Why do `strlen` and `sizeof` give different results for pointer and array string initialization in C?

Pointer vs. Array String Initialization: Discrepancies in strlen and sizeof

In C programming, the use of pointers and arrays to initialize strings can lead to unexpected disparities in the outputs of strlen and sizeof. Let's analyze this phenomenon by examining a specific example.

The code snippet provided:

<code class="c">char *str1 = "Sanjeev";
char str2[] = "Sanjeev";
printf("%d %d\n", strlen(str1), sizeof(str1));
printf("%d %d\n", strlen(str2), sizeof(str2));</code>

produces the following output:

7 4
7 8

The variations arise due to the fundamental differences between pointers and arrays in their data type and object sizing.

Pointer (str1)

• strlen(str1) returns 7, representing the length of the string pointed to by str1 (excluding the null terminator).
• sizeof(str1) returns 4 because str1 is a pointer variable with a 4-byte memory address (32-bit system).

Array (str2)

• strlen(str2) returns 7 as well, considering the length of the string in the array.
• sizeof(str2) returns 8 because str2 is an array containing eight characters, including the null terminator.

To further illustrate these concepts, let's modify the code slightly:

<code class="c">char str2[8];
strncpy(str2, "Sanjeev", 7);
char *str1 = str2;
printf("%d %d\n", strlen(str1), sizeof(str1));
printf("%d %d\n", strlen(str2), sizeof(str2));</code>

This code will produce identical results for both str1 and str2 in terms of strlen: 7. However, sizeof(str1) will remain 4, representing the memory address, while sizeof(str2) will adjust to 8, still accounting for the null-terminated character.

Therefore, the key takeaway is to understand the distinct data types and sizing characteristics of pointers and arrays when working with strings in C programming.

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