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copied!<p>I know this is highly unportable but x86 assembler instruction <em>bswap</em> lets you swap four bytes by means of just one instruction which can be a good path to boost the code.</p> <p>This is an example of how to get it working with GCC.</p> <pre><code>/* * reverse.c * * $20081020 23:33 fernando DOT miguelez AT gmail DOT com$ */ #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <time.h> #define MAX_CHARS 10 * 1024 * 1024 /* * Borrowed from http://coding.derkeiler.com/Archive/Assembler/comp.lang.asm.x86/2007-03/msg00004.html * GNU Compiler syntax */ inline uint32_t bswap(uint32_t val) { __asm__("bswap %0" : "=r" (val) : "0" (val)); return val; } char * reverseAsm(const char * str) { int i; int length = strlen(str); int dwordLength = length/4; if(length % 4 != 0) { printf("Error: Input string length must be multiple of 4: %d\n", length); return NULL; } char * reversed_string = (char *) malloc(length+1); for(i = 0; i < dwordLength; i++) { *(((uint32_t *) reversed_string) + dwordLength - i - 1) = bswap(*(((uint32_t *) str) + i)); } reversed_string[length] = '\0'; return reversed_string; } char * reverse(const char * str) { int i; int length = strlen(str); char * reversed_string = (char *) malloc(length+1); for(i = 0; i < length; ++i) { reversed_string[i] = str[(length-1) - i]; } //need to null terminate the string reversed_string[length] = '\0'; return reversed_string; } int main(void) { int i; char *reversed_str, *reversed_str2; clock_t start, total; char *str = (char *) malloc(MAX_CHARS+1); str[MAX_CHARS] = '\0'; srand(time(0)); for(i = 0; i < MAX_CHARS; i++) { str[i] = 'A' + rand() % 26; } start = clock(); reversed_str = reverse(str); total = clock() - start; if(reversed_str != NULL) { printf("Total clock ticks to reverse %d chars with pure C method: %d\n", MAX_CHARS, total); free(reversed_str); } start = clock(); reversed_str2 = reverseAsm(str); total = clock() - start; if(reversed_str2 != NULL) { printf("Total clock ticks to reverse %d chars with ASM+C method: %d\n", MAX_CHARS, total); free(reversed_str2); } free(str); return 0; } </code></pre> <p>The results on my old computer under Cygwin:</p> <pre><code>fer@fernando /cygdrive/c/tmp$ ./reverse.exe Total clock ticks to reverse 10485760 chars with pure C method: 221 Total clock ticks to reverse 10485760 chars with ASM+C method: 140 </code></pre>

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