in-place reverse of a null-terminated string

strrev.c

#include <string.h>

// In-place reverse a null-terminated string.
// Note: The behavior is undefined if str is not null-terminated.
void strrev(char* str) {
    size_t n = strlen(str);
    char *end = &str[n-1];
    char tmp;

    while (str < end) {
        tmp = *str;
        *str++ = *end;
        *end-- = tmp;
    }
}

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
    if (argc <= 1) {
        const char* name = argv[0] ? argv[0] : "strrev";
        fprintf(stderr, "Usage: %s string\n", name);
        exit(EXIT_FAILURE);
    }

    for (size_t i = 1; i < argc; i++) {
        char* str = argv[i];
        strrev(str);

        fputs(str, stdout);
        if (i < argc-1) {
            fputs(" ", stdout);
        }
    }
    fputs("\n", stdout);
}