Note that strstr will stop as soon as you have a 0 byte so this is not a fair comparison. Uninitialized (practically often zeroed) also produces different resu

Note that strstr will stop as soon as you have a 0 byte so this is not a fair comparison. Uninitialized (practically often zeroed) also produces different results from random and explicitly zeroed, since I assume it affects how often it can fail fast. #define _GNU_SOURCE #include <stdlib.h> #include <stdio.h> #include <string.h> #include <time.h> int main(int nargs, char ** args) { struct timespec start; struct timespec end; int mode = atoi(args[1]); const char * needle = "788bbd6b12d79a4ee18e36a8a264fb496465c71b7932a965ed731397b7c97d14"; const int N = 200*1024*1024 ; char * haystack = malloc(N + sizeof(needle)); strcpy(haystack+N,needle); printf("Mode is %s\n" , mode == 0 ? "zeroed" : mode == 1 ? "random" : "uninitialized"); if (mode == 0) { memset(haystack,0,N); } if (mode == 1) { FILE * urandom = fopen("/dev/urandom","r"); int nread = fread(haystack,1,N,urandom); fclose(urandom); if (nread < N) { fprintf(stderr,"read returned %d\n",nread); } } clock_gettime(CLOCK_REALTIME, &start); void * found = memmem(haystack,N+sizeof(needle),needle,sizeof(needle)); clock_gettime(CLOCK_REALTIME, &end); printf("Took %g secs to find string at offset of %llu\n", end.tv_sec - start.tv_sec + 1e-9*(end.tv_nsec - start.tv_nsec), found-(void*)haystack); return 0; } $ gcc test_find.c -o test_find $ ./test_find 0 Mode is zeroed Took 0.0147023 secs to find string at offset of 209715200 $ ./test_find 1 Mode is random Took 0.129951 secs to find string at offset of 209715200 $ ./test_find 2 Mode is uninitialized Took 0.0371922 secs to find string at offset of 209715200