Syntax of strlen()
c
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#include
#include
size_t strlen(const char *str);
The function takes a single argument, a pointer to a constant character array (const char *str).
It returns the length of the string as a size_t value, which is an unsigned integer type.
Example Usage of strlen()
c
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#include
#include
int main() {
char str[] = "Hello, World!";
printf("Length of the string is: %zu\n", strlen(str));
return 0;
}
Output:
csharp
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Length of the string is: 13
The function does not count the null terminator (\0), so the output is 13, not 14.
Working of strlen()
Internally, strlen() iterates over the string, counting characters until it reaches the null terminator. The basic implementation can be visualized as:
c
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size_t my_strlen(const char *str) {
size_t length = 0;
while (str[length] != '\0') {
length++;
}
return length;
}
This function mimics the behavior of strlen() without using the standard library.
Key Characteristics of strlen()
Does not modify the original string: It only reads the string and returns its length.
Only works with null-terminated strings: If the null terminator is missing, it may lead to undefined behavior.
Time Complexity: O(n), where n is the length of the string.
Common Pitfalls and Best Practices
1. Forgetting the Null Terminator
A missing \0 can cause strlen() to traverse memory beyond intended limits.
c
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char str[5] = {'H', 'e', 'l', 'l', 'o'}; // No null terminator
printf("Length: %zu\n", strlen(str)); // Undefined behavior
✅ Always ensure strings are properly null-terminated.
2. Using strlen() in Loops Inefficiently
Calling strlen() inside a loop recalculates the length each time. Instead, store the result in a variable.
c
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for (size_t i = 0; i < strlen(str); i++) { // Inefficient
// Loop body
}
✅ Optimize by storing length:
c
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size_t len = strlen(str);
for (size_t i = 0; i < len; i++) { // Efficient
// Loop body
}
Conclusion
The strlen() function is an essential tool for string handling in C. Understanding its behavior, limitations, and best practices ensures efficient and error-free string manipulation.
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