fseek | 易学教程

C-fopen,fwrite,fread,fseek,fgets,popen,access笔记

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FILE * fopen(const char * path,const char * mode); 所需库： <stdio.h> 返回值 FILE是C语言定义的标准数据结构,如果open()失败,则返回NULL path 路径 mode 打开模式,包括有以下几种 r 以只读方式打开文件，该文件必须存在。 r+ 以读/写方式打开文件，该文件必须存在。 rb+ 以读/写方式打开一个二进制文件，只允许读/写数据。 rt+ 以读/写方式打开一个文本文件，允许读和写。 w 打开只写文件，若文件存在则长度清为 0，即该文件内容消失，若不存在则创建该文件。 w+ 打开可读/写文件，若文件存在则文件长度清为零，即该文件内容会消失。若文件不存在则建立该文件。 a 以附加的方式打开只写文件。若文件不存在，则会建立该文件，如果文件存在，写入的数据会被加到文件尾，即文件原先的内容会被保留（EOF 符保留）。 a+ 以附加方式打开可读/写的文件。若文件不存在，则会建立该文件，如果文件存在，则写入的数据会被加到文件尾后，即文件原先的内容会被保留（原来的 EOF 符不保留）。 wb 以只写方式打开或新建一个二进制文件，只允许写数据。 wb+ 以读/写方式打开或建立一个二进制文件，允许读和写。 wt+ 以读/写方式打开或建立一个文本文件，允许读写。 at+ 以读/写方式打开一个文本文件，允许读或在文本末追加数据

use fseek with structure array in c

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问题 everyone.so I have this binary file with data written to it using an array of structures. Now I want to update a specific record in it. however, i am unsure of how to achieve this based on using fseek. Could anyone please offer some assistance? struct clientData{ char cliName[16]; char persIdNum[10]; int pos; char empID[6]; }; FILE* fptr; //FILE* f; char compName[15]; printf("Enter company name: "); scanf(" %s", compName); fptr = fopen(strcat(compName,".dat"), "wb"); struct clientData data[5]

behaviour of fseek and SEEK_END

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问题 If I have a text file with the following content opened as binary 1234567890 a call like this: fseek(fp, 5L, SEEK_SET); give me 6 when I call (char)fgetc(fp) because I offset 5 byte from byte 0 (not start from 1 but from 2) but If I do: fseek(fp, -3L, SEEK_END); give me 8 and not 7 when I call (char)fgetc(fp) . Why? It seems as with SEEK_END the offset doesn't start from the previous byte after the last. 回答1: SEEK_END searches from the one-past last byte of the file: 1234567890 <--- bytes

MaxMind GeoIP API: fseek() [function.fseek]: stream does not support seeking in geoip.inc

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问题 I'm using MaxMind's GeoIP service as such: <?php include("geoip/geoip.inc"); include("geoip/geoipcity.inc"); include('geoip/geoipregionvars.php'); $gi = geoip_open("http://watchandrepeat.com/GeoLiteCity.dat", GEOIP_STANDARD); ?> It turns out that the geopi_open API uses fseek, and it is failing on my web server: Warning: fseek() [function.fseek]: stream does not support seeking in /hsphere/local/home/yudaluz/watchandrepeat.com/geoip/geoip.inc on line 319 As of this writing, you can see it

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

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问题 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

PHP appending to file from specific position

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问题 In php i am opening a text file and appending to it. However I need to append 3 chars before the end of file. In other words i need to append/write from a specific place in the file. Can any one help? Best Regards Luben 回答1: You need to open the file for edit, seek to the desired position and then write to the file, eg.: <?php $file = fopen($filename, "c"); fseek($file, -3, SEEK_END); fwrite($file, "whatever you want to write"); fclose($file); ?> Further reference at php.net - fseek doc Hope

Flushing fopen()'ed files opened in update mode,between read and write operations.Explicit flushing needed?

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问题 I have read this about the switch between read and write operations(and vice-versa) for files opened for update using fopen() (LINK) "For files open for update (those which include a "+" sign), on which both input and output operations are allowed, the stream should be flushed (fflush) or repositioned (fseek, fsetpos, rewind) between either a writing operation followed by a reading operation or a reading operation which did not reach the end-of-file followed by a writing operation." There are

fseek on a position beyond EOF does not trigger EOF using feof, how come?

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问题 I'm reading data from a file to memory that is opened with: FILE *f = fopen(path, "rb"); Before I start copying bytes from the file I seek to a start position using: /** * Goes to the given position of the given file. * * - Returns 0 on success * - Returns -1 on EOF * - Returns -2 if an error occured, see errno for error code * - Returns -3 if none of the above applies. This should never happen! */ static int8_t goto_pos(FILE *f, uint64_t pos) { int err = fseek(f, pos, SEEK_SET); if (err != 0

ftello/fseeko vs fgetpos/fsetpos

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问题 What is the difference between ftello/fseeko and fgetpos/fsetpos? Both seem to be file pointer getting/setting functions that use opaque offset types to sometimes allow 64 bit offsets. Are they supported on different platforms or by different standards? Is one more flexible in the type of the offset it uses? And, by the way, I am aware of fgetpos/fsetpos and ftell/fseek, but this is not a duplicate. That question asks about ftell/fseek, and the answer is not applicable to ftello/fseeko. 回答1:

Append to the end of a file in C

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问题 I'm trying to append the contents of a file myfile.txt to the end of a second file myfile2.txt in c. I can copy the contents, but I can't find a way to append. Here's my code: FILE *pFile; FILE *pFile2; char buffer[256]; pFile=fopen("myfile.txt", "r"); pFile2=fopen("myfile2.txt", r+); if(pFile==NULL) { perror("Error opening file."); } else { while(!feof(pFile)) { if(fgets(buffer, 100, pFile) != NULL) { fseek(pFile2, -100, SEEK_END); fprintf(pFile2, buffer); } } fclose(pFile); fclose(pFile2);