我有一个使用openSSL库用C编写的sha256的最简单示例。

// compile with: gcc -o sha256 sha256.c -lcrypto

#include <openssl/sha.h>
#include <stdio.h>
int main(int argc, char **argv)
{
    unsigned char buffer[BUFSIZ];
    FILE *f;
    SHA256_CTX ctx;
    size_t len;
    if (argc < 2) {
        fprintf(stderr, "usage: %s <file>\n", argv[0]);
        return 1;
    }

    f = fopen(argv[1], "r");
    if (!f) {
        fprintf(stderr, "couldn't open %s\n", argv[1]);
        return 1;
    }

    SHA256_Init(&ctx);

    do {
        len = fread(buffer, 1, BUFSIZ, f);
        SHA256_Update(&ctx, buffer, len);
    } while (len == BUFSIZ);

    SHA256_Final(buffer, &ctx);

    fclose(f);

    for (len = 0; len < SHA256_DIGEST_LENGTH; ++len)
        printf("%02x", buffer[len]);
    putchar('\n');
    return 0;
}

我需要同样的sha1，但我找不到类似的简单示例，用SHA1替换上面代码中出现的SHA256的简单方法显然不起作用。
如何修改SHA1的程序？

更新

正如@dbush所建议的，我已经使用了他的EVP代码，并将其集成到我的程序中。我的程序现在看起来像这样：

#include <stdio.h>
#include <openssl/sha.h>
#include <openssl/evp.h>
#include <openssl/err.h>

int main(int argc, char **argv)
{

FILE *f;
size_t len;
unsigned char buffer[BUFSIZ];

if (argc < 2) {
    fprintf(stderr, "usage: %s <file>\n", argv[0]);
    return 1;
}

f = fopen(argv[1], "r");

if (!f) {
    fprintf(stderr, "couldn't open %s\n", argv[1]);
    return 1;
}

EVP_MD_CTX hashctx;
//EVP_MD *hashptr = EVP_get_digestbyname("SHA256");
EVP_MD *hashptr = EVP_get_digestbyname("SHA1");

EVP_MD_CTX_init(&hashctx);
EVP_DigestInit_ex(&hashctx, hashptr, NULL));

do {
    len = fread(buffer, 1, BUFSIZ, f);
    EVP_DigestUpdate(&hashctx, buffer, len);
} while (len == BUFSIZ);

EVP_DigestFinal_ex(&hashctx, buffer, &len);
EVP_MD_CTX_cleanup(&hashctx);

fclose(f);

int i;
for (i = 0; i < len; ++i)
    printf("%02x", buffer[i]);

    return 0;
}

当我使用gcc -o evp evp.c -lcrypto编译它时，我得到了一些错误，例如：

evp.c: In function ‘main’:
evp.c:29:19: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
evp.c:32:43: error: expected ‘;’ before ‘)’ token
evp.c:32:43: error: expected statement before ‘)’ token
evp.c:39:1: warning: passing argument 3 of ‘EVP_DigestFinal_ex’ from incompatible pointer type [enabled by default]
In file included from evp.c:4:0:
/usr/include/openssl/evp.h:574:5: note: expected ‘unsigned int *’ but argument is of type ‘size_t *’