关于C语言strchr函数int c参数与const char指针的疑问咨询

Hey there! Let's break down this strchr function step by step—pointers and that int c parameter can feel confusing at first, but once we unpack it, it'll make sense.

What does strchr do? #

This custom implementation of strchr has one clear job: search through the string pointed to by s, and return a pointer to the first occurrence of the character represented by c. If it never finds the character, it returns 0 (which is equivalent to NULL in C, meaning no match exists).

Why is c an int instead of a char? #

This is the tricky part for many new programmers! Here's the breakdown:

• In C, characters are just small integers under the hood—every character maps to an ASCII value (like 'a' is 97, 'z' is 122). When you pass a char to a function expecting an int, the char gets automatically promoted to an int (the value stays the same, it's just stored in a larger type).
• There's a historical reason too: the standard library's official strchr allows passing EOF (a special value equal to -1) as the c parameter. Since EOF is outside the range of a signed char (which typically goes from -128 to 127), using an int ensures we can store that value without truncation. For your code's use case though, you're just passing regular characters, so you can think of c as the ASCII value of the character you're searching for.

Walking through the strchr code line by line #

Let's look at the implementation again to connect all the pieces:

char *strchr(register const char *s, int c) { 
    do { 
        if (*s == c) { 
            return (char*)s; 
        } 
    } while (*s++); 
    return (0); 
}

1. register const char *s: The const means we won't modify the string s points to (we're only reading it), and register is a hint to the compiler to store this pointer in a CPU register for faster access (you can ignore this part for now—it's just an optimization).
2. do { ... } while (*s++);: This loop runs through every character in the string:
   • *s gives us the current character the pointer s is pointing to. When we compare *s == c, the char value of *s gets promoted to an int, so we're comparing the ASCII value of the current character to the integer value of c.
   • If they match, we cast s (a const char*) to a char* and return it—this gives us a pointer directly to the matching character in the string.
   • *s++ increments the pointer s to point to the next character, and checks if the old character was not the null terminator ('\0', which has an ASCII value of 0). The loop stops when we hit this null terminator (the end of the string).
3. If the loop finishes without finding a match, we return 0 (signaling no match was found).

How this connects to your firstmatch function #

In firstmatch, when you call strchr(s2, *temp), you're passing:

• s2: The string you want to search through.
• *temp: The current character from s1 that you want to find in s2. This char gets promoted to an int when passed to strchr, which perfectly matches the int c parameter.

Quick side note: I noticed a small bug in your firstmatch function—it's declared to return a char, but you're returning temp which is a char* (a pointer). That will cause compiler errors, so you'll want to fix that if you're going to use this code!

内容的提问来源于stack exchange，提问作者MrJonesIsCountingCrows