lib/zip_source_file_common.c - third_party/libzip - Git at Google

 /*
   zip_source_file_common.c -- create data source from file
   Copyright (C) 1999-2021 Dieter Baron and Thomas Klausner

   This file is part of libzip, a library to manipulate ZIP archives.
   The authors can be contacted at <libzip@nih.at>

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:
   1. Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.
   2. Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in
      the documentation and/or other materials provided with the
      distribution.
   3. The names of the authors may not be used to endorse or promote
      products derived from this software without specific prior
      written permission.

   THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``AS IS'' AND ANY EXPRESS
   OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
   WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
   DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
   GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
   INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
   IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
   OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
   IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 #include "zipint.h"

 #include "zip_source_file.h"

 static zip_int64_t read_file(void *state, void *data, zip_uint64_t len, zip_source_cmd_t cmd);

 static void
 zip_source_file_stat_init(zip_source_file_stat_t *st) {
     st->size = 0;
     st->mtime = time(NULL);
     st->exists = false;
     st->regular_file = false;
 }

 zip_source_t *
 zip_source_file_common_new(const char *fname, void *file, zip_uint64_t start, zip_int64_t len, const zip_stat_t *st, zip_source_file_operations_t *ops, void *ops_userdata, zip_error_t *error) {
     zip_source_file_context_t *ctx;
     zip_source_t *zs;
     zip_source_file_stat_t sb;

     if (ops == NULL) {
         zip_error_set(error, ZIP_ER_INVAL, 0);
         return NULL;
     }

     if (ops->close == NULL || ops->read == NULL || ops->seek == NULL || ops->stat == NULL) {
         zip_error_set(error, ZIP_ER_INTERNAL, 0);
         return NULL;
     }

     if (ops->write != NULL && (ops->commit_write == NULL || ops->create_temp_output == NULL || ops->remove == NULL || ops->rollback_write == NULL || ops->tell == NULL)) {
         zip_error_set(error, ZIP_ER_INTERNAL, 0);
         return NULL;
     }

     if (fname != NULL) {
         if (ops->open == NULL || ops->string_duplicate == NULL) {
             zip_error_set(error, ZIP_ER_INTERNAL, 0);
             return NULL;
         }
     }
     else if (file == NULL) {
         zip_error_set(error, ZIP_ER_INVAL, 0);
         return NULL;
     }

     if (len < 0) {
         len = 0;
     }

     if (start > ZIP_INT64_MAX || start + (zip_uint64_t)len < start) {
         zip_error_set(error, ZIP_ER_INVAL, 0);
         return NULL;
     }

     if ((ctx = (zip_source_file_context_t *)malloc(sizeof(zip_source_file_context_t))) == NULL) {
         zip_error_set(error, ZIP_ER_MEMORY, 0);
         return NULL;
     }

     ctx->ops = ops;
     ctx->ops_userdata = ops_userdata;
     ctx->fname = NULL;
     if (fname) {
         if ((ctx->fname = ops->string_duplicate(ctx, fname)) == NULL) {
             zip_error_set(error, ZIP_ER_MEMORY, 0);
             free(ctx);
             return NULL;
         }
     }
     ctx->f = file;
     ctx->start = start;
     ctx->len = (zip_uint64_t)len;
     if (st) {
         memcpy(&ctx->st, st, sizeof(ctx->st));
         ctx->st.name = NULL;
         ctx->st.valid &= ~ZIP_STAT_NAME;
     }
     else {
         zip_stat_init(&ctx->st);
     }

     if (ctx->len > 0) {
         ctx->st.size = ctx->len;
         ctx->st.valid |= ZIP_STAT_SIZE;
     }

     zip_error_init(&ctx->stat_error);

     ctx->tmpname = NULL;
     ctx->fout = NULL;

     zip_error_init(&ctx->error);
     zip_file_attributes_init(&ctx->attributes);

     ctx->supports = ZIP_SOURCE_SUPPORTS_READABLE | zip_source_make_command_bitmap(ZIP_SOURCE_SUPPORTS, ZIP_SOURCE_TELL, -1);

