fseek

Why is fseek or fflush always required between reading and writing in the update modes?

由 老子叫甜甜 提交于 2019-12-17 02:49:13
问题 Q: I'm trying to update a file in place, by using fopen mode "r+" , reading a certain string, and writing back a modified string, but it's not working. A: Be sure to call fseek before you write, both to seek back to the beginning of the string you're trying to overwrite, and because an fseek or fflush is always required between reading and writing in the read/write "+" modes. My question is why fseek or fflush is always required between reading and writing in the read/write "+" modes? Section

C binary read and write file

由 为君一笑 提交于 2019-12-14 04:13:17
问题 I am using a binary file for reading an array of integers, then each even integer x should become 2 * x and each odd integer x should become 3 * x. When I am doing this it always read the 2nd integer (which is 2 ). Any idea? #include <stdio.h> #include <stdlib.h> int main(void) { FILE *f; f = fopen("inputData.txt", "w+b"); int n = 5; int i; for (i = 1; i <= n; ++i) { fwrite(&i, sizeof(int), 1, f); } int x; fseek(f, 0, SEEK_SET); while (fread(&x, sizeof(int), 1, f) == 1) { printf("%d ", x); if

fread fwrite fseek in C

由 北城余情 提交于 2019-12-11 18:59:45
问题 Hello what i am trying to do is to reverse a binary file. The type of the file is wav so for example if the channel number is 2 and bits per sample are 16 each time i will copy 32/8 = 4 bytes. The first think to do is copy the header as it is(that part is ok) and then reverse he data. I've created a code to copy the header and then part of the data from the end this 10 times(for testing) but instead of copying 40 bytes it stops at 20 for some reason(even if it would do it 20 times it would

XML XSLT Result-Document fseek

由 ぐ巨炮叔叔 提交于 2019-12-11 11:05:36
问题 Using XSLT transformation I want to convert an XML file, that in turn represents various chunks of another file, into a HTML file with a link to the file that was represented. Input XML File: <File> <Name>foo.jpg<Name> <Chunk> <Offset>200</Offset> <Length>100</Length> <Data> <![CDATA[data bytes, encoded in base64, can be greater than length 100 too, but first 100 decoded bytes are valid.]]> </Data> </Chunk> <Chunk> ... </File> The output should be a html file that has a valid link to foo.jpg,

How in portable C to seek forward when reading from a pipe

由 三世轮回 提交于 2019-12-10 03:16:57
问题 Since fseek() does not work on pipes what methods exist for simulating seeking forward? The naive approach is to use fread() and throw away the contents read into the memory buffer. For huge seeks to avoid huge buffers you would use the same buffer over and over with the final read using just a part of the buffer. But is this the only approach? Is there another way which avoids the buffer and the potential multiple read? 回答1: Yes, it is the only way. I would use a buffer somewhere around 1k

What's the intended use of _fread_nolock, _fseek_nolock?

由 久未见 提交于 2019-12-08 09:14:14
问题 we have a C++ class which basically reads and writes vectors from a binary file. An exemplary read function that loads a single vector into memory looks like this: int load (const __int64 index, T* values) const { int re = _fseeki64(_file, index * _vectorSize + _offsetData, SEEK_SET); assert(re == 0); size_t read = fread(values, sizeof(T), _vectorElements, _file); assert(read == _vectorElements); return 0;} Out programs are multithreaded with OpenMP and multiple threads access the same file

转载-C语言FILE类型与标准I/O流

由 自闭症网瘾萝莉.ら 提交于 2019-12-07 02:55:07
权威资料 <cstdio> (stdio.h) - C++ Reference http://www.cplusplus.com/reference/cstdio/ 先来了解下什么是标准ＩＯ以及文件ＩＯ。 标准ＩＯ以及文件ＩＯ。 标准ＩＯ：标准I/O是ANSI C建立的一个标准I/O模型，是一个标准函数包和stdio.h头文件中的定义，具有一定的可移植性。标准IO库处理很多细节。例如缓存分配，以优化长度执行IO等。标准的IO提供了三种类型的缓存。 （1）全缓存：当填满标准IO缓存后才进行实际的IO操作。 （2）行缓存：当输入或输出中遇到新行符时，标准IO库执行IO操作。 （3）不带缓存：stderr就是了。 文件ＩＯ：文件ＩＯ称之为 不带缓存的IO （unbuffered I/O)。不带缓存指的是每个read，write都调用内核中的一个系统调用。也就是一般所说的 低级I/O —— 操作系统提供的基本IO服务 ，与os绑定，特定于linix或unix平台。 ２区别 首先：两者一个显著的不同点在于， 标准I/O默认采用了缓冲机制 ，比如调用fopen函数， 不仅打开一个文件，而且建立了一个缓冲区 （读写模式下将建立两个缓冲区），还创建了一个 包含文件和缓冲区相关数据的数据结构 。 低级I/O一般没有采用缓冲，需要自己创建缓冲区， 不过其实在linix或unix系统中

C reading/writing to a file in binary mode

由 自闭症网瘾萝莉.ら 提交于 2019-12-06 15:37:45
问题 I created a File of 4000 blocks with a blocksize of 4096 Bytes. Now I want to manipulate single blocks and read them again without changeing the files' size. Actually I want to write blocks out of another file to specific blocks in the file I created. Therefore I am opening the Files in binarymode like this: FILE * storeFile=fopen(targetFile, "wb"); // this one I created before FILE * sourceFILE=fopen(sourceFile,"rb"); now I am trying to read stuff to a pointer char * ptr=malloc(4096); ...

Are there cases where fseek/ftell can give the wrong file size?

由 家住魔仙堡 提交于 2019-12-05 15:07:01
In C or C++, the following can be used to return a file size: const unsigned long long at_beg = (unsigned long long) ftell(filePtr); fseek(filePtr, 0, SEEK_END); const unsigned long long at_end = (unsigned long long) ftell(filePtr); const unsigned long long length_in_bytes = at_end - at_beg; fprintf(stdout, "file size: %llu\n", length_in_bytes); Are there development environments, compilers, or OSes which can return the wrong file size from this code, based on padding or other information that is situation-specific? Were there changes in the C or C++ specification around 1999, which would have

Inserting data to file in c

由 耗尽温柔 提交于 2019-12-05 02:19:08
问题 I need to add a string before the 45 th byte in an existing file. I tried using fseek as shown below. int main() { FILE *fp; char str[] = "test"; fp = fopen(FILEPATH,"a"); fseek(fp,-45, SEEK_END); fprintf(fp,"%s",str); fclose(fp); return(0); } I expected that this code will add "test" before the 45 th char from EOF, instead, it just appends "test" to the EOF. Please help me to find the solution. This is continuation of my previous question Append item to a file before last line in c 回答1: Open
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