main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>
#include <ctype.h>
#include <stdbool.h>

char *filepath;
char *prop_name;
char *value;
enum Mode {
    SET = 1, GET = 2, SHOW = 3, ERROR = -1, NONE = 0
};
enum Mode mode = NONE;

size_t id3_readSize(FILE *pIobuf);

char *parse_bin_string(char *content, size_t length);

void id3_writeSize(FILE *pIobuf, size_t size);

int main(int argc, char *argv[]) {
    static const struct option longopts[] = {
            {.name = "filepath", .has_arg = required_argument, .val = 'f'},
            {.name = "show", .has_arg = no_argument, .val = 'p'},
            {.name = "get", .has_arg = required_argument, .val = 'g'},
            {.name = "set", .has_arg = required_argument, .val = 's'},
            {.name = "value", .has_arg = required_argument, .val = 'v'},
            {},
    };
    for (;;) {
        int opt = getopt_long(argc, argv, "bf", longopts, NULL);
        if (opt == -1)
            break;
        switch (opt) {
            case 'f':
                fprintf(stderr, "Got --filepath: %s\n", optarg);
                filepath = optarg;
                break;
            case 's':
                fprintf(stderr, "Got --set: %s\n", optarg);
                prop_name = optarg;
                mode = SET;
                break;
            case 'v':
                fprintf(stderr, "Got --value: %s\n", optarg);
                if (mode != SET)
                    mode = ERROR;
                value = optarg;
                break;
            case 'g':
                fprintf(stderr, "Got --get: %s\n", optarg);
                prop_name = optarg;
                if (mode != NONE)
                    mode = ERROR;
                else
                    mode = GET;
                break;
            case 'p':
                fprintf(stderr, "Got --show\n");
                if (mode != NONE)
                    mode = ERROR;
                else
                    mode = SHOW;
                break;
            default:
                /* Unexpected option */
                return 1;
        }
    }
    if (mode == ERROR) {
        fprintf(stderr, "Invalid arguments.\n");
        return 1;
    }

    if (!filepath) {
        fprintf(stderr, "Filename required.\n");
        return 1;
    }

    FILE *inFile = fopen(filepath, "rb");
    if (inFile == NULL) {
        fprintf(stderr, "File with specified name is not present.\n");
        return 1;
    }

    size_t tmpSize = 3;
    char id3_marker[tmpSize + 1];
    id3_marker[tmpSize] = 0x00;
    for (int i = 0; i < tmpSize; ++i) {
        id3_marker[i] = (char) getc(inFile);
    }

    if (strcmp(id3_marker, "ID3") != 0) {
        fprintf(stderr, "Invalid file\n");
        return 1;
    }

    for (int i = 0; i < 3; ++i) {  // Skip subversion etc
        getc(inFile);
    }

    int frameSize = id3_readSize(inFile);

    if (mode == SHOW) {
        while (ftell(inFile) < frameSize - 10) {
            char name[5];
            name[4] = 0x00;
            for (int i = 0; i < 4; ++i) {
                name[i] = (char) getc(inFile);
            }
            size_t tagSize = id3_readSize(inFile);

            // Skip flags
            getc(inFile);
            getc(inFile);
            if (name[0] == 'T' || (strcmp(name, "COMM") == 0)) {
                char *content = calloc(sizeof(char), tagSize + 1);
                content[tagSize] = 0x00;
                content[0] = ' ';
                getc(inFile);
                for (int i = 0; i < tagSize - 1; ++i) {
                    content[i] = (char) getc(inFile);
                }
                char *text = parse_bin_string(content, tagSize - 1);
                printf("%s -> %s\n", name, text);
                free(text);
                free(content);
            } else {
                fseek(inFile, (long) tagSize, SEEK_CUR);
            }
        }
    }
    if (mode == SET) {
        bool found = false;
        size_t size = 0;
        while (ftell(inFile) < frameSize - 10) {
            char name[5];
            name[4] = 0x00;
            for (int i = 0; i < 4; ++i) {
                name[i] = (char) getc(inFile);
            }
            size_t tagSize = id3_readSize(inFile);

            // Skip flags
            getc(inFile);
            getc(inFile);
            if (strcmp(name, prop_name) == 0) {
                found = true;
                size = tagSize;
            } else {
                fseek(inFile, (long) tagSize, SEEK_CUR);
            }
            if (found)
                break;
        }
        if (!found || size == 0) {
            printf("Prop %s is not present in the file.\n", prop_name);
            return 0;
        }

        int pos = ftell(inFile) - 6; // Remember the tag start
        fseek(inFile, 0, SEEK_SET);  // Reset cursor to copy the file

        FILE *outFile = fopen("newTmpFile.mp3", "wb");

        for (int i = 0; i < 6; i++) // Copy header
        {
            putc((char) getc(inFile), outFile);
        }

        size_t newSize = frameSize - size + strlen(value);  // Calculate and write new len
        id3_writeSize(outFile, newSize);

        fseek(inFile, ftell(outFile), SEEK_SET); // Sync the two files until THE frame
        while (ftell(outFile) != pos)
            putc((char) getc(inFile), outFile);

        // Write frame len
        id3_writeSize(outFile, strlen(value) + 1);

        // Flags and encoding (set to ASCII)
        for (int i = 0; i < 3; ++i) {
            putc(0x00, outFile);
        }

        for(int i = 0; i < strlen(value); i++)
            putc(value[i] & 0xFF, outFile);

        fseek(inFile, 4 + 2 + size, SEEK_CUR);

        while(1)
        {
            char c = (char)getc(inFile);
            if(c == EOF)
                if(feof(inFile) != 0)
                    break;
            putc(c, outFile);
        }

        fclose(inFile);
        fclose(outFile);
        remove(filepath);
        rename("newTmpFile.mp3", filepath);


    }
    if (mode == GET) {
        bool found = false;
        while (ftell(inFile) < frameSize - 10) {
            char name[5];
            name[4] = 0x00;
            for (int i = 0; i < 4; ++i) {
                name[i] = (char) getc(inFile);
            }
            size_t tagSize = id3_readSize(inFile);

            // Skip flags
            getc(inFile);
            getc(inFile);
            if (strcmp(name, prop_name) == 0) {
                found = true;
                char *content = calloc(sizeof(char), tagSize + 1);
                content[tagSize] = 0x00;
                content[0] = ' ';
                getc(inFile);
                for (int i = 0; i < tagSize - 1; ++i) {
                    content[i] = (char) getc(inFile);
                }
                char *text = parse_bin_string(content, tagSize - 1);
                printf("%s -> %s\n", name, text);
                free(text);
                free(content);
            } else {
                fseek(inFile, (long) tagSize, SEEK_CUR);
            }
        }
        if (!found)
            printf("Prop %s is not present in the file.\n", prop_name);
    }
    return 0;
}

void id3_writeSize(FILE *pIobuf, size_t size) {
    char buf[4];
    for (int i = 0; i < 4; i++) {
        buf[i] = (char) ((size >> ((3 - i) * 7)) & 0xFF);
        buf[i] = (char) (buf[i] & 0x7F);
        putc(buf[i], pIobuf);
    }
}

char *parse_bin_string(char *content, size_t length) {
    char *result = calloc(sizeof(char), length);
    int i = 0;
    for (int j = 0; j < length; ++j) {
        if (isprint(content[j]))
            result[i++] = content[j];
    }
    return result;
}

size_t id3_readSize(FILE *pIobuf) {
    size_t res = 0;
    char tmp;
    for (int i = 0; i < 4; ++i) {
        tmp = (char) getc(pIobuf);
        res = res << 7;
        res |= tmp;
    }
    return res;
}