     zip_source_file_stat_init(&sb);
     if (!ops->stat(ctx, &sb)) {
         _zip_error_copy(error, &ctx->error);
         free(ctx->fname);
         free(ctx);
         return NULL;
     }

     if (!sb.exists) {
         if (ctx->fname && ctx->start == 0 && ctx->len == 0 && ops->write != NULL) {
             ctx->supports = ZIP_SOURCE_SUPPORTS_WRITABLE;
             /* zip_open_from_source checks for this to detect non-existing files */
             zip_error_set(&ctx->stat_error, ZIP_ER_READ, ENOENT);
         }
         else {
             zip_error_set(&ctx->stat_error, ZIP_ER_READ, ENOENT);
             free(ctx->fname);
             free(ctx);
             return NULL;
         }
     }
     else {
         if ((ctx->st.valid & ZIP_STAT_MTIME) == 0) {
             ctx->st.mtime = sb.mtime;
             ctx->st.valid |= ZIP_STAT_MTIME;
         }
         if (sb.regular_file) {
             ctx->supports = ZIP_SOURCE_SUPPORTS_SEEKABLE;

             if (ctx->start + ctx->len > sb.size) {
                 zip_error_set(error, ZIP_ER_INVAL, 0);
                 free(ctx->fname);
                 free(ctx);
                 return NULL;
             }

             if (ctx->len == 0) {
                 ctx->len = sb.size - ctx->start;
                 ctx->st.size = ctx->len;
                 ctx->st.valid |= ZIP_STAT_SIZE;

                 /* when using a partial file, don't allow writing */
                 if (ctx->fname && start == 0 && ops->write != NULL) {
                     ctx->supports = ZIP_SOURCE_SUPPORTS_WRITABLE;
                 }
             }
         }

         ctx->supports |= ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_GET_FILE_ATTRIBUTES);
     }

     ctx->supports |= ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_ACCEPT_EMPTY);
     if (ops->create_temp_output_cloning != NULL) {
         if (ctx->supports & ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_BEGIN_WRITE)) {
             ctx->supports |= ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_BEGIN_WRITE_CLONING);
         }
     }

     if ((zs = zip_source_function_create(read_file, ctx, error)) == NULL) {
         free(ctx->fname);
         free(ctx);
         return NULL;
     }

     return zs;
 }


 static zip_int64_t
 read_file(void *state, void *data, zip_uint64_t len, zip_source_cmd_t cmd) {
     zip_source_file_context_t *ctx;
     char *buf;

     ctx = (zip_source_file_context_t *)state;
     buf = (char *)data;

     switch (cmd) {
     case ZIP_SOURCE_ACCEPT_EMPTY:
         return 0;

     case ZIP_SOURCE_BEGIN_WRITE:
         /* write support should not be set if fname is NULL */
         if (ctx->fname == NULL) {
             zip_error_set(&ctx->error, ZIP_ER_INTERNAL, 0);
             return -1;
         }
         return ctx->ops->create_temp_output(ctx);

     case ZIP_SOURCE_BEGIN_WRITE_CLONING:
         /* write support should not be set if fname is NULL */
         if (ctx->fname == NULL) {
             zip_error_set(&ctx->error, ZIP_ER_INTERNAL, 0);
             return -1;
         }
         return ctx->ops->create_temp_output_cloning(ctx, len);

     case ZIP_SOURCE_CLOSE:
         if (ctx->fname) {
             ctx->ops->close(ctx);
             ctx->f = NULL;
         }
         return 0;

     case ZIP_SOURCE_COMMIT_WRITE: {
         zip_int64_t ret = ctx->ops->commit_write(ctx);
         ctx->fout = NULL;
         if (ret == 0) {
             free(ctx->tmpname);
             ctx->tmpname = NULL;
         }
         return ret;
     }

     case ZIP_SOURCE_ERROR:
         return zip_error_to_data(&ctx->error, data, len);

     case ZIP_SOURCE_FREE:
         free(ctx->fname);
         free(ctx->tmpname);
         if (ctx->f) {
             ctx->ops->close(ctx);
         }
         free(ctx);
         return 0;

