Read the binary file (any file will do; this article uses binary as an example) and read the entire contents of the binary file into a char* string. With fseek() and fread() functions to achieve file reading advanced methods.
需求
- 使用fwrite(dbdata, dblength, 1,fp)把字节流写入二进制文件。在新程序读取二进制文件遇到问题：二进制内容不能向文本一样行读取，也不知道二进制文件长度，在fread()函数中无从下手。
1.creat FILE pointer and set mode as ‘rb’
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| FILE *f = fopen(inputFN, "rb");
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2.check the FILE pointer is not null
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| if (!f) {
fprintf(stderr, "ERROR: unable to open file \"%s\": %s\n", inputFN,strerror(errno));
return NULL;
}
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3.use fseek/ftell to get data length
- fseek(f,0,SEEK_END) put the pointer to the end of the file.
- ftell(f) can get the current offset.
- then use fseek(f,0,SEEK_SET) put the pointer to the start of file.
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| if (fseek(f, 0, SEEK_END) != 0) {
fprintf(stderr, "ERROR: unable to seek file \"%s\": %s\n", inputFN,
strerror(errno));
fclose(f);
return NULL;
}
long datalen = ftell(f);
if (dataLen < 0) {
fprintf(stderr, "ERROR: ftell() failed: %s\n", strerror(errno));
fclose(f);
return NULL;
}
if (fseek(f, 0, SEEK_SET) != 0) {
fprintf(stderr, "ERROR: unable to seek file \"%s\": %s\n", inputFN,
strerror(errno));
fclose(f);
return NULL;
}
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4.check the datalen
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| if ((unsigned long)dataLen > UINT_MAX) {
dataLen = UINT_MAX;
printf("WARNING: clipping data to %ld bytes\n", dataLen);
} else if (dataLen == 0) {
fprintf(stderr, "ERROR: input file \"%s\" is empty\n", inputFN);
fclose(f);
return NULL;
}
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| char *inputData = static_cast<char *>(malloc(dataLen));
if (!inputData) {
fprintf(stderr, "ERROR: unable to malloc %ld bytes\n", dataLen);
fclose(f);
return NULL;
}
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6.read the bin data
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| // create a pointer p to point the begin of the inputData
char *p = inputData;
// create a bytesLeft to record the moving of offset
size_t bytesLeft = dataLen;
while (bytesLeft) {
//fread will return the bytes of read
size_t bytesRead = fread(p, 1, bytesLeft, f);
bytesLeft -= bytesRead;
p += bytesRead;
if (ferror(f) != 0) {
fprintf(stderr, "ERROR: fread() failed\n");
free(inputData);
fclose(f);
return NULL;
}
}
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7.close the File stream
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| //change the parameter of &length
*length = (unsigned int)dataLen;
return inputData;
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9.完整代码
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| FILE *f = fopen(inputFN, "rb");
if (!f) {
fprintf(stderr, "ERROR: unable to open file \"%s\": %s\n", inputFN,strerror(errno));
return NULL;
}
if (fseek(f, 0, SEEK_END) != 0) {
fprintf(stderr, "ERROR: unable to seek file \"%s\": %s\n", inputFN,
strerror(errno));
fclose(f);
return NULL;
}
long dataLen = ftell(f);
if (dataLen < 0) {
fprintf(stderr, "ERROR: ftell() failed: %s\n", strerror(errno));
fclose(f);
return NULL;
}
if (fseek(f, 0, SEEK_SET) != 0) {
fprintf(stderr, "ERROR: unable to seek file \"%s\": %s\n", inputFN,
strerror(errno));
fclose(f);
return NULL;
}
if ((unsigned long)dataLen > UINT_MAX) {
dataLen = UINT_MAX;
printf("WARNING: clipping data to %ld bytes\n", dataLen);
} else if (dataLen == 0) {
fprintf(stderr, "ERROR: input file \"%s\" is empty\n", inputFN);
fclose(f);
return NULL;
}
char *inputData = static_cast<char *>(malloc(dataLen));
if (!inputData) {
fprintf(stderr, "ERROR: unable to malloc %ld bytes\n", dataLen);
fclose(f);
return NULL;
}
char *p = inputData;
size_t bytesLeft = dataLen;
while (bytesLeft) {
size_t bytesRead = fread(p, 1, bytesLeft, f);
bytesLeft -= bytesRead;
p += bytesRead;
if (ferror(f) != 0) {
fprintf(stderr, "ERROR: fread() failed\n");
free(inputData);
fclose(f);
return NULL;
}
}
fclose(f);
*length = (unsigned int)dataLen;
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