setbuf() Function

The setbuf() function in C stdio.h sets the buffering mode for a file stream, either by assigning a user-defined buffer to the stream or by disabling buffering altogether. This function ensures that the stream becomes fully buffered when a valid buffer is provided, or unbuffered when a null pointer is specified, affecting how input/output operations are handled.

Syntax of setbuf()

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void setbuf(FILE *stream, char *buffer);

Parameters

ParameterDescription
streamPointer to a FILE object that identifies an open stream.
bufferUser-allocated buffer which must be at least BUFSIZ bytes in size, or a null pointer to disable buffering.

This function should be called after a stream is associated with an open file but before any input or output operations are performed. If a valid buffer is provided, the stream becomes fully buffered, using the entire buffer; if a null pointer is passed, buffering is disabled and the stream becomes unbuffered. The behavior is equivalent to calling setvbuf() with _IOFBF and BUFSIZ for a non-null buffer, or _IONBF for a null pointer.

Return Value

setbuf() does not return a value.

Examples for setbuf()

Example 1: Assigning a Custom Buffer to a File Stream

This example demonstrates how to assign a custom buffer to a file stream to enable full buffering. A user-defined buffer is used, and the stream operations will utilize this buffer.

Program

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#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *fp = fopen("example.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    char buf[BUFSIZ];
    setbuf(fp, buf);  /* Assign a custom buffer to fp */

    fprintf(fp, "This is a test using a custom buffer.\n");
    fclose(fp);

    printf("Data written to file with custom buffering.\n");
    return 0;
}

Explanation:

  1. The program opens a file “example.txt” for writing.
  2. A buffer buf of size BUFSIZ is declared.
  3. setbuf() is called to assign the custom buffer to the file stream, making it fully buffered.
  4. Data is written to the file, and the stream is closed, which flushes the buffer.
  5. A confirmation message is printed on the console.

Program Output:

Data written to file with custom buffering.

Example 2: Disabling Buffering for a File Stream

This example illustrates how to disable buffering for a file stream by passing a null pointer to setbuf(). With buffering disabled, every I/O operation is performed directly.

Program

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#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *fp = fopen("nobuffer.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    setbuf(fp, NULL);  /* Disable buffering for fp */

    fprintf(fp, "Buffering is disabled.\n");
    fclose(fp);

    printf("Data written to file without buffering.\n");
    return 0;
}

Explanation:

  1. The file “nobuffer.txt” is opened for writing.
  2. setbuf() is called with a null pointer, disabling buffering for this stream.
  3. The message “Buffering is disabled.” is directly written to the file.
  4. The stream is closed, ensuring that the data is immediately flushed.
  5. A confirmation message is printed on the console.

Example 3: Demonstrating Buffer Flushing with a Custom Buffer

This example shows how data is buffered and then flushed when using a custom buffer. It emphasizes the role of the buffer in reducing the number of physical I/O operations.

Program

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#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *fp = fopen("flushbuffer.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return EXIT_FAILURE;
    }

    char buf[BUFSIZ];
    setbuf(fp, buf);  /* Assign a custom buffer to the stream */

    /* Write data to the file without explicitly flushing */
    fprintf(fp, "Line 1: Buffered output.\n");
    fprintf(fp, "Line 2: More buffered output.\n");

    /* Flushing the stream to force write the buffered data */
    fflush(fp);
    fclose(fp);

    printf("Buffered data flushed to file.\n");
    return 0;
}

Explanation:

  1. The program opens a file “flushbuffer.txt” for writing.
  2. A custom buffer of size BUFSIZ is allocated and assigned to the file stream using setbuf().
  3. Two lines of text are written to the file; these remain in the buffer until flushed.
  4. fflush() is called to flush the buffer, ensuring that all data is written to the file.
  5. The stream is closed and a message is printed to indicate that the buffered data has been flushed.

Program Output:

Buffered data flushed to file.