     case ZIP_SOURCE_GET_FILE_ATTRIBUTES:
         if (len < sizeof(ctx->attributes)) {
             zip_error_set(&ctx->error, ZIP_ER_INVAL, 0);
             return -1;
         }
         memcpy(data, &ctx->attributes, sizeof(ctx->attributes));
         return sizeof(ctx->attributes);

     case ZIP_SOURCE_OPEN:
         if (ctx->fname) {
             if (ctx->ops->open(ctx) == false) {
                 return -1;
             }
         }

         if (ctx->start > 0) { // TODO: rewind on re-open
             if (ctx->ops->seek(ctx, ctx->f, (zip_int64_t)ctx->start, SEEK_SET) == false) {
                 /* TODO: skip by reading */
                 return -1;
             }
         }
         ctx->offset = 0;
         return 0;

     case ZIP_SOURCE_READ: {
         zip_int64_t i;
         zip_uint64_t n;

         if (ctx->len > 0) {
             n = ZIP_MIN(ctx->len - ctx->offset, len);
         }
         else {
             n = len;
         }

         if ((i = ctx->ops->read(ctx, buf, n)) < 0) {
             zip_error_set(&ctx->error, ZIP_ER_READ, errno);
             return -1;
         }
         ctx->offset += (zip_uint64_t)i;

         return i;
     }

     case ZIP_SOURCE_REMOVE:
         return ctx->ops->remove(ctx);

     case ZIP_SOURCE_ROLLBACK_WRITE:
         ctx->ops->rollback_write(ctx);
         ctx->fout = NULL;
         free(ctx->tmpname);
         ctx->tmpname = NULL;
         return 0;

     case ZIP_SOURCE_SEEK: {
         zip_int64_t new_offset = zip_source_seek_compute_offset(ctx->offset, ctx->len, data, len, &ctx->error);

         if (new_offset < 0) {
             return -1;
         }

         /* The actual offset inside the file must be representable as zip_int64_t. */
         if (new_offset > ZIP_INT64_MAX - (zip_int64_t)ctx->start) {
             zip_error_set(&ctx->error, ZIP_ER_SEEK, EOVERFLOW);
             return -1;
         }

         ctx->offset = (zip_uint64_t)new_offset;

         if (ctx->ops->seek(ctx, ctx->f, (zip_int64_t)(ctx->offset + ctx->start), SEEK_SET) == false) {
             return -1;
         }
         return 0;
     }

     case ZIP_SOURCE_SEEK_WRITE: {
         zip_source_args_seek_t *args;

         args = ZIP_SOURCE_GET_ARGS(zip_source_args_seek_t, data, len, &ctx->error);
         if (args == NULL) {
             return -1;
         }

         if (ctx->ops->seek(ctx, ctx->fout, args->offset, args->whence) == false) {
             return -1;
         }
         return 0;
     }

     case ZIP_SOURCE_STAT: {
         if (len < sizeof(ctx->st))
             return -1;

         if (zip_error_code_zip(&ctx->stat_error) != 0) {
             zip_error_set(&ctx->error, zip_error_code_zip(&ctx->stat_error), zip_error_code_system(&ctx->stat_error));
             return -1;
         }

         memcpy(data, &ctx->st, sizeof(ctx->st));
         return sizeof(ctx->st);
     }

     case ZIP_SOURCE_SUPPORTS:
         return ctx->supports;

     case ZIP_SOURCE_TELL:
         return (zip_int64_t)ctx->offset;

     case ZIP_SOURCE_TELL_WRITE:
         return ctx->ops->tell(ctx, ctx->fout);

     case ZIP_SOURCE_WRITE:
         return ctx->ops->write(ctx, data, len);

     default:
         zip_error_set(&ctx->error, ZIP_ER_OPNOTSUPP, 0);
         return -1;
     }
 }
	/*
	zip_source_file_common.c -- create data source from file
	Copyright (C) 1999-2021 Dieter Baron and Thomas Klausner

	This file is part of libzip, a library to manipulate ZIP archives.
	The authors can be contacted at <libzip@nih.at>

	Redistribution and use in source and binary forms, with or without
	modification, are permitted provided that the following conditions
	are met:
	1. Redistributions of source code must retain the above copyright
	notice, this list of conditions and the following disclaimer.
	2. Redistributions in binary form must reproduce the above copyright
	notice, this list of conditions and the following disclaimer in
	the documentation and/or other materials provided with the
	distribution.
	3. The names of the authors may not be used to endorse or promote
	products derived from this software without specific prior
	written permission.

