diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 3eb4d8e..3865e2f 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -4,4 +4,7 @@ target_link_libraries(list PRIVATE minitar)
 add_executable(untar EXCLUDE_FROM_ALL untar.c)
 target_link_libraries(untar PRIVATE minitar)
 
-add_custom_target(examples DEPENDS list untar)
\ No newline at end of file
+add_executable(pack EXCLUDE_FROM_ALL pack.c)
+target_link_libraries(pack PRIVATE minitar)
+
+add_custom_target(examples DEPENDS list untar pack)
\ No newline at end of file
diff --git a/examples/pack.c b/examples/pack.c
new file mode 100644
index 0000000..810ae75
--- /dev/null
+++ b/examples/pack.c
@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2023, apio.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * pack.c: Example utility which creates a tar archive (POSIX only).
+ */
+
+#define _XOPEN_SOURCE 700
+#include <minitar.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+
+int main(int argc, char** argv)
+{
+    if (argc < 3)
+    {
+        fprintf(stderr, "Usage: %s [output] files\n", argv[0]);
+        return 1;
+    }
+    struct minitar_w mp;
+    if (minitar_open_w(argv[1], &mp, MTAR_OVERWRITE) != 0)
+    {
+        perror(argv[1]);
+        return 1;
+    }
+    int exit_status = 0;
+    struct minitar_entry_metadata metadata;
+    int arg = 2;
+    while (arg < argc)
+    {
+        FILE* fp = fopen(argv[arg], "r");
+        if (!fp)
+        {
+            perror("fopen");
+            exit_status = 1;
+            break;
+        }
+
+        // Get the file length.
+        fseek(fp, 0, SEEK_END);
+        size_t length = ftell(fp);
+        fseek(fp, 0, SEEK_SET);
+
+        char* buf = malloc(length);
+        fread(buf, 1, length, fp);
+        if (ferror(fp))
+        {
+            perror("fread");
+            exit_status = 1;
+            break;
+        }
+
+        struct stat st;
+        int rc = fstat(fileno(fp), &st);
+        if (rc < 0)
+        {
+            perror("fstat");
+            exit_status = 1;
+            break;
+        }
+
+        struct minitar_entry_metadata metadata;
+        strncpy(metadata.path, argv[arg], sizeof(metadata.path));
+        metadata.uid = st.st_uid;
+        metadata.gid = st.st_gid;
+        metadata.mtime = st.st_mtim.tv_sec;
+        metadata.size = length;
+        metadata.type = MTAR_REGULAR;
+        metadata.mode = st.st_mode & ~S_IFMT;
+
+        rc = minitar_write_file_entry(&mp, &metadata, buf);
+        free(buf);
+
+        if (rc != 0)
+        {
+            perror("write entry failed");
+            exit_status = 1;
+            break;
+        }
+
+        arg++;
+    }
+    minitar_close_w(&mp);
+    return exit_status;
+}
\ No newline at end of file
diff --git a/minitar.h b/minitar.h
index 8a8aeee..d9ccac5 100644
--- a/minitar.h
+++ b/minitar.h
@@ -17,6 +17,17 @@ struct minitar
     FILE* stream;
 };
 
+struct minitar_w
+{
+    FILE* stream;
+};
+
+enum minitar_write_mode
+{
+    MTAR_APPEND,
+    MTAR_OVERWRITE
+};
+
 enum minitar_file_type
 {
     MTAR_REGULAR,
@@ -62,13 +73,17 @@ extern "C"
 #endif
 
     int minitar_open(const char* pathname, struct minitar* out);
+    int minitar_open_w(const char* pathname, struct minitar_w* out, enum minitar_write_mode mode);
     int minitar_read_entry(struct minitar* mp, struct minitar_entry* out);
+    int minitar_write_file_entry(struct minitar_w* mp, const struct minitar_entry_metadata* metadata, char* buf);
+    int minitar_write_special_entry(struct minitar_w* mp, const struct minitar_entry_metadata* metadata);
     void minitar_rewind(struct minitar* mp);
     int minitar_find_by_name(struct minitar* mp, const char* name, struct minitar_entry* out);
     int minitar_find_by_path(struct minitar* mp, const char* path, struct minitar_entry* out);
     int minitar_find_any_of(struct minitar* mp, enum minitar_file_type type, struct minitar_entry* out);
     size_t minitar_read_contents(struct minitar* mp, const struct minitar_entry* entry, char* buf, size_t max);
     int minitar_close(struct minitar* mp);
+    int minitar_close_w(struct minitar_w* mp);
 
 #ifdef __cplusplus
 }
diff --git a/src/tar.c b/src/tar.c
index 7cc8123..ed01004 100644
--- a/src/tar.c
+++ b/src/tar.c
@@ -11,10 +11,18 @@
 #include <stdio.h>
 #include <string.h>
 
+#ifndef __TINYC__
+#include <stdnoreturn.h>
+#else
+#define noreturn _Noreturn
+#endif
+
 // all of these are defined in util.c
 int minitar_read_header(struct minitar*, struct tar_header*);
+noreturn void minitar_handle_panic(const char*);
 int minitar_validate_header(const struct tar_header*);
 void minitar_parse_metadata_from_tar_header(const struct tar_header*, struct minitar_entry_metadata*);
+void minitar_construct_header_from_metadata(struct tar_header*, const struct minitar_entry_metadata*);
 size_t minitar_align_up_to_block_size(size_t);
 
 int minitar_open(const char* pathname, struct minitar* out)
@@ -27,11 +35,31 @@ int minitar_open(const char* pathname, struct minitar* out)
     return 0;
 }
 
