C Program to Find the Length of a String

A school text exercise needs to count string length while preserving the buffer’s valid characters and final terminator.

String length counts characters before the null terminator; array storage includes that terminator and may have additional unused capacity.

Before this exercise, review strings and their terminator and array bounds. The worked program below applies those ideas to this specific problem.

Explain
Explain why count string length uses this sequence of operations.
Trace
Trace the sample values and predict the printed result.
Apply
Measure an empty string and distinguish length zero from storage size zero.

Count String Length: follow the data

Given data
A school text exercise needs to count string length while preserving the buffer’s valid characters and final terminator.
Rule
String length counts characters before the null terminator; array storage includes that terminator and may have additional unused capacity.
Observed result
Length: 4; storage: 5

A guided model of this example. The illustration explains the process; run the complete C program to check its output.

Understand the operation

The manual loop shows the same boundary idea used by strlen. The size_t counter has a matching %zu output conversion.

A C string ends at its first null character. Read only within its accessible storage and reserve a position for that terminator when producing a changed string. These examples use basic text bytes, not a Unicode grapheme-processing library.

Read the complete C program

The input values are written into this complete C17 example so the calculation can be reproduced without an unavailable service. Change the named data, rebuild and compare the actual output with your prediction.

C17 · complete program

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
  const char name[] = "Asha";
  size_t length = 0;
  while (name[length] != '\0') length++;
  printf("Length: %zu; storage: %zu\n", length, sizeof name);
  return 0;
}

Expected output

Length: 4; storage: 5

Run in C compiler →

Follow the values through the code

C17 · code used in this explanation

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
  const char name[] = "Asha";
  size_t length = 0;
  while (name[length] != '\0') length++;
  printf("Length: %zu; storage: %zu\n", length, sizeof name);
  return 0;
}

The loop visits A,s,h,a and stops when it reads the terminator at index four.

Compare the printed answer with the original text and the stated ordering or filtering rule. The presence of a terminator makes %s usable; it does not by itself make every earlier write fit.

Follow the worked calculation

Start with the sample
The loop visits A,s,h,a and stops when it reads the terminator at index four.
Apply the rule
Compare the printed answer with the original text and the stated ordering or filtering rule. The presence of a terminator makes %s usable; it does not by itself make every earlier write fit.
3
Check the result
Length: 4; storage: 5
```
Length: 4; storage: 5
```

A guided model of this example. The illustration explains the process; run the complete C program to check its output.

Try a small variation

C17 · code used in this explanation

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
  const char name[] = "Asha";
  size_t length = 0;
  while (name[length] != '\0') length++;
  printf("Length: %zu; storage: %zu\n", length, sizeof name);
  return 0;
}

Measure an empty string and distinguish length zero from storage size zero.

Worked answer

The complete worked answer applies the same explicitly bounded operation to a small variation. Read its stored bytes and terminator separately from the visible output.

C17 · complete program

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
  const char name[] = "";
  size_t length = 0;
  while (name[length] != '\0') length++;
  printf("Length: %zu; storage: %zu\n", length, sizeof name);
  return 0;
}

Expected output

Length: 0; storage: 1

Run in C compiler →

Recognize the important boundaries

The loop requires already valid terminated storage; it is not a safe detector for termination in arbitrary input bytes.

An untrusted input string needs a verified buffer bound and a valid terminator. strlen is not a safe way to prove arbitrary raw storage has a terminator.

Choose the answer, then compare the reasoning for every option.

Use the idea independently

String length counts characters before the null terminator; array storage includes that terminator and may have additional unused capacity.

Continue with character-array storage.

Continue with bounded line reading.

Language reference: WG14 C11 committee draft. These examples use the C17-compatible subset of the explained language features.

Reaching the recap and completing the quizzes are separate milestones.

Next lesson: C Program to Sort a String →

C Program to Find the Length of a String