	THIS SOFTWARE IS PROVIDED BY THE AUTHORS ``AS IS'' AND ANY EXPRESS
	OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
	WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
	DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
	GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
	IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
	OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
	IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*/

	#include <stdio.h>
	#include <stdlib.h>
	#include <string.h>

	#include "zipint.h"

	#include "zip_source_file.h"

	static zip_int64_t read_file(void state, void data, zip_uint64_t len, zip_source_cmd_t cmd);

	static void
	zip_source_file_stat_init(zip_source_file_stat_t *st) {
	st->size = 0;
	st->mtime = time(NULL);
	st->exists = false;
	st->regular_file = false;
	}

	zip_source_t *
	zip_source_file_common_new(const char fname, void file, zip_uint64_t start, zip_int64_t len, const zip_stat_t st, zip_source_file_operations_t ops, void ops_userdata, zip_error_t error) {
	zip_source_file_context_t *ctx;
	zip_source_t *zs;
	zip_source_file_stat_t sb;

	if (ops == NULL) {
	zip_error_set(error, ZIP_ER_INVAL, 0);
	return NULL;
	}

	if (ops->close == NULL \|\| ops->read == NULL \|\| ops->seek == NULL \|\| ops->stat == NULL) {
	zip_error_set(error, ZIP_ER_INTERNAL, 0);
	return NULL;
	}

	if (ops->write != NULL && (ops->commit_write == NULL \|\| ops->create_temp_output == NULL \|\| ops->remove == NULL \|\| ops->rollback_write == NULL \|\| ops->tell == NULL)) {
	zip_error_set(error, ZIP_ER_INTERNAL, 0);
	return NULL;
	}

	if (fname != NULL) {
	if (ops->open == NULL \|\| ops->string_duplicate == NULL) {
	zip_error_set(error, ZIP_ER_INTERNAL, 0);
	return NULL;
	}
	}
	else if (file == NULL) {
	zip_error_set(error, ZIP_ER_INVAL, 0);
	return NULL;
	}

	if (len < 0) {
	len = 0;
	}

	if (start > ZIP_INT64_MAX \|\| start + (zip_uint64_t)len < start) {
	zip_error_set(error, ZIP_ER_INVAL, 0);
	return NULL;
	}

	if ((ctx = (zip_source_file_context_t *)malloc(sizeof(zip_source_file_context_t))) == NULL) {
	zip_error_set(error, ZIP_ER_MEMORY, 0);
	return NULL;
	}

	ctx->ops = ops;
	ctx->ops_userdata = ops_userdata;
	ctx->fname = NULL;
	if (fname) {
	if ((ctx->fname = ops->string_duplicate(ctx, fname)) == NULL) {
	zip_error_set(error, ZIP_ER_MEMORY, 0);
	free(ctx);
	return NULL;
	}
	}
	ctx->f = file;
	ctx->start = start;
	ctx->len = (zip_uint64_t)len;
	if (st) {
	memcpy(&ctx->st, st, sizeof(ctx->st));
	ctx->st.name = NULL;
	ctx->st.valid &= ~ZIP_STAT_NAME;
	}
	else {
	zip_stat_init(&ctx->st);
	}

	if (ctx->len > 0) {
	ctx->st.size = ctx->len;
	ctx->st.valid \|= ZIP_STAT_SIZE;
	}

	zip_error_init(&ctx->stat_error);

	ctx->tmpname = NULL;
	ctx->fout = NULL;

	zip_error_init(&ctx->error);
	zip_file_attributes_init(&ctx->attributes);

