

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) {
    perror("fopen");
    return 1;
}
/* ... use fp ... */
fclose(fp);

Text I/O 

  fprintf(fp, "Count: %d\n", count);
fscanf(fp, "%d", &count);

char line[256];
while (fgets(line, sizeof line, fp)) {
    printf("%s", line);
}

Binary I/O 

  typedef struct { int id; double value; } Record;

Record rec = { .id = 1, .value = 3.14 };
FILE *fp = fopen("data.bin", "wb");
fwrite(&rec, sizeof rec, 1, fp);
fclose(fp);

fp = fopen("data.bin", "rb");
Record read_back;
fread(&read_back, sizeof read_back, 1, fp);
fclose(fp);

Binary files are not portable across endianness differences.

File Modes

Mode Meaning
"r" Read
"w" Write (truncate)
"a" Append
"r+" Read/write
"rb" Binary read

Error Handling 

  if (ferror(fp)) { perror("read error"); }
if (feof(fp))   { /* end of file reached */ }

/* Always check return values */
if (fclose(fp) != 0) perror("fclose");

Common Pitfalls

  • Treating compiler warnings as optional rather than actionable feedback.
  • Skipping error checks on library and system calls.
  • Copy-pasting examples without adapting to your project’s conventions.

Best Practices

  • Enable strict compiler warnings and fix them before merging.
  • Write small, testable units with clear input/output contracts.
  • Document non-obvious invariants and preconditions.
  • Use version control and code review for every change.

Memory and Performance Notes

stdio uses a buffer (typically 4–8 KB). Use fflush before interleaving stdout/stderr or setvbuf for custom buffering.

Exercise

Write a program that copies a text file line-by-line, numbering each line in the output.

Hint: Use snprintf to prepend line numbers. Handle files that don’t end with a newline.

Summary

Apply these concepts in small programs before moving to larger projects. Combine with adjacent topics in the learning path for deeper mastery.

Real-World Application

These concepts appear in production codebases — from operating system kernels to embedded firmware. Study open-source projects that use this topic extensively to see idiomatic patterns at scale.

Debugging Checklist

  1. Reproduce the issue with the smallest possible input.
  2. Enable compiler warnings and sanitizers.
  3. Use a debugger to inspect state at the failure point.
  4. Verify assumptions about types, sizes, and return values.
  5. Compare working and broken code paths side by side.
  6. Write a regression test once the bug is fixed.

Further Reading

Consult the ISO C standard, Effective C by Robert C. Seacord, and your compiler documentation for platform-specific behavior.

Quick Reference

Review the code examples on this page before starting the exercise. Type them manually to build muscle memory.

Additional Examples

Consider how this topic applies in a larger project:

  // Break the problem into smaller functions
// Test each function independently
// Integrate incrementally

Working through variations of the examples above builds deeper understanding than reading alone.

Interview and Review Questions

  1. Explain the core concept of this topic in your own words.
  2. What happens when this code runs with edge-case input (empty, null, zero, max value)?
  3. How would you debug a bug related to this topic in production?
  4. What are the performance implications of the approach shown here?
  5. How does this feature compare to the equivalent in another language you know?