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4147562
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2010-11-10T18:05:48.520
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2012-01-20T19:50:30.693
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First, let's do a hex dump to see what is really stored in the file. <code>hexdump -C b.txt</code> or <code>od -t x2 -t c b.txt</code> are two examples (od is for octal dump, more common, less pretty output in my opinion) <pre><code>00000000 34 20 35 0a |4 5.| 00000004 </code></pre> That's is what the file looks like if it was a created as an ASCII text file (such as using a text editor like vi). You can use <code>man ascii</code> to double check the hexadecimal values. Now if you had a binary file that only contains two 8-bit bytes, in the system's native byte ordering (e.g. little-endian for x86, big endian for MIPS, PA-RISC, 680x0) then the hexdump would look like: <pre><code>00000000 04 05 |..| 00000004 </code></pre> Here is the code to both create (open & write) a binary file, and read it back. <pre><code>#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <stdint.h> /* uint32_t */ #include <sys/stat.h> #include <fcntl.h> #include <string.h> #include <errno.h> /* User has read & write perms, group and others have read permission */ const mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH; typedef struct prova { uint32_t first; uint32_t second; } prova_t; #define FILENAME "file.b" /* 'Safe' write */ int safewrite( int fd, const void *p, size_t want) { int ret; errno = 0; while (want) { ret = write(fd, (uint8_t *)p, want); if (ret <= 0) { if (errno != EINTR && errno != EAGAIN) { return -1; } errno = 0; continue; } want -= ret; p = (uint8_t*) p + ret; } return 0; } int saferead(int fd, const void *p, size_t want) { int ret; errno = 0; while (want) { ret = read(fd, (uint8_t*)p, want); if( ret == 0 ) return -1; /* EOF */ if (ret <= 0) { if( errno != EINTR && errno != EAGAIN ) { return -1; } errno = 0; continue; } want -= ret; p = (uint8_t*) p + ret; } return 0; } int main(int argc, char **argv) { int fd; prova_t result; size_t nbytes; /* Create file */ fd = creat(FILENAME, mode); if (fd < 0) { fprintf(stderr, "Unable to open " FILENAME ": %s\n", strerror(errno)); exit(EXIT_FAILURE); } nbytes = sizeof(prova_t); result.first = 4; result.second = 5; if (0 != safewrite(fd, &result, nbytes)) { fprintf(stderr, "Unable to write to " FILENAME ": %s\n", strerror(errno)); exit(EXIT_FAILURE); } close(fd); fd = -1; /* Reopen and read from binary file */ fd = open(FILENAME, O_RDONLY); nbytes = sizeof(prova_t); if (0 != saferead(fd, &result, nbytes)) { fprintf(stderr, "Unable to read file \"" FILENAME "\": %s\n", strerror(errno)); exit(EXIT_FAILURE); } close(fd); printf( "Read: %d %d (%#.02x%.02x)\n", result.first, result.second, result.first, result.second); return EXIT_SUCCESS; } </code></pre> Now the data file contents look like: <pre><code>00000000 04 00 00 00 05 00 00 00 |........| 00000008 </code></pre> Because the integers were specified as 32-bit integers (32-bits / 8 bits per byte = 4 bytes). I'm using a 64-bit system (little endian, x86), so I wanted to be explicit so the your results should match, assuming little-endian.
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1. PORead and write struct with unistd.h read/write
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1. USmctylr
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1. USmctylr
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1. PORead and write struct with unistd.h read/write
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1. COI should note the very important detail, for when dealing with binary files in *read life*, is that you cannot rely on structures to be written as you want when dumping them to disk. In C, the compiler has the right to silently pad a structure to align structure members so as to be accessible on memory boundaries. Ref: http://c-faq.com/struct/align.html and Michael Crawford's http://www.goingware.com/tips/getting-started/alignment.html
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2. CO+1 for actually doing the right thing, i.e. checking the number of bytes actually read or written.
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