	ctx->supports = ZIP_SOURCE_SUPPORTS_READABLE \| zip_source_make_command_bitmap(ZIP_SOURCE_SUPPORTS, ZIP_SOURCE_TELL, -1);

	zip_source_file_stat_init(&sb);
	if (!ops->stat(ctx, &sb)) {
	_zip_error_copy(error, &ctx->error);
	free(ctx->fname);
	free(ctx);
	return NULL;
	}

	if (!sb.exists) {
	if (ctx->fname && ctx->start == 0 && ctx->len == 0 && ops->write != NULL) {
	ctx->supports = ZIP_SOURCE_SUPPORTS_WRITABLE;
	/* zip_open_from_source checks for this to detect non-existing files */
	zip_error_set(&ctx->stat_error, ZIP_ER_READ, ENOENT);
	}
	else {
	zip_error_set(&ctx->stat_error, ZIP_ER_READ, ENOENT);
	free(ctx->fname);
	free(ctx);
	return NULL;
	}
	}
	else {
	if ((ctx->st.valid & ZIP_STAT_MTIME) == 0) {
	ctx->st.mtime = sb.mtime;
	ctx->st.valid \|= ZIP_STAT_MTIME;
	}
	if (sb.regular_file) {
	ctx->supports = ZIP_SOURCE_SUPPORTS_SEEKABLE;

	if (ctx->start + ctx->len > sb.size) {
	zip_error_set(error, ZIP_ER_INVAL, 0);
	free(ctx->fname);
	free(ctx);
	return NULL;
	}

	if (ctx->len == 0) {
	ctx->len = sb.size - ctx->start;
	ctx->st.size = ctx->len;
	ctx->st.valid \|= ZIP_STAT_SIZE;

	/* when using a partial file, don't allow writing */
	if (ctx->fname && start == 0 && ops->write != NULL) {
	ctx->supports = ZIP_SOURCE_SUPPORTS_WRITABLE;
	}
	}
	}

	ctx->supports \|= ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_GET_FILE_ATTRIBUTES);
	}

	ctx->supports \|= ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_ACCEPT_EMPTY);
	if (ops->create_temp_output_cloning != NULL) {
	if (ctx->supports & ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_BEGIN_WRITE)) {
	ctx->supports \|= ZIP_SOURCE_MAKE_COMMAND_BITMASK(ZIP_SOURCE_BEGIN_WRITE_CLONING);
	}
	}

	if ((zs = zip_source_function_create(read_file, ctx, error)) == NULL) {
	free(ctx->fname);
	free(ctx);
	return NULL;
	}

	return zs;
	}


	static zip_int64_t
	read_file(void state, void data, zip_uint64_t len, zip_source_cmd_t cmd) {
	zip_source_file_context_t *ctx;
	char *buf;

	ctx = (zip_source_file_context_t *)state;
	buf = (char *)data;

	switch (cmd) {
	case ZIP_SOURCE_ACCEPT_EMPTY:
	return 0;

	case ZIP_SOURCE_BEGIN_WRITE:
	/* write support should not be set if fname is NULL */
	if (ctx->fname == NULL) {
	zip_error_set(&ctx->error, ZIP_ER_INTERNAL, 0);
	return -1;
	}
	return ctx->ops->create_temp_output(ctx);

	case ZIP_SOURCE_BEGIN_WRITE_CLONING:
	/* write support should not be set if fname is NULL */
	if (ctx->fname == NULL) {
	zip_error_set(&ctx->error, ZIP_ER_INTERNAL, 0);
	return -1;
	}
	return ctx->ops->create_temp_output_cloning(ctx, len);

	case ZIP_SOURCE_CLOSE:
	if (ctx->fname) {
	ctx->ops->close(ctx);
	ctx->f = NULL;
	}
	return 0;

	case ZIP_SOURCE_COMMIT_WRITE: {
	zip_int64_t ret = ctx->ops->commit_write(ctx);
	ctx->fout = NULL;
	if (ret == 0) {
	free(ctx->tmpname);
	ctx->tmpname = NULL;
	}
	return ret;
	}

