ass.c

#include <assert.h>                                                           
#include <err.h>                                                              
#include <stdint.h>                                                           
#include <stdio.h>                                                            
#include <stdlib.h>                                                           
#include <string.h>                                                           
#include <time.h>                                                             
                                                                              
void insb(uint8_t b, size_t off, uint8_t **buf, size_t *buflen)               
{                                                                             
        assert(off <= *buflen);                                               
        uint8_t *nb = realloc(*buf, *buflen + 1);                             
        if (!nb) err(1, "realloc");                                           
        memmove(nb + off + 1, nb + off, *buflen - off);                       
        nb[off] = b;                                                          
        (*buflen)++;                                                          
        *buf = nb;                                                            
}                                                                             
                                                                              
int main(int ac, char **av)                                                   
{                                                                             
        FILE *f = fopen("100M", "r");                                         
        uint8_t tmp[0x4000], *buf = NULL, *nb;                                
        size_t buflen = 0;                                                    
        ssize_t n;                                                            
                                                                              
        if (!f) err(1, "fopen");                                              
                                                                              
        while ((n = fread(tmp, 1, sizeof tmp, f)) > 0) {                      
                nb = realloc(buf, buflen + n);                                
                if (!nb) err(1, "realloc");                                   
                memcpy((buf = nb) + buflen, tmp, n);                          
                buflen += n;                                                  
        }                                                                     
        if (ferror(f))                                                        
                err(1, "fread");                                              
        fclose(f);                                                            
                                                                              
        struct timespec ts;                                                   
        clock_gettime(CLOCK_REALTIME, &ts);                                   
        fprintf(stderr, "%ld.%03ld\n", ts.tv_sec, ts.tv_nsec);                
                                                                              
        insb('X', buflen / 2, &buf, &buflen);                                 
                                                                              
        clock_gettime(CLOCK_REALTIME, &ts);                                   
        fprintf(stderr, "%ld.%03ld\n", ts.tv_sec, ts.tv_nsec);                
                                                                              
        fwrite(buf, 1, buflen, stdout);                                       
        return 0;                                                             
}

/*
$ time ./a.out > 100M1
1576370676.960357900
1576370676.964517730

real    0m0.084s
user    0m0.006s
sys     0m0.078s
*/