C Programming Code Examples
C > File Operations Code Examples
C Program to Create Employee Record and Update it
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/* C Program to Create Employee Record and Update it */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define size 200
struct emp
{
int id;
char *name;
}*emp1, *emp3;
void display();
void create();
void update();
FILE *fp, *fp1;
int count = 0;
void main(int argc, char **argv)
{
int i, n, ch;
printf("1] Create a Record\n");
printf("2] Display Records\n");
printf("3] Update Records\n");
printf("4] Exit");
while (1)
{
printf("\nEnter your choice : ");
scanf("%d", &ch);
switch (ch)
{
case 1:
fp = fopen(argv[1], "a");
create();
break;
case 2:
fp1 = fopen(argv[1],"rb");
display();
break;
case 3:
fp1 = fopen(argv[1], "r+");
update();
break;
case 4:
exit(0);
}
}
}
/* To create an employee record */
void create()
{
int i;
char *p;
emp1 = (struct emp *)malloc(sizeof(struct emp));
emp1->name = (char *)malloc((size)*(sizeof(char)));
printf("Enter name of employee : ");
scanf(" %[^\n]s", emp1->name);
printf("Enter emp id : ");
scanf(" %d", &emp1->id);
fwrite(&emp1->id, sizeof(emp1->id), 1, fp);
fwrite(emp1->name, size, 1, fp);
count++; // count to number of entries of records
fclose(fp);
}
/* Display the records in the file */
void display()
{
emp3=(struct emp *)malloc(1*sizeof(struct emp));
emp3->name=(char *)malloc(size*sizeof(char));
int i = 1;
if (fp1 == NULL)
printf("\nFile not opened for reading");
while (i <= count)
{
fread(&emp3->id, sizeof(emp3->id), 1, fp1);
fread(emp3->name, size, 1, fp1);
printf("\n%d %s",emp3->id,emp3->name);
i++;
}
fclose(fp1);
free(emp3->name);
free(emp3);
}
void update()
{
int id, flag = 0, i = 1;
char s[size];
if (fp1 == NULL)
{
printf("File cant be opened");
return;
}
printf("Enter employee id to update : ");
scanf("%d", &id);
emp3 = (struct emp *)malloc(1*sizeof(struct emp));
emp3->name=(char *)malloc(size*sizeof(char));
while(i<=count)
{
fread(&emp3->id, sizeof(emp3->id), 1, fp1);
fread(emp3->name,size,1,fp1);
if (id == emp3->id)
{
printf("Enter new name of emplyee to update : ");
scanf(" %[^\n]s", s);
fseek(fp1, -204L, SEEK_CUR);
fwrite(&emp3->id, sizeof(emp3->id), 1, fp1);
fwrite(s, size, 1, fp1);
flag = 1;
break;
}
i++;
}
if (flag != 1)
{
printf("No employee record found");
flag = 0;
}
fclose(fp1);
free(emp3->name); /* to free allocated memory */
free(emp3);
}
In C, the "main" function is treated the same as every function, it has a return type (and in some cases accepts inputs via parameters). The only difference is that the main function is "called" by the operating system when the user runs the program. Thus the main function is always the first code executed when a program starts. main() function is a user defined, body of the function is defined by the programmer or we can say main() is programmer/user implemented function, whose prototype is predefined in the compiler. Hence we can say that main() in c programming is user defined as well as predefined because it's prototype is predefined. main() is a system (compiler) declared function whose defined by the user, which is invoked automatically by the operating system when program is being executed.
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The if-else statement is used to perform two operations for a single condition. The if-else statement is an extension to the if statement using which, we can perform two different operations, i.e., one is for the correctness of that condition, and the other is for the incorrectness of the condition. Here, we must notice that if and else block cannot be executed simiulteneously. Using if-else statement is always preferable since it always invokes an otherwise case with every if condition.
While loop is also known as a pre-tested loop. In general, a while loop allows a part of the code to be executed multiple times depending upon a given boolean condition. It can be viewed as a repeating if statement. The while loop is mostly used in the case where the number of iterations is not known in advance. The while loop evaluates the test expression inside the parentheses (). If test expression is true, statements inside the body of while loop are executed. Then, test expression is evaluated again. The process goes on until test expression is evaluated to false. If test expression is false, the loop terminates.
Open file. Opens the file whose name is specified in the parameter filename and associates it with a stream that can be identified in future operations by the FILE pointer returned. The operations that are allowed on the stream and how these are performed are defined by the mode parameter. The returned stream is fully buffered by default if it is known to not refer to an interactive device (see setbuf). The returned pointer can be disassociated from the file by calling fclose or freopen. All opened files are automatically closed on normal program termination.
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#include is a way of including a standard or user-defined file in the program and is mostly written at the beginning of any C/C++ program. This directive is read by the preprocessor and orders it to insert the content of a user-defined or system header file into the following program. These files are mainly imported from an outside source into the current program. The process of importing such files that might be system-defined or user-defined is known as File Inclusion. This type of preprocessor directive tells the compiler to include a file in the source code program. Here are the two types of file that can be included using #include:
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Allocate memory block. Allocates a block of size bytes of memory, returning a pointer to the beginning of the block. The content of the newly allocated block of memory is not initialized, remaining with indeterminate values. If size is zero, the return value depends on the particular library implementation (it may or may not be a null pointer), but the returned pointer shall not be dereferenced. The "malloc" or "memory allocation" method in C is used to dynamically allocate a single large block of memory with the specified size. It returns a pointer of type void which can be cast into a pointer of any form. It doesn't Iniatialize memory at execution time so that it has initializes each block with the default garbage value initially.
Close file. Closes the file associated with the stream and disassociates it. All internal buffers associated with the stream are disassociated from it and flushed: the content of any unwritten output buffer is written and the content of any unread input buffer is discarded. Even if the call fails, the stream passed as parameter will no longer be associated with the file nor its buffers.
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