What is the proper way to efficiently use digital signatures?

dsa_example.c

#include <openssl/dsa.h>
#include <openssl/engine.h>
#include <stdio.h>
#include <openssl/evp.h>
int N=3000;
int main()
{
    DSA *set=DSA_new();
    int a;
    a=DSA_generate_parameters_ex(set,1024,NULL,1,NULL,NULL,NULL);
    printf("%d\n",a);
    a=DSA_generate_key(set);
    printf("%d\n",a);
    unsigned char msg[]="I am watching you!I am watching you!";
    unsigned char sign[256];
    unsigned int size;
    for(int i=0;i<N;i++) {
        size=sizeof(sign); /* reset to the size of the buffer */
        a=DSA_sign(1,msg,32,sign,&size,set);
        printf("%d %d\n",a,size);
    }
    printf("DONE\n");
    DSA_free(set); /* free the memory allocated for the key */
}

evp_example.c

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <openssl/pem.h>
#include <openssl/err.h>
#include <openssl/evp.h>

#define KEYFILE "private_key.pem"
#define N 3000
#define BUFFSIZE 80

EVP_PKEY *read_secret_key_from_file(const char * fname)
{
    EVP_PKEY *key = NULL;
    FILE *fp = fopen(fname, "r");
    if(!fp) {
        perror(fname); return NULL;
    }
    key = PEM_read_PrivateKey(fp, NULL, NULL, NULL);
    fclose(fp);
    return key;
}

int do_sign(EVP_PKEY *key, const unsigned char *msg, const size_t mlen,
            unsigned char **sig, size_t *slen)
{
    EVP_MD_CTX *mdctx = NULL;
    int ret = 0;

    /* Create the Message Digest Context */
    if(!(mdctx = EVP_MD_CTX_create())) goto err;

    /* Initialise the DigestSign operation - SHA-256 has been selected
     * as the message digest function in this example */
    if(1 != EVP_DigestSignInit(mdctx, NULL, EVP_sha256(), NULL, key))
        goto err;

    /* Call update with the message */
    if(1 != EVP_DigestSignUpdate(mdctx, msg, mlen)) goto err;

    /* Finalise the DigestSign operation */
    /* First call EVP_DigestSignFinal with a NULL sig parameter to
     * obtain the length of the signature. Length is returned in slen */
    if(1 != EVP_DigestSignFinal(mdctx, NULL, slen)) goto err;
    /* Allocate memory for the signature based on size in slen */
    if(!(*sig = OPENSSL_malloc(*slen))) goto err;
    /* Obtain the signature */
    if(1 != EVP_DigestSignFinal(mdctx, *sig, slen)) goto err;

    /* Success */
    ret = 1;

err:
    if(ret != 1)
    {
        /* Do some error handling */
    }

    /* Clean up */
    if(*sig && !ret) OPENSSL_free(*sig);
    if(mdctx) EVP_MD_CTX_destroy(mdctx);

    return ret;
}

int main()
{
    int ret = EXIT_FAILURE;
    const char *str = "I am watching you!I am watching you!";
    unsigned char *sig = NULL;
    size_t slen = 0;
    unsigned char msg[BUFFSIZE];
    size_t mlen = 0;

    EVP_PKEY *key = read_secret_key_from_file(KEYFILE);
    if(!key) goto err;

    for(int i=0;i<N;i++) {
        if ( snprintf((char *)msg, BUFFSIZE, "%s %d", str, i+1) < 0 )
            goto err;
        mlen = strlen((const char*)msg);
        if (!do_sign(key, msg, mlen, &sig, &slen)) goto err;
        OPENSSL_free(sig); sig = NULL;
        printf("\"%s\" -> siglen=%lu\n", msg, slen);
    }

    printf("DONE\n");
    ret = EXIT_SUCCESS;

err:
    if (ret != EXIT_SUCCESS) {
        ERR_print_errors_fp(stderr);
        fprintf(stderr, "Something broke!\n");
    }

    if (key)
        EVP_PKEY_free(key);

    exit(ret);
}

make_output.txt

lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 94
Model name:            Intel(R) Core(TM) i5-6500 CPU @ 3.20GHz
Stepping:              3
CPU MHz:               3189.400
BogoMIPS:              6384.00
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              6144K
NUMA node0 CPU(s):     0-3
Flags:                 fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single pti retpoline intel_pt rsb_ctxsw spec_ctrl tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm mpx rdseed adx smap clflushopt xsaveopt xsavec xgetbv1 xsaves dtherm arat pln pts hwp hwp_notify hwp_act_window hwp_epp