	case ZIP_SOURCE_ERROR:
	return zip_error_to_data(&ctx->error, data, len);

	case ZIP_SOURCE_FREE:
	free(ctx->fname);
	free(ctx->tmpname);
	if (ctx->f) {
	ctx->ops->close(ctx);
	}
	free(ctx);
	return 0;

	case ZIP_SOURCE_GET_FILE_ATTRIBUTES:
	if (len < sizeof(ctx->attributes)) {
	zip_error_set(&ctx->error, ZIP_ER_INVAL, 0);
	return -1;
	}
	memcpy(data, &ctx->attributes, sizeof(ctx->attributes));
	return sizeof(ctx->attributes);

	case ZIP_SOURCE_OPEN:
	if (ctx->fname) {
	if (ctx->ops->open(ctx) == false) {
	return -1;
	}
	}

	if (ctx->start > 0) { // TODO: rewind on re-open
	if (ctx->ops->seek(ctx, ctx->f, (zip_int64_t)ctx->start, SEEK_SET) == false) {
	/* TODO: skip by reading */
	return -1;
	}
	}
	ctx->offset = 0;
	return 0;

	case ZIP_SOURCE_READ: {
	zip_int64_t i;
	zip_uint64_t n;

	if (ctx->len > 0) {
	n = ZIP_MIN(ctx->len - ctx->offset, len);
	}
	else {
	n = len;
	}

	if ((i = ctx->ops->read(ctx, buf, n)) < 0) {
	zip_error_set(&ctx->error, ZIP_ER_READ, errno);
	return -1;
	}
	ctx->offset += (zip_uint64_t)i;

	return i;
	}

	case ZIP_SOURCE_REMOVE:
	return ctx->ops->remove(ctx);

	case ZIP_SOURCE_ROLLBACK_WRITE:
	ctx->ops->rollback_write(ctx);
	ctx->fout = NULL;
	free(ctx->tmpname);
	ctx->tmpname = NULL;
	return 0;

	case ZIP_SOURCE_SEEK: {
	zip_int64_t new_offset = zip_source_seek_compute_offset(ctx->offset, ctx->len, data, len, &ctx->error);

	if (new_offset < 0) {
	return -1;
	}

	/* The actual offset inside the file must be representable as zip_int64_t. */
	if (new_offset > ZIP_INT64_MAX - (zip_int64_t)ctx->start) {
	zip_error_set(&ctx->error, ZIP_ER_SEEK, EOVERFLOW);
	return -1;
	}

	ctx->offset = (zip_uint64_t)new_offset;

	if (ctx->ops->seek(ctx, ctx->f, (zip_int64_t)(ctx->offset + ctx->start), SEEK_SET) == false) {
	return -1;
	}
	return 0;
	}

	case ZIP_SOURCE_SEEK_WRITE: {
	zip_source_args_seek_t *args;

	args = ZIP_SOURCE_GET_ARGS(zip_source_args_seek_t, data, len, &ctx->error);
	if (args == NULL) {
	return -1;
	}

	if (ctx->ops->seek(ctx, ctx->fout, args->offset, args->whence) == false) {
	return -1;
	}
	return 0;
	}

	case ZIP_SOURCE_STAT: {
	if (len < sizeof(ctx->st))
	return -1;

	if (zip_error_code_zip(&ctx->stat_error) != 0) {
	zip_error_set(&ctx->error, zip_error_code_zip(&ctx->stat_error), zip_error_code_system(&ctx->stat_error));
	return -1;
	}

	memcpy(data, &ctx->st, sizeof(ctx->st));
	return sizeof(ctx->st);
	}

	case ZIP_SOURCE_SUPPORTS:
	return ctx->supports;

	case ZIP_SOURCE_TELL:
	return (zip_int64_t)ctx->offset;

	case ZIP_SOURCE_TELL_WRITE:
	return ctx->ops->tell(ctx, ctx->fout);

	case ZIP_SOURCE_WRITE:
	return ctx->ops->write(ctx, data, len);

	default:
	zip_error_set(&ctx->error, ZIP_ER_OPNOTSUPP, 0);
	return -1;
	}
	}