7-segment display in 50 lines of C99.

7segment.c

#include <stdio.h>

int const schematic[7][7] = {
    {0,1,1,1,1,0,0},
    {2,0,0,0,0,3,0},
    {2,0,0,0,0,3,0},
    {0,4,4,4,4,0,0},
    {5,0,0,0,0,6,0},
    {5,0,0,0,0,6,0},
    {0,7,7,7,7,0,0},
};

#define seg(i) (1<<(i))

int const digits[10] = {
    /* 0 */ seg(1) | seg(2) | seg(3) | seg(5) | seg(6) | seg(7),
    /* 1 */ seg(3) | seg(6),
    /* 2 */ seg(1) | seg(3) | seg(4) | seg(5) | seg(7),
    /* 3 */ seg(1) | seg(3) | seg(4) | seg(6) | seg(7),
    /* 4 */ seg(2) | seg(3) | seg(4) | seg(6),
    /* 5 */ seg(1) | seg(2) | seg(4) | seg(6) | seg(7),
    /* 6 */ seg(1) | seg(2) | seg(4) | seg(5) | seg(6) | seg(7),
    /* 7 */ seg(1) | seg(3) | seg(6),
    /* 8 */ seg(1) | seg(2) | seg(3) | seg(4) | seg(5) | seg(6) | seg(7),
    /* 9 */ seg(1) | seg(2) | seg(3) | seg(4) | seg(6)
};

void segmentize_print(int digit, int row)
{
    if (digit >= 0 && digit <= 10)
        for (int col = 0; col < 7; ++col)
            putchar((seg(schematic[row][col]) & digits[digit]) ? 'X' : ' ');
}

void segmentize(char const* number)
{
    for (int row = 0; row < 7; ++row)
    {
        char const* it = number;
        while (*it) segmentize_print(*it++ - '0', row);
        putchar('\n');
    }
}

int main(int argc, char const* argv[])
{
    if (argc > 1) segmentize(argv[1]);
    return 0;
}