/opt/openssl-110h/bin/openssl version
OpenSSL 1.1.0h  27 Mar 2018


Generating NIST P-256 private key
/opt/openssl-110h/bin/openssl ecparam -genkey -name prime256v1 -noout -out private_key.pem

cc op_example.c -O2 -g -I/opt/openssl-110h/include -L/opt/openssl-110h/lib -lcrypto -Wl,--enable-new-dtags,-rpath,/opt/openssl-110h/lib -o op_example
cc dsa_example.c -O2 -g -I/opt/openssl-110h/include -L/opt/openssl-110h/lib -lcrypto -Wl,--enable-new-dtags,-rpath,/opt/openssl-110h/lib -o dsa_example
cc evp_example.c -O2 -g -I/opt/openssl-110h/include -L/opt/openssl-110h/lib -lcrypto -Wl,--enable-new-dtags,-rpath,/opt/openssl-110h/lib -o evp_example

Running ./op_example
time ./op_example >/dev/null
0.32user 0.00system 0:00.32elapsed 100%CPU (0avgtext+0avgdata 3452maxresident)k
0inputs+0outputs (0major+153minor)pagefaults 0swaps

Running ./dsa_example
time ./dsa_example >/dev/null
0.42user 0.00system 0:00.42elapsed 100%CPU (0avgtext+0avgdata 3404maxresident)k
0inputs+0outputs (0major+153minor)pagefaults 0swaps

Running ./evp_example
time ./evp_example >/dev/null
0.12user 0.00system 0:00.12elapsed 99%CPU (0avgtext+0avgdata 3764maxresident)k
0inputs+0outputs (0major+157minor)pagefaults 0swaps

Makefile

KEYFILE=private_key.pem

ifeq ($(OPENSSL_PREFIX),)
OPENSSL_PREFIX := /usr
else
RPATH := -Wl,--enable-new-dtags,-rpath,$(OPENSSL_PREFIX)/lib
endif

CFLAGS := -O2 -g -I$(OPENSSL_PREFIX)/include
LDFLAGS := -L$(OPENSSL_PREFIX)/lib -lcrypto $(RPATH)

TARGETS=op_example dsa_example evp_example

all: $(TARGETS)
.PHONY: all

%: %.c
	$(CC) $< $(CFLAGS) $(LDFLAGS) -o $@

$(KEYFILE):
	@echo
	@echo Generating NIST P-256 private key
	$(OPENSSL_PREFIX)/bin/openssl ecparam -genkey -name prime256v1 -noout -out $@
	@echo

.DEFAULT_GOAL :=
benchmark: info $(KEYFILE) $(TARGETS)
	@echo
	@echo Running ./op_example
	time ./op_example >/dev/null
	@echo
	@echo Running ./dsa_example
	time ./dsa_example >/dev/null
	@echo
	@echo Running ./evp_example
	time ./evp_example >/dev/null
	@echo
.PHONY: benchmark

info: lscpu ossl_version
.PHONY: info

lscpu:
	lscpu
.PHONY: lscpu

ossl_version:
	@echo
	$(OPENSSL_PREFIX)/bin/openssl version
	@echo
.PHONY: ossl_version

clean:
	@rm -rf $(TARGETS) $(KEYFILE)
.PHONY: clean

op_example.c

#include <openssl/dsa.h>
#include <openssl/engine.h>
#include <stdio.h>
#include <openssl/evp.h>
int N=3000;
int main()
{
    DSA *set=DSA_new();
    int a;
    a=DSA_generate_parameters_ex(set,1024,NULL,1,NULL,NULL,NULL);
    printf("%d\n",a);
    a=DSA_generate_key(set);
    printf("%d\n",a);
    unsigned char msg[]="I am watching you!I am watching you!";
    unsigned char sign[256];
    unsigned int size;
    for(int i=0;i<N;i++)
        a=DSA_sign(1,msg,32,sign,&size,set);
    printf("%d %d\n",a,size);
}