Some additional thoughts after @Mat fine answer.

1. When implementing an algorithm that counts on a particular size of integer, better to use a size that is guaranteed to work, or consider fixed size types. unsigned may only be 16-bit. Insufficient for the task.

    #include <stdint.h>
   
    // unsigned finalfinal;
    // Options in order of (my) preference
    uint32_t      finalfinal;
    unsigned long finalfinal;
    uint_fast32_t finalfinal;
   

2. For maximum portability, consider that 16-bit machines are very popular in the embedded world in 2016, so do not assume unsigned/int math is at least 32-bit, code to C spec which says an unsigned may only be 16-bit. A 16-bit shift of text[x], which is promoted to int, may not work.

    // finalfinal = (text[x] << 16) | (text[x+1] << 8) | text[x+2];
    finalfinal = (text[x]*1UL << 16) | (text[x+1]*1UL << 8) | text[x+2];
    //  or 
    finalfinal = text[x];
    finalfinal <<= 8; 
    finalfinal |= text[x+1];
    finalfinal <<= 8; 
    finalfinal |= text[x+2];
   
    // final = text[text_len-1] << 16;
    final = text[text_len-1]*1UL << 16;
   

3. Recommend similar thoughts for the function parameters, variables too.

   // char get_base64_digit(const unsigned int base64, int digit_number)
   int get_base64_digit(const uint32_t base64, unsigned digit_number)
   

4. Extending the code to non-null character terminated sources. Current code works fine for test, yet all the following code should be wrapped in a function with the data pointer and size passed in.

    char* text = ...
    size_t text_len = strlen(text);
   
    // Something like this is the real function you need
    int status = NMK64_encode(text, text_len);
   

5. funcs.c is not distinctive. How about NMK_Base64.c?

6. Minor stuff follows

7. Use matching type. strlen() return type size_t, not int.

    // int text_len  = strlen(text);
    // int remainder = text_len % 3;
    size_t text_len  = strlen(text);
    size_t remainder = text_len % 3;
   

8. Make const

    // static char index_table[] = ...
    static const char index_table[] = ...
   

9. Make static. Would not expect this helper function to have global usage. Expect NMK64_encode(text, text_len) to be the global interface.

    // get_base64_digit(
    static int get_base64_digit( ...
   

Some additional thoughts after @Mat fine answer.

1. When implementing an algorithm that counts on a particular size of integer, better to use a size that is guaranteed to work, or consider fixed size types. unsigned may only be 16-bit. Insufficient for the task.

    #include <stdint.h>
   
    // unsigned finalfinal;
    // Options in order of (my) preference
    uint32_t      finalfinal;
    unsigned long finalfinal;
    uint_fast32_t finalfinal;
   

2. For maximum portability, consider that 16-bit machines are very popular in the embedded world in 2016, so do not assume unsigned/int math is at least 32-bit, code to C spec which says an unsigned may only be 16-bit. A 16-bit shift of text[x], which is promoted to int, may not work.

    // finalfinal = (text[x] << 16) | (text[x+1] << 8) | text[x+2];
    finalfinal = (text[x]*1UL << 16) | (text[x+1]*1UL << 8) | text[x+2];
    //  or 
    finalfinal = text[x];
    finalfinal <<= 8; 
    finalfinal |= text[x+1];
    finalfinal <<= 8; 
    finalfinal |= text[x+2];
   
    // final = text[text_len-1] << 16;
    final = text[text_len-1]*1UL << 16;
   

3. Recommend similar thoughts for the function parameters, variables too.

   // char get_base64_digit(const unsigned int base64, int digit_number)
   int get_base64_digit(const uint32_t base64, unsigned digit_number)
   

4. Extending the code to non-null character terminated sources. Current code works fine for test, yet all the following code should be wrapped in a function with the data pointer and size passed in.

    char* text = ...
    size_t text_len = strlen(text);
   
    // Something like this is the real function you need
    int status = NMK64_encode(text, text_len);
   

5. funcs.c is not distinctive. How about NMK_Base64.c?

6. Minor stuff follows

7. Use matching type. strlen() return type size_t, not int.

    // int text_len  = strlen(text);
    // int remainder = text_len % 3;
    size_t text_len  = strlen(text);
    size_t remainder = text_len % 3;
   

8. Make const

    // static char index_table[] = ...
    static const char index_table[] = ...
   

9. Make static. Would not expect this helper function to have global usage. Expect NMK64_encode(text, text_len) to be the global interface.

    // get_base64_digit(
    static int get_base64_digit( ...
   