+int minitar_open_w(const char* pathname, struct minitar_w* out, enum minitar_write_mode mode)
+{
+    const char* mode_string;
+    switch (mode)
+    {
+    case MTAR_APPEND: mode_string = "ab"; break;
+    case MTAR_OVERWRITE: mode_string = "wb"; break;
+    default: minitar_handle_panic("mode passed to minitar_open_w is not supported");
+    }
+    FILE* fp = fopen(pathname, mode_string);
+    if (!fp) return -1;
+    out->stream = fp;
+    return 0;
+}
+
 int minitar_close(struct minitar* mp)
 {
     return fclose(mp->stream);
 }
 
+int minitar_close_w(struct minitar_w* mp)
+{
+    return fclose(mp->stream);
+}
+
 // Try to read a valid header, and construct an entry from it. If the 512-byte block at the current read offset is not a
 // valid header, valid is set to 0 so we can try again with the next block. In any other case, valid is set to 1. This
 // helps distinguish valid return values, EOF, and invalid headers that we should just skip.
@@ -74,6 +102,47 @@ int minitar_read_entry(struct minitar* mp, struct minitar_entry* out)
     return result;
 }
 
+int minitar_write_file_entry(struct minitar_w* mp, const struct minitar_entry_metadata* metadata, char* buf)
+{
+    struct minitar_entry_metadata meta = *metadata;
+    meta.type = MTAR_REGULAR;
+
+    struct tar_header hdr;
+    minitar_construct_header_from_metadata(&hdr, &meta);
+    // Write the header.
+    size_t nwrite = fwrite(&hdr, sizeof(hdr), 1, mp->stream);
+    if (nwrite == 0 && ferror(mp->stream)) return -1;
+
+    // Write the file data.
+    nwrite = fwrite(buf, 1, meta.size, mp->stream);
+    if (nwrite == 0 && ferror(mp->stream)) return -1;
+
+    char zeroes[512];
+    memset(zeroes, 0, sizeof(zeroes));
+
+    // Write as many zeroes as necessary to finish a block.
+    size_t nzero = minitar_align_up_to_block_size(meta.size) - meta.size;
+    nwrite = fwrite(zeroes, 1, nzero, mp->stream);
+    if (nwrite == 0 && ferror(mp->stream)) return -1;
+
+    return 0;
+}
+
+int minitar_write_special_entry(struct minitar_w* mp, const struct minitar_entry_metadata* metadata)
+{
+    struct minitar_entry_metadata meta = *metadata;
+    if (meta.type == MTAR_REGULAR)
+        minitar_handle_panic("Trying to write a special entry, yet MTAR_REGULAR passed as the entry type");
+    meta.size = 0;
+
+    struct tar_header hdr;
+    minitar_construct_header_from_metadata(&hdr, &meta);
+    size_t nwrite = fwrite(&hdr, sizeof(hdr), 1, mp->stream);
+    if (nwrite == 0 && ferror(mp->stream)) return -1;
+
+    return 0;
+}
+
 void minitar_rewind(struct minitar* mp)
 {
     rewind(mp->stream);
diff --git a/src/util.c b/src/util.c
index 84bf178..5cc7d4c 100644
--- a/src/util.c
+++ b/src/util.c
@@ -234,6 +234,56 @@ uint32_t minitar_checksum_header(const struct tar_header* hdr)
     return sum;
 }
 
+void minitar_construct_header_from_metadata(struct tar_header* hdr, const struct minitar_entry_metadata* metadata)
+{
+    if (strlen(metadata->path) > 100)
+    {
+        minitar_handle_panic("FIXME: pathnames over 100 (using the prefix field) are unsupported for now");
+    }
+
+    // We intentionally want strncpy to not write a null terminator here if the path field is 100 bytes long.
+    strncpy(hdr->name, metadata->path, 100);
+
+    snprintf(hdr->mode, 8, "%.7o", metadata->mode);
+    snprintf(hdr->uid, 8, "%.7o", metadata->uid);
+    snprintf(hdr->gid, 8, "%.7o", metadata->gid);
+
+    // snprintf will write the null terminator past the size field. We don't care, as we will overwrite that zero later.
+    snprintf(hdr->size, 13, "%.12zo", metadata->size);
+    // Same here.
+    snprintf(hdr->mtime, 13, "%.12lo", metadata->mtime);
+
+    switch (metadata->type)
+    {
+    case MTAR_REGULAR: hdr->typeflag = '0'; break;
+    case MTAR_HARDLINK: hdr->typeflag = '1'; break;
+    case MTAR_SYMLINK: hdr->typeflag = '2'; break;
+    case MTAR_CHRDEV: hdr->typeflag = '3'; break;
+    case MTAR_BLKDEV: hdr->typeflag = '4'; break;
+    case MTAR_DIRECTORY: hdr->typeflag = '5'; break;
+    case MTAR_FIFO: hdr->typeflag = '6'; break;
+    }
+
+    strncpy(hdr->linkname, metadata->link, 100);
+
+    memcpy(hdr->magic, "ustar", 6);
+
+    hdr->version[0] = '0';
+    hdr->version[1] = '0';
+
+    strncpy(hdr->uname, metadata->uname, 32);
+    strncpy(hdr->gname, metadata->gname, 32);
+
+    snprintf(hdr->devmajor, 8, "%.7o", metadata->devmajor);
+    snprintf(hdr->devminor, 8, "%.7o", metadata->devminor);
+
+    memset(hdr->prefix, 0, sizeof(hdr->prefix));
+    memset(hdr->padding, 0, sizeof(hdr->padding));
+
+    uint32_t checksum = minitar_checksum_header(hdr);
+    snprintf(hdr->chksum, 8, "%.7o", checksum);
+}
+
 int minitar_validate_header(const struct tar_header* hdr)
 {
     if (hdr->typeflag != '\0' && hdr->typeflag != '0' && hdr->typeflag != '1' && hdr->typeflag != '2' &&