fread/fwrite string in C

Question

I have a binary file which contains records. The structure of the file is as such:

Structure (see below) Name String Address String

The structure in question:

typedef struct{
    char * name;
    char * address;
    short addressLength, nameLength;
    int phoneNumber;
}employeeRecord;
employeeRecord record;

I get the name as such:

char name[50];
printf("\nName:");
fgets(name,50,stdin);
record.nameLength = strlen(name)-1;
record.name = malloc(sizeof(char)*record.nameLength);
strcpy(record.name,name);

I write the structure, the the name, then the address (as mentioned above).

fwrite(&record.name,sizeof(char),record.nameLength,fp);

where fp is a file pointer.

Now i close the file. However, if i then want to read from the file to get this data back, I believe I need to read in the structure, read the nameLength variable, malloc enough memory for the name to sit in, then fread the name into the variable.

Like so:

char *nameString = malloc(sizeof(char)*record.nameLength);
fread(nameString,sizeof(char),record.nameLength,fp);
printf("\nName: %s",nameString);

However, when i attempt this, i do not get valid data. Example:

Input name is: Joseph (6 characters)
Output data: 
Name length is 6 (correct), 
Name is  �A        � (aka garbage)

So obviously im doing something wrong. Could someone give me some help?

Solution 1

I see two problems with the write, you are setting record.nameLength to be too small, and you are passing the wrong pointer to fwrite for the name. record.name is already a pointer.

Change this

record.nameLength = strlen(name)-1;
...
fwrite(&record.name,sizeof(char),record.nameLength,fp);

to this

record.nameLength = strlen(name);
...
fwrite(record.name,sizeof(char),record.nameLength,fp);

You also have a problem on the read, since you aren't writing the terminating \0 from the strings into your file, when you read back, you need to add that terminator explicitly.

char *nameString = malloc(sizeof(char)* (record.nameLength + 1));
fread(nameString,sizeof(char),record.nameLength,fp);
nameString[record.NameLength] = '\0';

Solution 2

The problem is that you pass the pointer to the char* in your fwrite:

fwrite(&record.name,sizeof(char),record.nameLength,fp);

This means that instead of writing the name, you're writing the memory address of the name. Fwrite expects a pointer to the data to write—in your case, that's the pointer to the char data, not the pointer to the pointer of the char data.

Pass it record.name instead of &record.name and you should be set:

fwrite(record.name, sizeof(char), record.nameLength, fp);

Comments

[zneak]

It won't solve anything, but you could use strdup instead of strlen + malloc + strcpy.

[John Zwinck]

Is this a homework assignment?

[Blackbinary]

added homework tag, forgot about that.

[Blackbinary]

why do i need the null terminator? I know the size of what I'll be reading, so don't i just want to chop it off anyways?

[Blackbinary]

also, if i use the name 'M' for example, using record.nameLength=strlen(name) returns a length of 2, not 1.

[John Knoeller]

strlen doesn't include the null terminator in it's length. (unless you wrote your own strlen function?), strlen("M") will return 1, not 2.

[mctylr]

C strings end with a NULL character ('\0') so that the string related functions, including printf, know when the string ends, because C strings unlike Pascal strings, don't keep an internal string length, so the string "goes on" until it finds a '\0' character to terminate it. This is a common source of problems for new C programmers.