Some additional thoughts after @Mat fine answer.

1. When implementing an algorithm that counts on a particular size of integer, better to use a size that is guaranteed to work, or consider fixed size types. unsigned may only be 16-bit. Insufficient for the task.

    #include <stdint.h>
   
    // unsigned finalfinal;
    // Options in order of (my) preference
    uint32_t      finalfinal;
    unsigned long finalfinal;
    uint_fast32_t finalfinal;
   

2. For maximum portability, consider that 16-bit machines are very popular in the embedded world in 2016, so do not assume unsigned/int math is at least 32-bit, code to C spec which says an unsigned may only be 16-bit. A 16-bit shift of text[x], which is promoted to int, may not work.

    // finalfinal = (text[x] << 16) | (text[x+1] << 8) | text[x+2];
    finalfinal = (text[x]*1UL << 16) | (text[x+1]*1UL << 8) | text[x+2];
    //  or 
    finalfinal = text[x];
    finalfinal <<= 8; 
    finalfinal |= text[x+1];
    finalfinal <<= 8; 
    finalfinal |= text[x+2];
   
    // final = text[text_len-1] << 16;
    final = text[text_len-1]*1UL << 16;
   

3. Recommend similar thoughts for the function parameters, variables too.

   // char get_base64_digit(const unsigned int base64, int digit_number)
   int get_base64_digit(const uint32_t base64, unsigned digit_number)
   

4. Extending the code to non-nulll character terminated sources. Current code works fine for test, yet all the following code should be wrapped in a function with the data pointer and size passed in.

    char* text = ...
    size_t text_len = strlen(text);
   
    // Something like this is the real function you need
    int status = NMK64_encode(text, text_len);
   

5. funcs.c is not distinctive. How about NMK_Base64.c?

6. Minor stuff follows

7. Use matching type. strlen() return type size_t, not int.

    // int text_len  = strlen(text);
    // int remainder = text_len % 3;
    size_t text_len  = strlen(text);
    size_t remainder = text_len % 3;
   

8. Make const

    // static char index_table[] = ...
    static const char index_table[] = ...
   

9. Make static. Would not expect this helper function to have global usage. Expect NMK64_encode(text, text_len) to be the global interface.

    // get_base64_digit(
    static int get_base64_digit( ...
   

Some additional thoughts after @Mat fine answer.

1. When implementing an algorithm that counts on a particular size of integer, better to use a size that is guaranteed to work, or consider fixed size types. unsigned may only be 16-bit. Insufficient for the task.

    #include <stdint.h>
   
    // unsigned finalfinal;
    // Options in order of (my) preference
    uint32_t      finalfinal;
    unsigned long finalfinal;
    uint_fast32_t finalfinal;
   

2. For maximum portability, consider that 16-bit machines are very popular in the embedded world in 2016, so do not assume unsigned/int math is at least 32-bit, code to C spec which says an unsigned may only be 16-bit. A 16-bit shift of text[x], which is promoted to int, may not work.

    // finalfinal = (text[x] << 16) | (text[x+1] << 8) | text[x+2];
    finalfinal = (text[x]*1UL << 16) | (text[x+1]*1UL << 8) | text[x+2];
    //  or 
    finalfinal = text[x];
    finalfinal <<= 8; 
    finalfinal |= text[x+1];
    finalfinal <<= 8; 
    finalfinal |= text[x+2];
   
    // final = text[text_len-1] << 16;
    final = text[text_len-1]*1UL << 16;
   

3. Recommend similar thoughts for the function parameters, variables too.

   // char get_base64_digit(const unsigned int base64, int digit_number)
   int get_base64_digit(const uint32_t base64, unsigned digit_number)
   

4. Extending the code to non-null character terminated sources. Current code works fine for test, yet all the following code should be wrapped in a function with the data pointer and size passed in.

    char* text = ...
    size_t text_len = strlen(text);
   
    // Something like this is the real function you need
    int status = NMK64_encode(text, text_len);
   

5. funcs.c is not distinctive. How about NMK_Base64.c?

6. Minor stuff follows

7. Use matching type. strlen() return type size_t, not int.

    // int text_len  = strlen(text);
    // int remainder = text_len % 3;
    size_t text_len  = strlen(text);
    size_t remainder = text_len % 3;
   

8. Make const

    // static char index_table[] = ...
    static const char index_table[] = ...
   

9. Make static. Would not expect this helper function to have global usage. Expect NMK64_encode(text, text_len) to be the global interface.

    // get_base64_digit(
    static int get_base64_digit( ...