在C语言中，可以使用一些开源的库来实现txt文件的压缩，比如zlib库。以下是一个简单的示例代码，用来实现txt文件的压缩：

#include <stdio.h>#include <stdlib.h>#include <string.h>#include <zlib.h>#define CHUNK_SIZE 16384int compress_file(const char *source_file, const char *dest_file) {FILE *source = fopen(source_file, "rb");if (!source) {fprintf(stderr, "Error opening source file\n");return -1;}FILE *dest = fopen(dest_file, "wb");if (!dest) {fprintf(stderr, "Error opening destination file\n");fclose(source);return -1;}z_stream stream;stream.zalloc = Z_NULL;stream.zfree = Z_NULL;stream.opaque = Z_NULL;stream.avail_in = 0;stream.next_in = Z_NULL;if (deflateInit(&stream, Z_DEFAULT_COMPRESSION) != Z_OK) {fprintf(stderr, "Error initializing zlib\n");fclose(source);fclose(dest);return -1;}unsigned char in[CHUNK_SIZE];unsigned char out[CHUNK_SIZE];int ret;do {stream.avail_in = fread(in, 1, CHUNK_SIZE, source);if (ferror(source)) {fprintf(stderr, "Error reading from source file\n");deflateEnd(&stream);fclose(source);fclose(dest);return -1;}stream.next_in = in;do {stream.avail_out = CHUNK_SIZE;stream.next_out = out;ret = deflate(&stream, stream.avail_in ? Z_NO_FLUSH : Z_FINISH);if (ret == Z_STREAM_ERROR) {fprintf(stderr, "Error compressing data\n");deflateEnd(&stream);fclose(source);fclose(dest);return -1;}fwrite(out, 1, CHUNK_SIZE - stream.avail_out, dest);if (ferror(dest)) {fprintf(stderr, "Error writing to destination file\n");deflateEnd(&stream);fclose(source);fclose(dest);return -1;}} while (stream.avail_out == 0);} while (ret != Z_STREAM_END);deflateEnd(&stream);fclose(source);fclose(dest);return 0;}int main() {const char *source_file = "source.txt";const char *dest_file = "compressed.txt";if (compress_file(source_file, dest_file) != 0) {fprintf(stderr, "Error compressing file\n");return -1;}printf("File compressed successfully\n");return 0;}

在上面的代码中，我们使用了zlib库中的deflate函数来实现文件的压缩。首先打开源文件和目标文件，然后使用deflateInit函数初始化压缩流。接下来，我们使用一个循环来读取源文件的数据并进行压缩，最后写入目标文件。最后调用deflateEnd函数释放资源并关闭文件。

请注意，以上代码仅供参考，实际使用时需要根据实际情况进行适当的修改和调整。