fread | 易学教程

using fwrite() & fread() for binary in/output

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问题 I'm using a large array of floats. After a lot of fiddling I've managed to write it to a binary file. When opening that file at a later time, the reading process only reads a couple of handfuls of floats (according to the return-value of fread(), and it's all values 0.0f). The reading is supposed to put the floats into an (the original) array, and it does not contain the original values. I'm using Code::Blocks and MinGW doing a program in the 32bit realm on a 64bit pc .. and I'm not very

Does fread fail for large files?

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I have to analyze a 16 GB file. I am reading through the file sequentially using fread() and fseek() . Is it feasible? Will fread() work for such a large file? You don't mention a language, so I'm going to assume C. I don't see any problems with fread , but fseek and ftell may have issues. Those functions use long int as the data type to hold the file position, rather than something intelligent like fpos_t or even size_t . This means that they can fail to work on a file over 2 GB, and can certainly fail on a 16 GB file. You need to see how big long int is on your platform. If it's 64 bits, you

Javascript examples found on severial sites regardinf fopen is not working for me

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问题 I am trying to read a text file that is in the same directory as my html file using javascript so that I might include the contents of the text file in my html file. Here is the code I have to test the fopen and fread functions <html> <head> </head> <body> <script> fh = fopen('my.txt', 0); // Open the file for reading. if(fh!=-1) // Check if the file has been successfully opened. { length = flength(fh); // Get the length of the file. str = fread(fh, length); // Read in the entire file. fclose

Whats wrong with fread in this program?

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问题 I'm intermediate student of C. I'm trying to make a bank management program but first I need to make a login program, so I created one of the following. As I've recently learned about file I/O in C and don't know much about fread and fwrite. I have a file (data.txt) which format if as following. user1 1124 user2 3215 user3 5431 In the following program I've asked user to input user name and pin(4-digit password) and copy file data into a structure then compare these two for verifying

How many random / sequential access per fread / fwrite?

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问题 I have the following question regarding C file I/O. At a physical level (harddrive), is it valid to assume that every fread(n_blocks, size, length,FILE fp) operation should cost one random access for the first page (block) and n-1 sequential accesses for the next blocks of that same buffer ?? I assumed this because the OS has so many processes that is mostly sure that one of them is also writing to or reading from a file between each fread of the local program and by that assumption the hard

fread error “unexpected character ending field”

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问题 could you please help me? I am trying to load large TSV file (4 mln rows), and using for that 'fread' (enormous speed :) Problem is that when reaching certain line all program crashes. Last message from verbose is " Bumping column 12 from INT64 to REAL on data row 2220004, field contains '0.54'" I tried to copy only till that row with 'skip' option - it worked fine, but after when I was trying to copy last rows it immediately thrown another error: Unexpected character ("Ам) ending field 5 of

append multiple large data.table's; custom data coercion using colClasses and fread; named pipes

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[This is kind of multiple bug-reports/feature requests in one post, but they don't necessarily make sense in isolation. Apologies for the monster post in advance. Posting here as suggested by help(data.table). Also, I'm new to R; so apologies if I'm not following best practices in my code below. I'm trying.] 1. rbindlist crash on 6 * 8GB files (I have 128GB RAM) First I want to report that using rbindlist to append large data.tables causes R to segfault (ubuntu 13.10, the packaged R version 3.0.1-3ubuntu1, data.table installed from within R from CRAN). The machine has 128 GiB of RAM; so, I

Compare two files Byte by Byte

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问题 I have two binary files and I want to compare them Byte by Byte. I came up with the following code to do so: int CompareFiles(char *pFname1, char *pFname2) { FILE *pFile1,*pFile2; long lSize1, lSize2; // file length int i=0; char tmp1, tmp2; pFile1 = fopen(pFname1,"r"); pFile2 = fopen(pFname2,"r"); // obtain file size: fseek (pFile1 , 0 , SEEK_END); lSize1 = ftell (pFile1); rewind (pFile1); // obtain file size: fseek (pFile2 , 0 , SEEK_END); lSize2 = ftell (pFile2); rewind (pFile2); if

Why does fread mess with my byte order?

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问题 Im trying to parse a bmp file with fread() and when I begin to parse, it reverses the order of my bytes. typedef struct{ short magic_number; int file_size; short reserved_bytes[2]; int data_offset; }BMPHeader; ... BMPHeader header; ... The hex data is 42 4D 36 00 03 00 00 00 00 00 36 00 00 00 ; I am loading the hex data into the struct by fread(&header,14,1,fileIn); My problem is where the magic number should be 0x424d //'BM' fread() it flips the bytes to be 0x4d42 // 'MB' Why does fread() do

How to read line by line after i read a text into a buffer?

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First , I read a text into a buffer by calling fread, and then I want to read it line by line, how to do it? I try to use a sscanf , but it seems not to work. char textbuf[4096]; char line[256]; FILE *fp; fp = fopen(argv[1],"r"); memset(textbuf, 0, 4096); fread(textbuf, 1, 4096, fp); I know using fgets is a good way. I just want to know weather this method can do the same thing. Try this: fgets(textbuf, sizeof(textbuf), fp); For read line by line you can use: fgets(line, 128, fp) or getline(&line, &size, fp); EDIT If you want to read it from a variable, look at strtok() function: char * line =