错误消息未按预期显示

Question

对于我的以下代码，我希望当参数小于3时会出现错误消息，而我是否输入

./compress_uncompress -c compressedimg1 > image.bmp

或

./compress_uncompress -c compressedimg1

或

./compress_uncompress -c

我总是得到＆＃34;无法打开输入文件＆＃34;。另外，对于

./compress_uncompress -e compressedimg1 > image.bmp 

我希望收到错误信息＆＃34;无效的argv [1]＆＃34;我成功解压缩后没有任何错误信息。

unsigned long (*compressBound)(unsigned long length);
int (*compress)(void *dest, unsigned long* destLen, const void* source,       unsigned long sourceLen);
int (*uncompress)(void *dest, unsigned long* destLen, const void* source, unsigned long sourceLen);

void error_exit(const char *error_message)
{
    printf("Error: %s\n", error_message);
    exit(1);
}

int main(int argc, const char* argv[]){
    FILE *fp;
    unsigned long uncompressed_size;
    unsigned long compressed_size;
    char* uncompressed_buffer;
    char* compressed_buffer;
    int compress_result;
    int uncompress_result;

    void* lib = dlopen("libz.so", RTLD_NOW);
    if(lib == NULL)
    {
        error_exit("couldn’t open libz.so");
    }

    compressBound = dlsym(lib, "compressBound");
    if(compressBound == NULL)
    {
        error_exit("compress bound is null!");
    }

    compress = dlsym(lib, "compress");
    if(compress == NULL)
    {
        error_exit("couldn't load the compress symbol from zlib");
    }

    uncompress = dlsym(lib, "uncompress");
    if(uncompress == NULL)
    {
        error_exit("couldn't load the uncompress symbol from zlib");
    }

    fp = fopen(argv[2], "rb");
    if(fp == NULL){
        error_exit("couldn’t open input file");
    }

    if(argc < 3){
        error_exit("too few program arguments");
    }

    if(strcmp(argv[1],"-c") == 0){
        fseek(fp, 0, SEEK_END); //to the end of the file
        uncompressed_size = ftell(fp); //find the size of the uncompressed file
        fseek(fp, 0, SEEK_SET); //back to the begining
        uncompressed_buffer = malloc(uncompressed_size + 1);
        fread(uncompressed_buffer, uncompressed_size, 1, fp);
        compressed_size = compressBound(uncompressed_size);
        compressed_buffer = malloc(compressed_size);
        compress_result = compress(compressed_buffer, &compressed_size, uncompressed_buffer, uncompressed_size);
        if(compress_result < 0){
            error_exit("compress failed");
        }
        fwrite(&uncompressed_size, sizeof(unsigned long), 1, stdout);
        fwrite(&compressed_size, sizeof(unsigned long), 1, stdout);
        fwrite(compressed_buffer, compressed_size, 1, stdout);
    } else if(strcmp(argv[1],"-d") == 0){
        fread(&uncompressed_size, sizeof(unsigned long), 1, fp);
        fread(&compressed_size, sizeof(unsigned long), 1, fp);
        compressed_buffer = malloc(compressed_size);
        fread(compressed_buffer, compressed_size, 1, fp);
        uncompressed_buffer = malloc(uncompressed_size);
        uncompress_result = uncompress(uncompressed_buffer, &uncompressed_size, compressed_buffer, compressed_size);
        if(uncompress_result < 0){
            error_exit("uncompress failed");
        }
        fwrite(uncompressed_buffer, uncompressed_size, 1, stdout);
    } else{
        error_exit("invalid argv[1]");
    }
    free(uncompressed_buffer);
    free(compressed_buffer);
    fclose(fp);
    return 0;
}